How To Manage an SQL Database

How To Manage an SQL Database

SQL databases come installed with all the commands you need to add, modify, delete, and query your data. This cheat sheet-style guide provides a quick reference to some of the most commonly-used SQL commands.

SQL databases come installed with all the commands you need to add, modify, delete, and query your data. This cheat sheet-style guide provides a quick reference to some of the most commonly-used SQL commands.

An SQL Cheat Sheet


SQL databases come installed with all the commands you need to add, modify, delete, and query your data. This cheat sheet-style guide provides a quick reference to some of the most commonly-used SQL commands.

How to Use This Guide:

  • This guide is in cheat sheet format with self-contained command-line snippets
  • Jump to any section that is relevant to the task you are trying to complete
  • When you see <span class="highlight">highlighted text</span> in this guide’s commands, keep in mind that this text should refer to the columns, tables, and data in your own database.
  • Throughout this guide, the example data values given are all wrapped in apostrophes ('). In SQL, it is necessary to wrap any data values that consist of strings in apostrophes. This isn’t required for numeric data, but it also won’t cause any issues if you do include apostrophes.

Please note that, while SQL is recognized as a standard, most SQL database programs have their own proprietary extensions. This guide uses MySQL as the example relational database management system (RDBMS), but the commands given will work with other relational database programs, including PostgreSQL, MariaDB, and SQLite. Where there are significant differences between RDBMSs, we have included the alternative commands.

Opening up the Database Prompt (using Socket/Trust Authentication)

By default on Ubuntu 18.04, the root MySQL user can authenticate without a password using the following command:

sudo mysql

To open up a PostgreSQL prompt, use the following command. This example will log you in as the postgres user, which is the included superuser role, but you can replace that with any already-created role:

sudo -u postgres psql

Opening up the Database Prompt (using Password Authentication)

If your root MySQL user is set to authenticate with a password, you can do so with the following command:

mysql -u root -p

If you’ve already set up a non-root user account for your database, you can also use this method to log in as that user:

mysql -u user -p

The above command will prompt you for your password after you run it. If you’d like to supply your password as part of the command, immediately follow the -p option with your password, with no space between them:

mysql -u root -ppassword

Creating a Database

The following command creates a database with default settings.

CREATE DATABASE database_name;

If you want your database to use a character set and collation different than the defaults, you can specify those using this syntax:

CREATE DATABASE database_name CHARACTER SET character_set COLLATE collation;

Listing Databases

To see what databases exist in your MySQL or MariaDB installation, run the following command:


In PostgreSQL, you can see what databases have been created with the following command:


Deleting a Database

To delete a database, including any tables and data held within it, run a command that follows this structure:


Creating a User

To create a user profile for your database without specifying any privileges for it, run the following command:

CREATE USER username IDENTIFIED BY 'password';

PostgreSQL uses a similar, but slightly different, syntax:


If you want to create a new user and grant them privileges in one command, you can do so by issuing a GRANT statement. The following command creates a new user and grants them full privileges to every database and table in the RDBMS:

GRANT ALL PRIVILEGES ON *.* TO 'username'@'localhost' IDENTIFIED BY 'password';

Note the PRIVILEGES keyword in this previous GRANT statement. In most RDBMSs, this keyword is optional, and this statement can be equivalently written as:

GRANT ALL ON *.* TO 'username'@'localhost' IDENTIFIED BY 'password';

Be aware, though, that the PRIVILEGES keyword is required for granting privileges like this when Strict SQL mode is turned on.

Deleting a User

Use the following syntax to delete a database user profile:


Note that this command will not by default delete any tables created by the deleted user, and attempts to access such tables may result in errors.

Selecting a Database

Before you can create a table, you first have to tell the RDBMS the database in which you’d like to create it. In MySQL and MariaDB, do so with the following syntax:

USE database;

In PostgreSQL, you must use the following command to select your desired database:

\connect database

Creating a Table

The following command structure creates a new table with the name <span class="highlight">table</span>, and includes two columns, each with their own specific data type:

CREATE TABLE table ( column_1 column_1_data_type, column_2 column_2_data_taype );

Deleting a Table

To delete a table entirely, including all its data, run the following:


Inserting Data into a Table

Use the following syntax to populate a table with one row of data:

INSERT INTO table ( column_A, column_B, column_C ) VALUES ( 'data_A', 'data_B', 'data_C' );

You can also populate a table with multiple rows of data using a single command, like this:

INSERT INTO table ( column_A, column_B, column_C ) VALUES ( 'data_1A', 'data_1B', 'data_1C' ),  ( 'data_2A', 'data_2B', 'data_2C' ), ( 'data_3A', 'data_3B', 'data_3C' );

Deleting Data from a Table

To delete a row of data from a table, use the following command structure. Note that <span class="highlight">value</span> should be the value held in the specified <span class="highlight">column</span> in the row that you want to delete:

DELETE FROM table WHERE column='value';

Note: If you do not include a WHERE clause in a DELETE statement, as in the following example, it will delete all the data held in a table, but not the columns or the table itself:


Changing Data in a Table

Use the following syntax to update the data held in a given row. Note that the WHERE clause at the end of the command tells SQL which row to update. <span class="highlight">value</span> is the value held in <span class="highlight">column_A</span> that aligns with the row you want to change.

Note: If you fail to include a WHERE clause in an UPDATE statement, the command will replace the data held in every row of the table.

UPDATE table SET column_1 = value_1, column_2 = value_2 WHERE column_A=value;

Inserting a Column

The following command syntax will add a new column to a table:

ALTER TABLE table ADD COLUMN column data_type;

Deleting a Column

A command following this structure will delete a column from a table:


Performing Basic Queries

To view all the data from a single column in a table, use the following syntax:

SELECT column FROM table;

To query multiple columns from the same table, separate the column names with a comma:

SELECT column_1, column_2 FROM table;

You can also query every column in a table by replacing the names of the columns with an asterisk (*). In SQL, asterisks act as placeholders to represent “all”:

SELECT * FROM table;

Using WHERE Clauses

You can narrow down the results of a query by appending the SELECT statement with a WHERE clause, like this:

SELECT column FROM table WHERE conditions_that_apply;

For example, you can query all the data from a single row with a syntax like the following. Note that <span class="highlight">value</span> should be a value held in both the specified <span class="highlight">column</span> and the row you want to query:

SELECT * FROM table WHERE column = value;

Working with Comparison Operators

A comparison operator in a WHERE clause defines how the specified column should be compared against the value. Here are some common SQL comparison operators:

Operator What it does = tests for equality != tests for inequality < tests for less-than > tests for greater-than <= tests for less-than or equal-to >= tests for greater-than or equal-to BETWEEN tests whether a value lies within a given range IN tests whether a row’s value is contained in a set of specified values EXISTS tests whether rows exist, given the specified conditions LIKE tests whether a value matches a specified string IS NULL tests for NULL values IS NOT NULL tests for all values other than NULL Working with Wildcards

SQL allows the use of wildcard characters. These are useful if you’re trying to find a specific entry in a table, but aren’t sure of what that entry is exactly.

Asterisks (*) are placeholders that represent “all,” this will query every column in a table:

SELECT * FROM table;

Percentage signs (%) represent zero or more unknown characters.

SELECT * FROM table WHERE column LIKE val%;

Underscores (_) are used to represent a single unknown character:

SELECT * FROM table WHERE column LIKE v_lue;

Counting Entries in a Column

The COUNT function is used to find the number of entries in a given column. The following syntax will return the total number of values held in <span class="highlight">column</span>:

SELECT COUNT(column) FROM table;

You can narrow down the results of a COUNT function by appending a WHERE clause, like this:

SELECT COUNT(column) FROM table WHERE column=value;

Finding the Average Value in a Column

The AVG function is used to find the average (in this case, the mean) amongst values held in a specific column. Note that the AVG function will only work with columns holding numeric values; when used on a column holding string values, it may return an error or 0:

SELECT AVG(column) FROM table;

Finding the Sum of Values in a Column

The SUM function is used to find the sum total of all the numeric values held in a column:

SELECT SUM(column) FROM table;

As with the AVG function, if you run the SUM function on a column holding string values it may return an error or just 0, depending on your RDBMS.

Finding the Largest Value in a Column

To find the largest numeric value in a column or the last value alphabetically, use the MAX function:

SELECT MAX(column) FROM table;

Finding the Smallest Value in a Column

To find the smallest numeric value in a column or the first value alphabetically, use the MIN function:

SELECT MIN(column) FROM table;

Sorting Results with ORDER BY Clauses

An ORDER BY clause is used to sort query results. The following query syntax returns the values from <span class="highlight">column_1</span> and <span class="highlight">column_2</span> and sorts the results by the values held in <span class="highlight">column_1</span> in ascending order or, for string values, in alphabetical order:

SELECT column_1, column_2 FROM table ORDER BY column_1;

To perform the same action, but order the results in descending or reverse alphabetical order, append the query with DESC:

SELECT column_1, column_2 FROM table ORDER BY column_1 DESC;

Sorting Results with GROUP BY Clauses

The GROUP BY clause is similar to the ORDER BY clause, but it is used to sort the results of a query that includes an aggregate function such as COUNT, MAX, MIN, or SUM. On their own, the aggregate functions described in the previous section will only return a single value. However, you can view the results of an aggregate function performed on every matching value in a column by including a GROUP BY clause.

The following syntax will count the number of matching values in <span class="highlight">column_2</span> and group them in ascending or alphabetical order:

SELECT COUNT(column_1), column_2 FROM table GROUP BY column_2;

To perform the same action, but group the results in descending or reverse alphabetical order, append the query with DESC:

SELECT COUNT(column_1), column_2 FROM table GROUP BY column_2 DESC;

Querying Multiple Tables with JOIN Clauses

JOIN clauses are used to create result sets that combine rows from two or more tables. A JOIN clause will only work if the two tables each have a column with an identical name and data type, as in this example:

SELECT table_1.column_1, table_2.column_2 FROM table_1 JOIN table_2 ON table_1.common_column=table_2.common_column;

This is an example of an INNER JOIN clause. An INNER JOIN will return all the records that have matching values in both tables, but won’t show any records that don’t have matching values.

It’s possible to return all the records from one of two tables, including values that do not have a corresponding match in the other table, by using an outer JOIN clause. Outer JOIN clauses are written as either LEFT JOIN or RIGHT JOIN.

A LEFT JOIN clause returns all the records from the “left” table and only the matching records from the “right” table. In the context of outer JOIN clauses, the left table is the one referenced in the FROM clause, and the right table is any other table referenced after the JOIN statement. The following will show every record from <span class="highlight">table_1</span> and only the matching values from <span class="highlight">table_2</span>. Any values that do not have a match in <span class="highlight">table_2</span> will appear as NULL in the result set:

SELECT table_1.column_1, table_2.column_2 FROM table_1 LEFT JOIN table_2 ON table_1.common_column=table_2.common_column;

A RIGHT JOIN clause functions the same as a LEFT JOIN, but it prints the all the results from the right table, and only the matching values from the left:

SELECT table_1.column_1, table_2.column_2 FROM table_1 RIGHT JOIN table_2 ON table_1.common_column=table_2.common_column;

Combining Multiple SELECT Statements with UNION Clauses

A UNION operator is useful for combining the results of two (or more) SELECT statements into a single result set:

SELECT column_1 FROM table UNION SELECT column_2 FROM table;

Additionally, the UNION clause can combine two (or more) SELECT statements querying different tables into the same result set:

SELECT column FROM table_1 UNION SELECT column FROM table_2;


This guide covers some of the more common commands in SQL used to manage databases, users, and tables, and query the contents held in those tables. There are, however, many combinations of clauses and operators that all produce unique result sets. If you’re looking for a more comprehensive guide to working with SQL, we encourage you to check out Oracle’s Database SQL Reference.

Additionally, if there are common SQL commands you’d like to see in this guide, please ask or make suggestions in the comments below.

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What are the differences between Standard SQL and Transact-SQL?

What are the differences between Standard SQL and Transact-SQL?

In this article, we'll explain syntax differences between standard SQL and the Transact-SQL language dedicated to interacting with the SQL

#1 Names of Database Objects

In relational database systems, we name tables, views, and columns, but sometimes we need to use the same name as a keyword or use special characters. In standard SQL, you can place this kind of name in quotation marks (""), but in T-SQL, you can also place it in brackets ([]). Look at these examples for the name of a table in T-SQL:

CREATE TABLE dbo.test.“first name” ( Id INT, Name VARCHAR(100));
CREATE TABLE dbo.test.[first name]  ( Id INT, Name VARCHAR(100));

Only the first delimiter (the quotation marks) for the special name is also part of the SQL standard.

What Is Different in a SELECT Statement?#2 Returning Values

The SQL standard does not have a syntax for a query returning values or values coming from expressions without referring to any columns of a table, but MS SQL Server does allow for this type of expression. How? You can use a SELECT statement alone with an expression or with other values not coming from columns of the table. In T-SQL, it looks like the example below:

SELECT 12/6 ;

In this expression, we don’t need a table to evaluate 12 divided by 6, therefore, the FROM statement and the name of the table can be omitted.

#3 Limiting Records in a Result Set

In the SQL standard, you can limit the number of records in the results by using the syntax illustrated below:


T-SQL implements this syntax in a different way. The example below shows the MS SQL Server syntax:


As you notice, this uses an ORDER BY clause. Another way to select rows, but without ORDER BY, is by using the TOP clause in T-SQL:

#4 Automatically Generating Values

The SQL standard enables you to create columns with automatically generated values. The syntax to do this is shown below:


In T-SQL we can also automatically generate values, but in this way:

#5 Math Functions

Several common mathematical functions are part of the SQL standard. One of these math functions is CEIL(x), which we don’t find in T-SQL. Instead, T-SQL provides the following non-standard functions: SIGN(x), ROUND(x,[,d]) to round decimal value x to the number of decimal positions, TRUNC(x) for truncating to given number of decimal places, LOG(x) to return the natural logarithm for a value x, and RANDOM() to generate random numbers. The highest or lowest number in a list in the SQL standard is returned by MAX(list) and MIN(list) functions, but in Transact-SQL, you use the GREATEST(list) and LEAST(list) functions.

T-SQL function ROUND:


#6 Aggregate Functions

We find another syntax difference with the aggregate functions. The functions COUNT, SUM, and AVG all take an argument related to a count. T-SQL allows the use of DISTINCT before these argument values so that rows are counted only if the values are different from other rows. The SQL standard doesn't allow for the use of DISTINCT in these functions.

Standard SQL:



But in T-SQL we don’t find a population covariance function: COVAR_POP(x,y), which is defined in the SQL standard.

#7 Retrieving Parts of Dates and Times

Most relational database systems deliver many functions to operate on dates and times.

In standard SQL, the EXTRACT(YEAR FROM x) function and similar functions to select parts of dates are different from the T-SQL functions like YEAR(x) or DATEPART(year, x).

There is also a difference in getting the current date and time. Standard SQL allows you to get the current date with the CURRENT_DATE function, but in MS SQL Server, there is not a similar function, so we have to use the GETDATE function as an argument in the CAST function to convert to a DATE data type.

#8 Operating on Strings

Using functions to operate on strings is also different between the SQL standard and T-SQL. The main difference is found in removing trailing and leading spaces from a string. In standard SQL, there is the TRIM function, but in T-SQL, there are several related functions: TRIM (removing trailing and leading spaces), LTRIM (removing leading spaces), and RTRIM (removing trailing spaces).

Another very-often-used string function is SUBSTRING.

The standard SQL syntax for the SUBSTRING function looks like:

SUBSTRING(str FROM start [FOR len])

but in T-SQL, the syntax of this function looks like:

SUBSTRING(str, start, length)

There are reasons sometimes to add values coming from other columns and/or additional strings. Standard SQL enables the following syntax to do this:

As you can see, this syntax makes use of the || operator to add one string to another.

But the equivalent operator in T-SQL is the plus sign character. Look at this example:

SELECT col1 + col2  FROM tab;

In SQL Server, we also have the possibility to use the CONCAT function concatenates a list of strings:

SELECT CONCAT(col1, str1, col2, ...)  FROM tab;

We can also repeat one character several times. Standard SQL defines the function REPEAT(str, n) to do this. Transact-SQL provides the REPLICATE function. For example:


where x indicates how many times to repeat the string or character.

#9 Inequality Operator

During filtering records in a SELECT statement, sometimes we have to use an inequality operator. Standard SQL defines <> as this operator, while T-SQL allows for both the standard operator and the != operator:

SELECT col3 FROM tab WHERE col1 != col2;
#10 ISNULL Function

In T-SQL, we have the ability to replace NULL values coming from a column using the ISNULL function. This is a function that is specific to T-SQL and is not in the SQL standard.

Which Parts of DML Syntax Are Different?

In T-SQL, the basic syntax of DELETE, UPDATE, and INSERT queries is the same as the SQL standard, but differences appear in more advanced queries. Let’s look at them.

#11 OUTPUT Keyword

The OUTPUT keyword occurs in DELETE, UPDATE, and INSERT statements. It is not defined in standard SQL.

Using T-SQL, we can see extra information returned by a query. It returns both old and new values in UPDATE or the values added using INSERT or deleted using DELETE. To see this information, we have to use prefixes in INSERT, UPDATE, and DELETE.

UPDATE tab SET col='new value'
OUTPUT Deleted.col, Inserted.col;

We see the result of changing records with the previous and new values in an updated column. The SQL standard does not support this feature.

#12 Syntax for INSERT INTO ... SELECT

Another structure of an INSERT query is INSERT INTO … SELECT. T-SQL allows you to insert data from another table into a destination table. Look at this query:

INSERT INTO tab SELECT col1,col2,... FROM tab_source;

It is not a standard feature but a feature characteristic of SQL Server.

#13 FROM Clause in DELETE and UPDATE

SQL Server provides extended syntax of the UPDATE and DELETE with FROM clauses. You can use DELETE with FROM to use the rows from one table to remove corresponding rows in another table by referring to a primary key and a foreign key. Similarly, you can use UPDATE with FROM update rows from one table by referring to the rows of another table using common values (primary key in one table and foreign key in second, e.g. the same city name). Here is an example:

FROM Author
WHERE Author.Id=Book.AuthorId AND Author.Name IS NULL;

SET Book.Price=Book.Price*0.2
FROM Author
WHERE Book.AuthorId=Author.Id AND Author.Id=12;

The SQL standard doesn’t provide this syntax.


You can also use INSERT, UPDATE, and DELETE using JOIN to connect to another table. An example of this is:

DELETE ItemOrder FROM ItemOrder
JOIN Item ON ItemOrder.ItemId=Item.Id
WHERE YEAR(Item.DeliveredDate) <= 2017;

This feature is not in the SQL standard.


This article does not cover all the issues about syntax differences between the SQL standard and T-SQL using the MS SQL Server system. However, this guide helps point out some basic features characteristic only of Transact-SQL and what SQL standard syntax isn’t implemented by MS SQL Server.

Thanks for reading. If you liked this post, share it with all of your programming buddies!

Originally published on

SQL Tutorial – Learn SQL Programming Online from Experts

This SQL tutorial will help you learn SQL basics, so you can become a successful SQL developer. You will find out what are the SQL commands, syntax, data types, operators, creation & dropping of tables, inserting and selecting query. Through this tutorial you will learn SQL for working with a relational database. Learn SQL from Intellipaat SQL training and fast-track your career.

SQL Tutorial for Beginners

This SQL Tutorial for Beginners is a complete package for how to learn SQL online. In this SQL tutorial , you will learn SQL programming to get a clear idea of what Structured Query Language is and how you deploy SQL to work with a relational database system.

So, a structured query language is a language that is used to operate the relational databases. Some of the major ways in which SQL is used in conjunction with a relational database is for the purposes of storing, retrieving and manipulating of data stored in a relational database.

Check out Intellipaat’s blog to get a fair understanding of SQL Optimization Techniques!

Here we have the list of topics if you want to jump right into a specific one:

Watch this SQL Tutorial for Beginners video:

What is SQL?

The language to communicate with the relational database is the SQL or Structured Query Language. SQL programming helps to operate the relational databases and derive information from it.

Some of the operations that the SQL does include creation of database, fetching, modifying, updating and deleting the rows along with storing, manipulating and retrieving data within the relational database. SQL programming is an ANSI standard language but there are a lot of versions of SQL in usage as well.

Why is SQL programming so widely used?

Structured Query Language or SQL programming is used so extensively for the following reasons.

  • SQL lets you access any data within the relational database
  • You can describe the data in the database using SQL
  • Using SQL you can manipulate the data with the relational database
  • SQL can embed within other languages through SQL modules & libraries
  • SQL lets you easily create and drop databases and tables
  • SQL allows you to create views, functions and stored procedures in databases
  • Using SQL you can set permissions on procedures, tables and views.

Wish to crack SQL job interviews? Intellipaat’s Top SQL Interview Questions are meant only for you!

Watch this SQL Interview Questions and Answers for Beginners video

Features of SQL

Here in this section of the SQL tutorial for beginners, we list some of the top features of SQL that make it so ubiquitous when it comes to managing relational databases.

  • SQL is very simple and easy to learn language
  • SQL is versatile as it works with database systems from Oracle, IBM, Microsoft, etc.
  • SQL is an ANSI and ISO standard language for database creation and manipulation
  • SQL has a well-defined structure as it uses long established standards
  • SQL is very fast in retrieving large amounts of data very efficiently
  • SQL lets you manage databases without knowing lot of coding

Applications of SQL

Here in this section of the SQL tutorial we will learn SQL applications that make it so important in a data-driven world where managing huge databases is the norm of the day.

  • SQL is used as a Data Definition Language (DDL) meaning you can independently create a database, define its structure, use it and then discard it when you are done with it
  • SQL is also used as a Data Manipulation Language (DML) which means you can use it for maintaining an already existing database. SQL is a powerful language for entering data, modifying data and extracting data with regard to a database
  • SQL is also deployed as a Data Control Language (DCL) which specifies how you can protect your database against corruption and misuse.
  • SQL is extensively used as a Client/Server language to connect the front-end with the back-end thus supporting the client/server architecture
  • SQL can also be used in the three-tier architecture of a client, an application server and a database which defines the Internet architecture.

Still have queries? Come to Intellipaat’s SQL Community, clarify all your doubts, and excel in your career!

Why should you learn SQL?

Today regardless of the relational databases systems by major corporations like Oracle, IBM, Microsoft and others, the one thing that is common to them is the Structured Query Language or SQL.

So if you learn SQL online then you will be able to pursue a very broad career spanning a lot of roles of responsibilities. Also if you are learning SQL then it is important for a data science career as well since a data scientist will also have to deal with relational databases and query it using the standard language SQL.

Originally published at in the tutorial "SQL Tutorial" on 09 Sept. 2019.

Storing a value from SQL database to a variable

I tried this code to extract a single value from my database and store it in a variable. But it extracts the value&nbsp;<code>Supplier ID</code>&nbsp;instead of&nbsp;<code>Quantity</code>.

I tried this code to extract a single value from my database and store it in a variable. But it extracts the value Supplier ID instead of Quantity.

string query3 = "SELECT Quantity FROM Supplier WHERE [Supplier ID]='"+supplierid+"'";
SqlCommand cmd3 = new SqlCommand(query3, con);
string temporaryquantity = cmd.ExecuteScalar().ToString();

How can I extract Quantity instead of Supplier ID?