Temporary Tables in SQL server

 Temporary Tables in SQL server

In SQL Server, temporary tables are a type of table that is created and used for a specific session or connection. They are used to store data temporarily that can be used for later processing or analysis. Temporary tables can be created in two ways: local and global. In this blog, we will discuss both types of temporary tables and their usage in SQL Server.

Local Temporary Tables:

A local temporary table is a table that is created within the current connection and is only available to the session that created it. It is automatically dropped when the session is closed. The table name begins with a single hash (#) sign. Here is an example of how to create a local temporary table:

CREATE TABLE #TempTable

(

    ID INT PRIMARY KEY,

    Name VARCHAR(50),

    Age INT

);

In the above example, we have created a local temporary table named #TempTable with three columns: ID, Name, and Age. Once the table is created, we can insert data into it using the INSERT INTO statement:

INSERT INTO #TempTable (ID, Name, Age)

VALUES (1, 'John Doe', 30),

       (2, 'Jane Smith', 25),

       (3, 'Bob Johnson', 40);

After inserting data, we can retrieve the data from the temporary table using a SELECT statement:

SELECT * FROM #TempTable;

Global Temporary Tables:

A global temporary table is a table that is created within the current session, but it is available to all sessions. It is dropped when all sessions referencing it have closed. The table name begins with two hash (##) signs. Here is an example of how to create a global temporary table:

CREATE TABLE ##TempTable

(

    ID INT PRIMARY KEY,

    Name VARCHAR(50),

    Age INT

);

In the above example, we have created a global temporary table named ##TempTable with three columns: ID, Name, and Age. Once the table is created, we can insert data into it using the INSERT INTO statement:

INSERT INTO ##TempTable (ID, Name, Age)

VALUES (1, 'John Doe', 30),

       (2, 'Jane Smith', 25),

       (3, 'Bob Johnson', 40);

After inserting data, we can retrieve the data from the global temporary table using a SELECT statement:

 

SELECT * FROM ##TempTable;

Temporary Tables with SELECT INTO:

Another way to create temporary tables is by using the SELECT INTO statement. This statement creates a new table with the same structure as the result set returned by a SELECT statement. Here is an example:

SELECT ID, Name, Age INTO #TempTable FROM Employees;

In the above example, we have created a local temporary table named #TempTable by selecting data from an existing table named Employees. The new table has the same structure as the Employees table, and the data is copied into it. Once the table is created, we can manipulate the data in the same way as any other table.

SQL Server temporary tables are a useful feature that allows you to create tables that exist only for the duration of a session or transaction. Like any feature, there are both advantages and disadvantages to using temporary tables.

Pros:

Ø  Temporary tables can be used to store intermediate results during complex query processing. This can help improve query performance and reduce the number of round trips between the client and server.

Ø  They allow you to break down complex queries into smaller, more manageable steps. This can make queries easier to understand and debug.

Ø  Temporary tables are often used in stored procedures to store data for later use. This can be useful when working with large amounts of data or when you need to reuse the same data across multiple queries.

Ø  Since temporary tables are only visible within the current session or transaction, they can help to reduce naming conflicts between different users and applications.

Cons:

Ø  Temporary tables can increase disk usage and memory consumption, especially when working with large datasets. This can lead to performance issues and may require additional resources to manage.

Ø  They can also be a source of contention when multiple users or applications are working with the same set of temporary tables. This can lead to issues with locking and blocking, which can impact query performance.

Ø  Since temporary tables are only visible within the current session or transaction, they cannot be used to share data between different sessions or transactions. This can make it more difficult to write code that relies on shared data.

Ø  Temporary tables can be more difficult to manage and maintain than regular tables. They require more attention to ensure that they are created and dropped at the appropriate times, and that they are not causing performance issues.

Ø  Overall, temporary tables are a useful feature that can help improve query performance and simplify complex queries. However, they should be used judiciously, and you should be aware of their potential downsides.