Cómo usar disparadores SQL

En este tutorial, aprenderá: ¿Qué son los disparadores de SQL, cómo crear disparadores, cómo soltarlos y cuándo usarlos para poder escribir mejores consultas.

MySQL Triggers son como escuchas de eventos de JavaScript. No se ejecutan hasta que ocurre una acción que se les ha dicho que escuchen.

Aquí hay una descripción útil de ellos de los documentos de MySQL:

Un disparador es un objeto de base de datos con nombre que está asociado con una tabla y que se activa cuando ocurre un evento particular para la tabla. Algunos usos de los disparadores son para realizar comprobaciones de valores que se insertarán en una tabla o para realizar cálculos sobre valores involucrados en una actualización. - MySQL.com

Los disparadores son útiles para automatizar tareas repetitivas. Simplemente puede configurar un disparador para hacer algún cálculo después de cada acción específica de la base de datos. También puede configurar un activador para realizar tareas de validación de datos en una tabla.

Un activador se activa cuando ocurre una operación o en una tabla de base de datos INSERT. Se activa un activador por fila, por lo que si se insertan o eliminan varias filas de datos, cada una activa la configuración de acción del activador. Se puede configurar un activador para que se dispare antes o después de una acción.UPDATEDELETE

En este tutorial, aprenderá cómo crear activadores, cómo colocarlos y cuándo son útiles.

Cómo crear un disparador

Para crear un nuevo disparador, use el CREATE TRIGGERcomando. Este comando tiene la siguiente estructura:

CREATE TRIGGER trigger_name
trigger_time 
trigger_event
ON table_name 
FOR EACH ROW
trigger_body

Así que aquí hay un desglose de cada línea:

• La palabra clave CREATE TRIGGERes obligatoria y va seguida del nombre del disparador. Utilizará este nombre para referirse al disparador en el futuro y para eliminar el disparador si alguna vez surge la necesidad. Este nombre debe ser único por base de datos.
• El trigger_timees un valor variable que solo puede ser BEFOREo AFTER. Esto determina si el activador se disparará antes o después de que haya ocurrido el evento.
• El trigger_eventes otra variable que tiene un número limitado de opciones posibles. Esta variable no puede tener ningún valor que no sea INSERT, UPDATEo DELETE. Especifica qué evento escuchar.
• table_namees el nombre de la tabla que debe vigilar el activador. Este tiene que ser el nombre de una tabla existente en su base de datos, pero puede ser una tabla vacía.
• La FOR EACH ROWes la otra parte obligatoria de la definición del disparador.
• trigger_bodyes la consulta SQL que desea que se ejecute cuando se active este activador.

Para crear un disparador de ejemplo, crearé una userstabla simple para practicar.

CREATE TABLE
    users (
        fullname VARCHAR(120),
        email VARCHAR(120),
        username VARCHAR(30),
        password VARCHAR(60)
    );

Ahora, podemos crear un disparador simple y adjuntarlo a esta tabla vacía. Un disparador que cifre las contraseñas de cadena antes de que se inserten usando la MD5función tendría sentido.

CREATE TRIGGER password_hasher BEFORE INSERT ON users FOR EACH ROW
SET
    NEW.password = MD5 (NEW.password);

Este ejemplo es bastante sencillo y se explica por sí mismo. Pero hay una NEWpalabra clave allí. Esta palabra clave le da acceso a los nuevos datos que se están creando y le permite usar o modificar los valores como desee.

Solo puede modificar estos valores si su conjunto event_timees BEFORE. Si se event_timeestablece en AFTER, los datos ya se almacenaron antes de llegar al activador, por lo que no se pueden volver a modificar.  

Puede utilizar la NEWpalabra clave en INSERTy UPDATEeventos, pero no el DELETEevento.

También está la OLDpalabra clave que puede usar DELETEy UPDATElos activadores de eventos que le dan acceso a los valores anteriores del registro afectado. No puede usar esta palabra clave en un INSERTevento porque no hay un registro anterior antes de que se creen nuevos datos.

Para probar este activador, inserte una fila en la userstabla:

INSERT INTO
    users
VALUES
    (
        'idris babu',
        'zubs@test.com',
        'zubby1',
        'password'
    );

Consulte su tabla para conocer los valores. Deberías tener algo como esto:

El valor de la contraseña se ha cifrado correctamente. 🥳

Cómo soltar un gatillo

Después de crear un activador, es posible que desee detener su ejecución por algún motivo. En este caso, puede soltar el gatillo.

Para soltar el gatillo, use el DROP TRIGGERcomando. El comando solo requiere el nombre del disparador. Puedes usar el comando así:

DROP TRIGGER password_hasher;

Ejecutar esta consulta eliminará el activador que creamos anteriormente y todos los registros insertados a partir de ahora no tendrán la contraseña cifrada.

Para probar esto, inserte el mismo registro que antes y verifique el resultado.

El registro recién creado no tiene una contraseña cifrada.

Algo a tener en cuenta: si suelta la tabla por completo, todos los desencadenantes asociados también se eliminan automáticamente.

Cuándo usar disparadores

1. Registro: puede tener un activador para escribir automáticamente en otra tabla al insertar, actualizar o eliminar un registro de una tabla.
2. Validación de datos: puede escribir un disparador para asegurarse de que los datos sean de cierto tipo y que se puedan establecer los valores correctos cuando sea necesario.
3. Sincronización de datos: puede usar un activador para mantener actualizadas las tablas relacionadas. Por ejemplo, en una tabla de comercio electrónico, cada vez que se crea un registro de ventas, un activador puede actualizar el saldo del proveedor. O si se elimina el registro del proveedor, un activador puede eliminar todos sus productos.

Resumen

Espero que ahora comprenda los disparadores de SQL y cuándo usarlos para que pueda escribir mejores consultas.

Fuente: https://www.freecodecamp.org

#sql 

What is GEEK

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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.

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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.

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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.

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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

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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

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