Cómo Aprender SQL En 10 Minutos

Como investigador de IA, una de las principales cosas con las que trato son los datos. Es un montón .

Con más de 2,5 exabytes de datos generados cada día, no es de extrañar que estos datos deban almacenarse en un lugar accesible cuando sea necesario.

En este artículo, usaremos una hoja de trucos pirateable para ayudarlo a iniciar y ejecutar SQL rápidamente.

¿Qué es SQL?

SQL son las siglas de Structured Query Language. Este es el lenguaje para los sistemas de administración de bases de datos relacionales. SQL se utiliza actualmente para almacenar, recuperar y manipular datos en bases de datos relacionales.

La base de datos relacional básica se ve así:

Fuente

Puede utilizar SQL para escribir consultas para manipular la base de datos .

Un ejemplo de consulta se ve así:

SELECT * FROM customers;

Esto, SELECTmediante el uso de la declaración, la consulta selecciona todos los datos de todas las columnas de la tabla de clientes y devuelve los datos de la siguiente manera.

Fuente: Guía de base de datos

El carácter comodín del asterisco (*) es " todos se refieren a" todos seleccionan la fila y la columna para usted. Puede reemplazarlo con nombres de columna específicos; aquí solo esas columnas son devueltas por la consulta

SELECT FirstName, LastName FROM customers;

WHEREPuede agregar cláusulas para filtrar lo que se devuelve.

SELECT * FROM customers WHERE age >= 30 ORDER BY age ASC;

Esta consulta devuelve todos los datos de la tabla de productos con valores de edad superiores a 30 .

ORDER BYEl uso de palabras clave significa que los resultados se ordenan de mínimo a máximo utilizando una columna de edad.

Puede INSERT INTOusar esta declaración para agregar nuevos datos a la tabla. Este es un ejemplo básico de cómo agregar un nuevo usuario a la tabla de clientes.

INSERT INTO customers(FirstName, LastName, address, email)
VALUES ('Jason', 'Dsouza', 'McLaren Vale, South Australia', 'test@fakeGmail.com');

Por supuesto, estos ejemplos muestran solo una pequeña parte de lo que puede hacer con el lenguaje SQL. Esta guía lo explicará en detalle.

¿Por qué aprender SQL?

Vivimos en la era del big data. Los macrodatos utilizan datos para encontrar conocimientos e informar estrategias, marketing, publicidad y muchas otras operaciones.

Grandes empresas como Google, Amazon y AirBnb dependen de grandes bases de datos relacionales como base para mejorar la experiencia del cliente. Comprender SQL es una gran habilidad para todos, no solo para los científicos y analistas de datos.

Hace solo unos minutos, cuando buscaba en Google sus zapatos favoritos, ¿qué piensa de un anuncio de YouTube que aparece repentinamente en sus zapatos? Este es SQL (o formato SQL).

SQL y MySQL

Antes de continuar, me gustaría aclarar un tema que a menudo es confuso: la diferencia entre SQL y MySQL. ¡Después de todo, son lo mismo que no !

SQL es un lenguaje, pero MySQL es un sistema para implementar SQL.

SQL describe la sintaxis para escribir consultas que administran bases de datos relacionales.

MySQL es un sistema de base de datos que se ejecuta en un servidor . Esto le permite escribir consultas y administrar su base de datos MySQL usando sintaxis SQL.

Además de MySQL, existen otros sistemas que implementan SQL. Algunos de los más populares son:

  • SQLite
  • Base de datos Oracle
  • PostgreSQL
  • Microsoft SQL Server

Cómo instalar MySQL

En la mayoría de los casos, se recomienda MySQL para su sistema de administración de bases de datos. Muchos sistemas de gestión de contenido populares (como Wordpress) utilizan MySQL de forma predeterminada, por lo que le recomendamos que utilice MySQL para gestionar estas aplicaciones.

Para usar MySQL, necesita tener MySQL instalado en su sistema.

Instalar MySQL en Windows

La forma recomendada de instalar MySQL en Windows es utilizar el instalador MSI en el sitio web de MySQL .

Este recurso lo guiará a través del proceso de instalación.

Instalar MySQL en Mac OS

En macOS, la instalación de MySQL también incluye la descarga del instalador .

Este recurso lo guiará a través del proceso de instalación.

Cómo utilizar MySQL

Ahora que MySQL está instalado en su sistema, le recomendamos que utilice algún tipo de aplicación de administración SQL para facilitar la administración de su base de datos .

Hay muchas aplicaciones que hacen casi el mismo trabajo, por lo que la que uses dependerá de tu gusto personal.

  • MySQL Workbench fue desarrollado por Oracle
  • phpMyAdmin (funciona en un navegador web)
  • HeidiSQL (recomendado para Windows)
  • Sequel Pro (recomendado para macOS)

Cuando esté listo para comenzar a escribir sus propias consultas SQL, considere importar datos ficticios en lugar de crear su propia base de datos.

Aquí hay algunas bases de datos ficticias que puede descargar de forma gratuita .

Hoja de trucos de SQL: guinda del pastel 

Palabra clave SQL

Aquí puede encontrar una colección de palabras clave utilizadas en declaraciones SQL, descripciones y ejemplos según sea necesario. Algunas de las palabras clave más avanzadas tienen sus propias secciones.

Si MySQL aparece junto al ejemplo, esto significa que este ejemplo solo es aplicable a bases de datos MySQL (a diferencia de otros sistemas de bases de datos).

ADD -- Adds a new column to an existing table

ADD CONSTRAINT -- Creates a new constraint on an existing table, which is used to specify rules for any data in the table.

ALTER TABLE -- Adds, deletes or edits columns in a table. It can also be used to add and delete constraints in a table, as per the above.

ALTER COLUMN -- Changes the data type of a table’s column.

ALL -- Returns true if all of the subquery values meet the passed condition.

AND -- Used to join separate conditions within a WHERE clause.

ANY -- Returns true if any of the subquery values meet the given condition.

AS -- Renames a table or column with an alias value which only exists for the duration of the query.

ASC -- Used with ORDER BY to return the data in ascending order.

BETWEEN -- Selects values within the given range.

CASE -- Changes query output depending on conditions.

CHECK -- Adds a constraint that limits the value which can be added to a column.

CREATE DATABASE -- Creates a new database.

CREATE TABLE -- Creates a new table. 

DEFAULT -- Sets a default value for a column

DELETE -- Delete data from a table.

DESC -- Used with ORDER BY to return the data in descending order.

DROP COLUMN -- Deletes a column from a table.

DROP DATABASE -- Deletes the entire database.

DROP DEAFULT -- Removes a default value for a column.

DROP TABLE -- Deletes a table from a database.

EXISTS -- Checks for the existence of any record within the subquery, returning true if one or more records are returned.

FROM -- Specifies which table to select or delete data from.

IN --  Used alongside a WHERE clause as a shorthand for multiple OR conditions.

INSERT INTO -- Adds new rows to a table.

IS NULL -- Tests for empty (NULL) values.

IS NOT NULL -- The reverse of NULL. Tests for values that aren’t empty / NULL.

LIKE -- Returns true if the operand value matches a pattern.

NOT -- Returns true if a record DOESN’T meet the condition.
 
OR -- Used alongside WHERE to include data when either condition is true.

ORDER BY -- Used to sort the result data in ascending (default) or descending order through the use of ASC or DESC keywords.

ROWNUM -- Returns results where the row number meets the passed condition.

SELECT -- Used to select data from a database, which is then returned in a results set.

SELECT DISTINCT -- Sames as SELECT, except duplicate values are excluded.

SELECT INTO -- Copies data from one table and inserts it into another.

SELECT TOP -- Allows you to return a set number of records to return from a table.

SET -- Used alongside UPDATE to update existing data in a table.

SOME -- Identical to ANY.

TOP -- Used alongside SELECT to return a set number of records from a table.

TRUNCATE TABLE -- Similar to DROP, but instead of deleting the table and its data, this deletes only the data.

UNION -- Combines the results from 2 or more SELECT statements and returns only distinct values.

UNION ALL -- The same as UNION, but includes duplicate values.

UNIQUE -- This constraint ensures all values in a column are unique.

UPDATE -- Updates existing data in a table.

VALUES -- Used alongside the INSERT INTO keyword to add new values to a table.

WHERE -- Filters results to only include data which meets the given condition.

Comentario SQL

Los comentarios se pueden utilizar para describir una sección de una instrucción SQL sin ejecutarla directamente.

Hay dos tipos de comentarios en SQL, una línea y varias líneas.

Comentario SQL de una línea

Los comentarios de una línea comienzan con "-". Se ignora todo el texto desde estos dos caracteres hasta el final de la línea.

-- This part is ignored

SELECT * FROM customers;

Comentario de varias líneas de SQL

Los comentarios de varias líneas comienzan con / * y terminan con * /. Se extienden a lo largo de varias líneas hasta que se encuentra el carácter final.

/*

This is a multiline comment.
It can span across multiple lines.

*/

SELECT * FROM customers;

/*

This is another comment. 
You can even put code within a comment to prevent its execution

SELECT * FROM icecreams;

*/

Tipo de datos MySQL

Cuando crea una tabla nueva o edita una tabla existente, debe especificar el tipo de datos que acepta cada columna.

En este ejemplo , los iddatos pasados ​​a la columna deben ser un entero (entero), FirstNamepero el VARCHARtipo de datos de la columna puede tener hasta 255 caracteres.

CREATE TABLE customers(
id int,
FirstName varchar(255)
);

1. Tipo de datos de cadena

CHAR(size) -- Fixed length string which can contain letters, numbers and special characters. The size parameter sets the maximum string length, from 0 – 255 with a default of 1.

VARCHAR(size) -- Variable length string similar to CHAR(), but with a maximum string length range from 0 to 65535.

BINARY(size) -- Similar to CHAR() but stores binary byte strings.

VARBINARY(size) -- Similar to VARCHAR() but for binary byte strings.

TINYBLOB -- Holds Binary Large Objects (BLOBs) with a max length of 255 bytes.

TINYTEXT -- Holds a string with a maximum length of 255 characters. Use VARCHAR() instead, as it’s fetched much faster.

TEXT(size) -- Holds a string with a maximum length of 65535 bytes. Again, better to use VARCHAR().

BLOB(size) -- Holds Binary Large Objects (BLOBs) with a max length of 65535 bytes.

MEDIUMTEXT -- Holds a string with a maximum length of 16,777,215 characters.

MEDIUMBLOB -- Holds Binary Large Objects (BLOBs) with a max length of 16,777,215 bytes.

LONGTEXT -- Holds a string with a maximum length of 4,294,967,295 characters.

LONGBLOB -- Holds Binary Large Objects (BLOBs) with a max length of 4,294,967,295 bytes.

ENUM(a, b, c, etc…) -- A string object that only has one value, which is chosen from a list of values which you define, up to a maximum of 65535 values. If a value is added which isn’t on this list, it’s replaced with a blank value instead.

SET(a, b, c, etc…) -- A string object that can have 0 or more values, which is chosen from a list of values which you define, up to a maximum of 64 values.

2. Tipo de datos numéricos

BIT(size) -- A bit-value type with a default of 1. The allowed number of bits in a value is set via the size parameter, which can hold values from 1 to 64.

TINYINT(size) -- A very small integer with a signed range of -128 to 127, and an unsigned range of 0 to 255. Here, the size parameter specifies the maximum allowed display width, which is 255.

BOOL -- Essentially a quick way of setting the column to TINYINT with a size of 1. 0 is considered false, whilst 1 is considered true.

BOOLEAN	-- Same as BOOL.

SMALLINT(size) -- A small integer with a signed range of -32768 to 32767, and an unsigned range from 0 to 65535. Here, the size parameter specifies the maximum allowed display width, which is 255.

MEDIUMINT(size) -- A medium integer with a signed range of -8388608 to 8388607, and an unsigned range from 0 to 16777215. Here, the size parameter specifies the maximum allowed display width, which is 255.

INT(size) -- A medium integer with a signed range of -2147483648 to 2147483647, and an unsigned range from 0 to 4294967295. Here, the size parameter specifies the maximum allowed display width, which is 255.

INTEGER(size) -- Same as INT.

BIGINT(size) -- A medium integer with a signed range of -9223372036854775808 to 9223372036854775807, and an unsigned range from 0 to 18446744073709551615. Here, the size parameter specifies the maximum allowed display width, which is 255.

FLOAT(p) -- A floating point number value. If the precision (p) parameter is between 0 to 24, then the data type is set to FLOAT(), whilst if it's from 25 to 53, the data type is set to DOUBLE(). This behaviour is to make the storage of values more efficient.

DOUBLE(size, d) -- A floating point number value where the total digits are set by the size parameter, and the number of digits after the decimal point is set by the d parameter.

DECIMAL(size, d) -- An exact fixed point number where the total number of digits is set by the size parameters, and the total number of digits after the decimal point is set by the d parameter.

DEC(size, d) -- Same as DECIMAL.

3. Tipo de datos de fecha / hora

DATE -- A simple date in YYYY-MM–DD format, with a supported range from ‘1000-01-01’ to ‘9999-12-31’.

DATETIME(fsp) -- A date time in YYYY-MM-DD hh:mm:ss format, with a supported range from ‘1000-01-01 00:00:00’ to ‘9999-12-31 23:59:59’. By adding DEFAULT and ON UPDATE to the column definition, it automatically sets to the current date/time.

TIMESTAMP(fsp) -- A Unix Timestamp, which is a value relative to the number of seconds since the Unix epoch (‘1970-01-01 00:00:00’ UTC). This has a supported range from ‘1970-01-01 00:00:01’ UTC to ‘2038-01-09 03:14:07’ UTC.
By adding DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT TIMESTAMP to the column definition, it automatically sets to current date/time.

TIME(fsp) -- A time in hh:mm:ss format, with a supported range from ‘-838:59:59’ to ‘838:59:59’.

YEAR -- A year, with a supported range of ‘1901’ to ‘2155’.

Operador SQL

1. Operador aritmético SQL

+ -- Add
– -- Subtract
* -- Multiply
/ -- Divide
% -- Modulus

2. Operador de bit SQL

& -- Bitwise AND
| -- Bitwise OR
^-- Bitwise XOR

3. Operador de comparación SQL

= -- Equal to
> -- Greater than
< -- Less than
>= -- Greater than or equal to
<= -- Less than or equal to
<> -- Not equal to

4. Operador compuesto de SQL

+= -- Add equals
-= -- Subtract equals
*= -- Multiply equals
/= -- Divide equals
%= -- Modulo equals
&= -- Bitwise AND equals
^-= -- Bitwise exclusive equals
|*= -- Bitwise OR equals

Función SQL

1. Función de cadena SQL

ASCII -- Returns the equivalent ASCII value for a specific character.

CHAR_LENGTH -- Returns the character length of a string.

CHARACTER_LENGTH -- Same as CHAR_LENGTH.

CONCAT -- Adds expressions together, with a minimum of 2.

CONCAT_WS -- Adds expressions together, but with a separator between each value.

FIELD -- Returns an index value relative to the position of a value within a list of values.

FIND IN SET -- Returns the position of a string in a list of strings.

FORMAT -- When passed a number, returns that number formatted to include commas (eg 3,400,000).

INSERT -- Allows you to insert one string into another at a certain point, for a certain number of characters.

INSTR -- Returns the position of the first time one string appears within another.

LCASE -- Converts a string to lowercase.

LEFT -- Starting from the left, extracts the given number of characters from a string and returns them as another.

LENGTH -- Returns the length of a string, but in bytes.

LOCATE -- Returns the first occurrence of one string within another,

LOWER -- Same as LCASE.

LPAD -- Left pads one string with another, to a specific length.

LTRIM -- Removes any leading spaces from the given string.

MID -- Extracts one string from another, starting from any position.

POSITION -- Returns the position of the first time one substring appears within another.

REPEAT -- Allows you to repeat a string

REPLACE -- Allows you to replace any instances of a substring within a string, with a new substring.

REVERSE	-- Reverses the string.

RIGHT -- Starting from the right, extracts the given number of characters from a string and returns them as another.

RPAD -- Right pads one string with another, to a specific length.

RTRIM -- Removes any trailing spaces from the given string.

SPACE -- Returns a string full of spaces equal to the amount you pass it.

STRCMP -- Compares 2 strings for differences

SUBSTR -- Extracts one substring from another, starting from any position.

SUBSTRING -- Same as SUBSTR

SUBSTRING_INDEX	-- Returns a substring from a string before the passed substring is found the number of times equals to the passed number.

TRIM --	Removes trailing and leading spaces from the given string. Same as if you were to run LTRIM and RTRIM together.

UCASE -- Converts a string to uppercase.

UPPER -- Same as UCASE.

2. Funciones numéricas SQL

ABS -- Returns the absolute value of the given number.

ACOS -- Returns the arc cosine of the given number.

ASIN -- Returns the arc sine of the given number.

ATAN -- Returns the arc tangent of one or 2 given numbers.

ATAN2 -- Returns the arc tangent of 2 given numbers.

AVG -- Returns the average value of the given expression.

CEIL -- Returns the closest whole number (integer) upwards from a given decimal point number.

CEILING -- Same as CEIL.

COS -- Returns the cosine of a given number.

COT -- Returns the cotangent of a given number.

COUNT -- Returns the amount of records that are returned by a SELECT query.

DEGREES -- Converts a radians value to degrees.

DIV -- Allows you to divide integers.

EXP -- Returns e to the power of the given number.

FLOOR -- Returns the closest whole number (integer) downwards from a given decimal point number.

GREATEST -- Returns the highest value in a list of arguments.

LEAST -- Returns the smallest value in a list of arguments.

LN -- Returns the natural logarithm of the given number.

LOG -- Returns the natural logarithm of the given number, or the logarithm of the given number to the given base.

LOG10 -- Does the same as LOG, but to base 10.

LOG2 -- Does the same as LOG, but to base 2.

MAX -- Returns the highest value from a set of values.

MIN -- Returns the lowest value from a set of values.

MOD -- Returns the remainder of the given number divided by the other given number.

PI -- Returns PI.

POW -- Returns the value of the given number raised to the power of the other given number.

POWER -- Same as POW.

RADIANS -- Converts a degrees value to radians.

RAND -- Returns a random number.

ROUND -- Rounds the given number to the given amount of decimal places.

SIGN -- Returns the sign of the given number.

SIN -- Returns the sine of the given number.

SQRT -- Returns the square root of the given number.

SUM -- Returns the value of the given set of values combined.

TAN -- Returns the tangent of the given number.

TRUNCATE -- Returns a number truncated to the given number of decimal places.

3. Función de fecha SQL

ADDDATE -- Adds a date interval (eg: 10 DAY) to a date (eg: 20/01/20) and returns the result (eg: 20/01/30).

ADDTIME -- Adds a time interval (eg: 02:00) to a time or datetime (05:00) and returns the result (07:00).

CURDATE -- Gets the current date.

CURRENT_DATE -- Same as CURDATE.

CURRENT_TIME -- Gest the current time.

CURRENT_TIMESTAMP -- Gets the current date and time.

CURTIME -- Same as CURRENT_TIME.

DATE -- Extracts the date from a datetime expression.

DATEDIFF -- Returns the number of days between the 2 given dates.

DATE_ADD -- Same as ADDDATE.

DATE_FORMAT -- Formats the date to the given pattern.

DATE_SUB -- Subtracts a date interval (eg: 10 DAY) to a date (eg: 20/01/20) and returns the result (eg: 20/01/10).

DAY -- Returns the day for the given date.

DAYNAME -- Returns the weekday name for the given date.

DAYOFWEEK -- Returns the index for the weekday for the given date.

DAYOFYEAR -- Returns the day of the year for the given date.

EXTRACT -- Extracts from the date the given part (eg MONTH for 20/01/20 = 01).

FROM DAYS -- Returns the date from the given numeric date value.

HOUR -- Returns the hour from the given date.

LAST DAY -- Gets the last day of the month for the given date.

LOCALTIME -- Gets the current local date and time.

LOCALTIMESTAMP -- Same as LOCALTIME.

MAKEDATE -- Creates a date and returns it, based on the given year and number of days values.

MAKETIME -- Creates a time and returns it, based on the given hour, minute and second values.

MICROSECOND -- Returns the microsecond of a given time or datetime.

MINUTE -- Returns the minute of the given time or datetime.

MONTH -- Returns the month of the given date.

MONTHNAME -- Returns the name of the month of the given date.

NOW -- Same as LOCALTIME.

PERIOD_ADD -- Adds the given number of months to the given period.

PERIOD_DIFF -- Returns the difference between 2 given periods.

QUARTER -- Returns the year quarter for the given date.

SECOND -- Returns the second of a given time or datetime.

SEC_TO_TIME -- Returns a time based on the given seconds.

STR_TO_DATE -- Creates a date and returns it based on the given string and format.

SUBDATE -- Same as DATE_SUB.

SUBTIME -- Subtracts a time interval (eg: 02:00) to a time or datetime (05:00) and returns the result (03:00).

SYSDATE -- Same as LOCALTIME.

TIME -- Returns the time from a given time or datetime.

TIME_FORMAT -- Returns the given time in the given format.

TIME_TO_SEC -- Converts and returns a time into seconds.

TIMEDIFF -- Returns the difference between 2 given time/datetime expressions.

TIMESTAMP -- Returns the datetime value of the given date or datetime.

TO_DAYS -- Returns the total number of days that have passed from ‘00-00-0000’ to the given date.

WEEK -- Returns the week number for the given date.

WEEKDAY -- Returns the weekday number for the given date.

WEEKOFYEAR -- Returns the week number for the given date.

YEAR -- Returns the year from the given date.

YEARWEEK -- Returns the year and week number for the given date.

4. Otras funciones de SQL

BIN -- Returns the given number in binary.

BINARY -- Returns the given value as a binary string.

CAST -- Converst one type into another.

COALESCE -- From a list of values, returns the first non-null value.

CONNECTION_ID -- For the current connection, returns the unique connection ID.

CONV -- Converts the given number from one numeric base system into another.

CONVERT -- Converts the given value into the given datatype or character set.

CURRENT_USER -- Returns the user and hostname which was used to authenticate with the server.

DATABASE -- Gets the name of the current database.

GROUP BY -- Used alongside aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the results.

HAVING -- Used in the place of WHERE with aggregate functions.

IF -- If the condition is true it returns a value, otherwise it returns another value.

IFNULL -- If the given expression equates to null, it returns the given value.

ISNULL -- If the expression is null, it returns 1, otherwise returns 0.

LAST_INSERT_ID -- For the last row which was added or updated in a table, returns the auto increment ID.

NULLIF -- Compares the 2 given expressions. If they are equal, NULL is returned, otherwise the first expression is returned.

SESSION_USER -- Returns the current user and hostnames.

SYSTEM_USER -- Same as SESSION_USER.

USER -- Same as SESSION_USER.

VERSION -- Returns the current version of the MySQL powering the database.

Carácter comodín SQL

En SQL, los comodines son caracteres especiales que se utilizan en LIKEy NOT LIKEpalabras clave. Esto le permite buscar datos con un alto patrón de manera bastante eficiente.

% -- Equates to zero or more characters.
-- Example: Find all customers with surnames ending in ‘ory’.
SELECT * FROM customers
WHERE surname LIKE '%ory';

_ -- Equates to any single character.
-- Example: Find all customers living in cities beginning with any 3 characters, followed by ‘vale’.
SELECT * FROM customers
WHERE city LIKE '_ _ _vale';

[charlist] -- Equates to any single character in the list.
-- Example: Find all customers with first names beginning with J, K or T.
SELECT * FROM customers
WHERE first_name LIKE '[jkt]%';

Clave SQL

Las bases de datos relacionales tienen el concepto de claves primarias y claves externas . En las tablas SQL, estos se incluyen como restricciones y la tabla puede tener una clave principal, una clave externa o ambas.

1. Clave principal de SQL

Puede utilizar el primario para identificar de forma única cada registro de la tabla. Puede tener solo una clave principal por tabla y puede asignar esta restricción a cualquier combinación de una sola columna o de una sola columna. Sin embargo, esto significa que cada valor de esta columna debe ser único.

En una tabla, la columna de ID suele ser la clave principal y suele estar AUTO_INCREMENTemparejada con una palabra clave. Esto significa que el valor aumentará automáticamente a medida que se creen nuevos registros.

Ejemplo (MySQL)

Cree una nueva tabla y establezca la clave principal en la columna ID.

CREATE TABLE customers (
id int NOT NULL AUTO_INCREMENT,
FirstName varchar(255),
Last Name varchar(255) NOT NULL,
address varchar(255),
email varchar(255),
PRIMARY KEY (id)
);

2. Clave externa de SQL

Puede aplicar una clave externa a una o más columnas. Use esto para vincular dos tablas en una base de datos relacional .

Las tablas que contienen claves externas se denominan claves secundarias y se denominan claves secundarias .

La tabla que contiene la clave a la que se hace referencia (o candidata) se denomina tabla principal .

Esto básicamente significa que los datos de la columna se comparten entre las dos tablas. Esto es para evitar que la clave externa inyecte datos no válidos que ni siquiera existen en la tabla principal.

Ejemplo (MySQL)

Cree una nueva tabla y convierta las columnas que hacen referencia a los ID de otras tablas en claves externas.

CREATE TABLE orders (
id int NOT NULL,
user_id int,
product_id int,
PRIMARY KEY (id),
FOREIGN KEY (user_id) REFERENCES users(id),
FOREIGN KEY (product_id) REFERENCES products(id)
);

Índice SQL

Los índices son atributos que se pueden asignar a las columnas que se buscan con frecuencia para que la recuperación de datos sea más rápida y eficiente.

CREATE INDEX -- Creates an index named ‘idx_test’ on the first_name and surname columns of the users table. In this instance, duplicate values are allowed.
CREATE INDEX idx_test
ON users (first_name, surname);
CREATE UNIQUE INDEX -- The same as the above, but no duplicate values.
CREATE UNIQUE INDEX idx_test
ON users (first_name, surname);
DROP INDEX -- Removes an index.
ALTER TABLE users
DROP INDEX idx_test;

Combinación SQL

SQL usa JOINcláusulas para devolver el resultado de combinar datos de varias tablas en función de una columna común contenida en ambas tablas.

Hay varias combinaciones diferentes que puede utilizar.

  • Unión interna (predeterminado): devuelve todos los registros con valores coincidentes en ambas tablas.
  • Unión a la izquierda: devuelve todos los registros de la primera tabla y los registros coincidentes de la segunda tabla.
  • Unión derecha: devuelve todos los registros de la segunda tabla, junto con los registros coincidentes de la primera tabla.
  • Unión completa: devuelve todos los registros de ambas tablas si coinciden.

A continuación, se muestra una forma común de visualizar cómo funcionan las combinaciones:

Fuente : configuración del sitio web

SELECT orders.id, users.FirstName, users.Surname, products.name as ‘product name’
FROM orders
INNER JOIN users on orders.user_id = users.id
INNER JOIN products on orders.product_id = products.id;

Vista SQL

La vista es esencialmente un conjunto de resultados de SQL, almacenado en la base de datos debajo de la etiqueta, por lo que puede volver a la vista más tarde sin tener que volver a consultar.

Son especialmente útiles si tiene costosas consultas SQL que puede necesitar muchas veces. Por lo tanto, en lugar de ejecutarlo varias veces para producir el mismo conjunto de resultados, puede ejecutarlo una vez y guardarlo como una vista.

Cómo crear una vista en SQL

Para crear una vista:

CREATE VIEW priority_users AS
SELECT * FROM users
WHERE country = ‘United Kingdom’;

Luego, si necesita acceder al conjunto de resultados guardado, puede acceder a él de la siguiente manera:

SELECT * FROM [priority_users];

Cómo reemplazar una vista con SQL

Puede CREATE OR REPLACEutilizar este comando para actualizar la vista de la siguiente manera:

CREATE OR REPLACE VIEW [priority_users] AS
SELECT * FROM users
WHERE country = ‘United Kingdom’ OR country=’USA’;

Cómo eliminar una vista en SQL

Para eliminar una vista, DROP VIEWsimplemente use el comando.

DROP VIEW priority_users;

en conclusión

La mayoría de los sitios web y aplicaciones utilizan bases de datos relacionales de alguna manera. Esto hace que sea muy valioso conocer SQL y le permite crear sistemas más complejos y funcionales.

 #sql 

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Cómo Aprender SQL En 10 Minutos
Cayla  Erdman

Cayla Erdman

1594369800

Introduction to Structured Query Language SQL pdf

SQL stands for Structured Query Language. SQL is a scripting language expected to store, control, and inquiry information put away in social databases. The main manifestation of SQL showed up in 1974, when a gathering in IBM built up the principal model of a social database. The primary business social database was discharged by Relational Software later turning out to be Oracle.

Models for SQL exist. In any case, the SQL that can be utilized on every last one of the major RDBMS today is in various flavors. This is because of two reasons:

1. The SQL order standard is genuinely intricate, and it isn’t handy to actualize the whole standard.

2. Every database seller needs an approach to separate its item from others.

Right now, contrasts are noted where fitting.

#programming books #beginning sql pdf #commands sql #download free sql full book pdf #introduction to sql pdf #introduction to sql ppt #introduction to sql #practical sql pdf #sql commands pdf with examples free download #sql commands #sql free bool download #sql guide #sql language #sql pdf #sql ppt #sql programming language #sql tutorial for beginners #sql tutorial pdf #sql #structured query language pdf #structured query language ppt #structured query language

Cayla  Erdman

Cayla Erdman

1596441660

Welcome Back the T-SQL Debugger with SQL Complete – SQL Debugger

When you develop large chunks of T-SQL code with the help of the SQL Server Management Studio tool, it is essential to test the “Live” behavior of your code by making sure that each small piece of code works fine and being able to allocate any error message that may cause a failure within that code.

The easiest way to perform that would be to use the T-SQL debugger feature, which used to be built-in over the SQL Server Management Studio tool. But since the T-SQL debugger feature was removed completely from SQL Server Management Studio 18 and later editions, we need a replacement for that feature. This is because we cannot keep using the old versions of SSMS just to support the T-SQL Debugger feature without “enjoying” the new features and bug fixes that are released in the new SSMS versions.

If you plan to wait for SSMS to bring back the T-SQL Debugger feature, vote in the Put Debugger back into SSMS 18 to ask Microsoft to reintroduce it.

As for me, I searched for an alternative tool for a T-SQL Debugger SSMS built-in feature and found that Devart company rolled out a new T-SQL Debugger feature to version 6.4 of SQL – Complete tool. SQL Complete is an add-in for Visual Studio and SSMS that offers scripts autocompletion capabilities, which help develop and debug your SQL database project.

The SQL Debugger feature of SQL Complete allows you to check the execution of your scripts, procedures, functions, and triggers step by step by adding breakpoints to the lines where you plan to start, suspend, evaluate, step through, and then to continue the execution of your script.

You can download SQL Complete from the dbForge Download page and install it on your machine using a straight-forward installation wizard. The wizard will ask you to specify the installation path for the SQL Complete tool and the versions of SSMS and Visual Studio that you plan to install the SQL Complete on, as an add-in, from the versions that are installed on your machine, as shown below:

Once SQL Complete is fully installed on your machine, the dbForge SQL Complete installation wizard will notify you of whether the installation was completed successfully or the wizard faced any specific issue that you can troubleshoot and fix easily. If there are no issues, the wizard will provide you with an option to open the SSMS tool and start using the SQL Complete tool, as displayed below:

When you open SSMS, you will see a new “Debug” tools menu, under which you can navigate the SQL Debugger feature options. Besides, you will see a list of icons that will be used to control the debug mode of the T-SQL query at the leftmost side of the SSMS tool. If you cannot see the list, you can go to View -> Toolbars -> Debugger to make these icons visible.

During the debugging session, the SQL Debugger icons will be as follows:

The functionality of these icons within the SQL Debugger can be summarized as:

  • Adding Breakpoints to control the execution pause of the T-SQL script at a specific statement allows you to check the debugging information of the T-SQL statements such as the values for the parameters and the variables.
  • Step Into is “navigate” through the script statements one by one, allowing you to check how each statement behaves.
  • Step Over is “execute” a specific stored procedure if you are sure that it contains no error.
  • Step Out is “return” from the stored procedure, function, or trigger to the main debugging window.
  • Continue executing the script until reaching the next breakpoint.
  • Stop Debugging is “terminate” the debugging session.
  • Restart “stop and start” the current debugging session.

#sql server #sql #sql debugger #sql server #sql server stored procedure #ssms #t-sql queries

Cayla  Erdman

Cayla Erdman

1596448980

The Easy Guide on How to Use Subqueries in SQL Server

Let’s say the chief credit and collections officer asks you to list down the names of people, their unpaid balances per month, and the current running balance and wants you to import this data array into Excel. The purpose is to analyze the data and come up with an offer making payments lighter to mitigate the effects of the COVID19 pandemic.

Do you opt to use a query and a nested subquery or a join? What decision will you make?

SQL Subqueries – What Are They?

Before we do a deep dive into syntax, performance impact, and caveats, why not define a subquery first?

In the simplest terms, a subquery is a query within a query. While a query that embodies a subquery is the outer query, we refer to a subquery as the inner query or inner select. And parentheses enclose a subquery similar to the structure below:

SELECT 
 col1
,col2
,(subquery) as col3
FROM table1
[JOIN table2 ON table1.col1 = table2.col2]
WHERE col1 <operator> (subquery)

We are going to look upon the following points in this post:

  • SQL subquery syntax depending on different subquery types and operators.
  • When and in what sort of statements one can use a subquery.
  • Performance implications vs. JOINs.
  • Common caveats when using SQL subqueries.

As is customary, we provide examples and illustrations to enhance understanding. But bear in mind that the main focus of this post is on subqueries in SQL Server.

Now, let’s get started.

Make SQL Subqueries That Are Self-Contained or Correlated

For one thing, subqueries are categorized based on their dependency on the outer query.

Let me describe what a self-contained subquery is.

Self-contained subqueries (or sometimes referred to as non-correlated or simple subqueries) are independent of the tables in the outer query. Let me illustrate this:

-- Get sales orders of customers from Southwest United States 
-- (TerritoryID = 4)

USE [AdventureWorks]
GO
SELECT CustomerID, SalesOrderID
FROM Sales.SalesOrderHeader
WHERE CustomerID IN (SELECT [CustomerID]
                     FROM [AdventureWorks].[Sales].[Customer]
                     WHERE TerritoryID = 4)

As demonstrated in the above code, the subquery (enclosed in parentheses below) has no references to any column in the outer query. Additionally, you can highlight the subquery in SQL Server Management Studio and execute it without getting any runtime errors.

Which, in turn, leads to easier debugging of self-contained subqueries.

The next thing to consider is correlated subqueries. Compared to its self-contained counterpart, this one has at least one column being referenced from the outer query. To clarify, I will provide an example:

USE [AdventureWorks]
GO
SELECT DISTINCT a.LastName, a.FirstName, b.BusinessEntityID
FROM Person.Person AS p
JOIN HumanResources.Employee AS e ON p.BusinessEntityID = e.BusinessEntityID
WHERE 1262000.00 IN
    (SELECT [SalesQuota]
    FROM Sales.SalesPersonQuotaHistory spq
    WHERE p.BusinessEntityID = spq.BusinessEntityID)

Were you attentive enough to notice the reference to BusinessEntityID from the Person table? Well done!

Once a column from the outer query is referenced in the subquery, it becomes a correlated subquery. One more point to consider: if you highlight a subquery and execute it, an error will occur.

And yes, you are absolutely right: this makes correlated subqueries pretty harder to debug.

To make debugging possible, follow these steps:

  • isolate the subquery.
  • replace the reference to the outer query with a constant value.

Isolating the subquery for debugging will make it look like this:

SELECT [SalesQuota]
    FROM Sales.SalesPersonQuotaHistory spq
    WHERE spq.BusinessEntityID = <constant value>

Now, let’s dig a little deeper into the output of subqueries.

Make SQL Subqueries With 3 Possible Returned Values

Well, first, let’s think of what returned values can we expect from SQL subqueries.

In fact, there are 3 possible outcomes:

  • A single value
  • Multiple values
  • Whole tables

Single Value

Let’s start with single-valued output. This type of subquery can appear anywhere in the outer query where an expression is expected, like the WHERE clause.

-- Output a single value which is the maximum or last TransactionID
USE [AdventureWorks]
GO
SELECT TransactionID, ProductID, TransactionDate, Quantity
FROM Production.TransactionHistory
WHERE TransactionID = (SELECT MAX(t.TransactionID) 
                       FROM Production.TransactionHistory t)

When you use a MAX() function, you retrieve a single value. That’s exactly what happened to our subquery above. Using the equal (=) operator tells SQL Server that you expect a single value. Another thing: if the subquery returns multiple values using the equals (=) operator, you get an error, similar to the one below:

Msg 512, Level 16, State 1, Line 20
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Multiple Values

Next, we examine the multi-valued output. This kind of subquery returns a list of values with a single column. Additionally, operators like IN and NOT IN will expect one or more values.

-- Output multiple values which is a list of customers with lastnames that --- start with 'I'

USE [AdventureWorks]
GO
SELECT [SalesOrderID], [OrderDate], [ShipDate], [CustomerID]
FROM Sales.SalesOrderHeader
WHERE [CustomerID] IN (SELECT c.[CustomerID] FROM Sales.Customer c
INNER JOIN Person.Person p ON c.PersonID = p.BusinessEntityID
WHERE p.lastname LIKE N'I%' AND p.PersonType='SC')

Whole Table Values

And last but not least, why not delve into whole table outputs.

-- Output a table of values based on sales orders
USE [AdventureWorks]
GO
SELECT [ShipYear],
COUNT(DISTINCT [CustomerID]) AS CustomerCount
FROM (SELECT YEAR([ShipDate]) AS [ShipYear], [CustomerID] 
      FROM Sales.SalesOrderHeader) AS Shipments
GROUP BY [ShipYear]
ORDER BY [ShipYear]

Have you noticed the FROM clause?

Instead of using a table, it used a subquery. This is called a derived table or a table subquery.

And now, let me present you some ground rules when using this sort of query:

  • All columns in the subquery should have unique names. Much like a physical table, a derived table should have unique column names.
  • ORDER BY is not allowed unless TOP is also specified. That’s because the derived table represents a relational table where rows have no defined order.

In this case, a derived table has the benefits of a physical table. That’s why in our example, we can use COUNT() in one of the columns of the derived table.

That’s about all regarding subquery outputs. But before we get any further, you may have noticed that the logic behind the example for multiple values and others as well can also be done using a JOIN.

-- Output multiple values which is a list of customers with lastnames that start with 'I'
USE [AdventureWorks]
GO
SELECT o.[SalesOrderID], o.[OrderDate], o.[ShipDate], o.[CustomerID]
FROM Sales.SalesOrderHeader o
INNER JOIN Sales.Customer c on o.CustomerID = c.CustomerID
INNER JOIN Person.Person p ON c.PersonID = p.BusinessEntityID
WHERE p.LastName LIKE N'I%' AND p.PersonType = 'SC'

In fact, the output will be the same. But which one performs better?

Before we get into that, let me tell you that I have dedicated a section to this hot topic. We’ll examine it with complete execution plans and have a look at illustrations.

So, bear with me for a moment. Let’s discuss another way to place your subqueries.

#sql server #sql query #sql server #sql subqueries #t-sql statements #sql

Ruth  Nabimanya

Ruth Nabimanya

1621850444

List of Available Database for Current User In SQL Server

Introduction

When working in the SQL Server, we may have to check some other databases other than the current one which we are working. In that scenario we may not be sure that does we have access to those Databases?. In this article we discuss the list of databases that are available for the current logged user in SQL Server

Get the list of database
Conclusion

#sql server #available databases for current user #check database has access #list of available database #sql #sql query #sql server database #sql tips #sql tips and tricks #tips

Introduction to Recursive CTE

This article will introduce the concept of SQL recursive. Recursive CTE is a really cool. We will see that it can often simplify our code, and avoid a cascade of SQL queries!

Why use a recursive CTE ?

The recursive queries are used to query hierarchical data. It avoids a cascade of SQL queries, you can only do one query to retrieve the hierarchical data.

What is recursive CTE ?

First, what is a CTE? A CTE (Common Table Expression) is a temporary named result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. For example, you can use CTE when, in a query, you will use the same subquery more than once.

A recursive CTE is one having a subquery that refers to its own name!

Recursive CTE is defined in the SQL standard.

How to make a recursive CTE?

A recursive CTE has this structure:

  • The WITH clause must begin with “WITH RECURSIVE”
  • The recursive CTE subquery has two parts, separated by “UNION [ALL]” or “UNION DISTINCT”:
  • The first part produces the initial row(s) for the CTE. This SELECT does not refer to the CTE name.
  • The second part recurses by referring to the CTE name in its FROM clause.

Practice / Example

In this example, we use hierarchical data. Each row can have zero or one parent. And it parent can also have a parent etc.

Create table test (id integer, parent_id integer);

insert into test (id, parent_id) values (1, null);

insert into test (id, parent_id) values (11, 1);
insert into test (id, parent_id) values (111, 11);

insert into test (id, parent_id) values (112, 11);

insert into test (id, parent_id) values (12, 1);

insert into test (id, parent_id) values (121, 12);

For example, the row with id 111 has as ancestors: 11 and 1.

Before knowing the recursive CTE, I was doing several queries to get all the ancestors of a row.

For example, to retrieve all the ancestors of the row with id 111.

While (has parent)

	Select id, parent_id from test where id = X

With recursive CTE, we can retrieve all ancestors of a row with only one SQL query :)

WITH RECURSIVE cte_test AS (
	SELECT id, parent_id FROM test WHERE id = 111
	UNION 
	SELECT test.id, test.parent_id FROM test JOIN cte_test ON cte_test.id = test.parent_id

) SELECT * FROM cte_test

Explanations:

  • “WITH RECURSIVE”:

It indicates we will make recursive

  • “SELECT id, parent_id FROM test WHERE id = 111”:

It is the initial query.

  • “UNION … JOIN cte_test” :

It is the recursive expression! We make a jointure with the current CTE!

Replay this example here

#sql #database #sql-server #sql-injection #writing-sql-queries #sql-beginner-tips #better-sql-querying-tips #sql-top-story