Dear Dev, if you are a SQL Server developer, you would know how crucial it is to work with temporary tables. These tables play an essential role in database development and management, making them a must-have tool in your arsenal. In this article, we’ll explore everything you need to know about SQL Server temporary tables, from their definition to their usage, optimization, and more.
What are SQL Server Temporary Tables?
SQL Server temporary tables are tables that are created and used for a specific session or task. Unlike permanent tables, which are stored in a database and remain there indefinitely, temporary tables exist only as long as the session or task that created them is active. Once the session is closed or the task is completed, the temporary tables are automatically dropped.
Temporary tables are useful when you need to store data temporarily, such as when you are performing complex calculations or joining multiple datasets. They are also handy when you need to manipulate data in multiple steps, allowing you to store intermediate results and reduce the load on the server.
Types of SQL Server Temporary Tables
There are two types of SQL Server temporary tables: local temporary tables and global temporary tables. Local temporary tables are created with a single pound sign (#) symbol and are only visible to the session that created them. Global temporary tables, on the other hand, are created with a double pound sign (##) symbol and are visible to all sessions. Let’s take a closer look at each type of temporary table:
Local Temporary Tables
Local temporary tables are created with a single pound sign (#) symbol and are only visible to the session that created them. They are stored in the tempdb database and are automatically dropped when the session that created them is closed. Here’s an example:
“`CREATE TABLE #TempTable(ID INT,Name VARCHAR(50))“`
In this example, we have created a local temporary table called #TempTable with two columns: ID and Name. This table will only be visible to the current session.
Global Temporary Tables
Global temporary tables are created with a double pound sign (##) symbol and are visible to all sessions. They are also stored in the tempdb database and are automatically dropped when the last session that referenced them is closed. Here’s an example:
“`CREATE TABLE ##TempTable(ID INT,Name VARCHAR(50))“`
In this example, we have created a global temporary table called ##TempTable with two columns: ID and Name. This table will be visible to all sessions.
Usage of SQL Server Temporary Tables
SQL Server temporary tables are used in a wide range of scenarios. Here are some common use cases:
Storing Intermediate Results
When performing complex calculations or joining multiple datasets, it’s often helpful to store intermediate results in temporary tables. This can help reduce the load on the server and simplify the code. Here’s an example:
“`CREATE TABLE #IntermediateTable(ID INT,Name VARCHAR(50))INSERT INTO #IntermediateTableSELECT ID, NameFROM Table1WHERE Column1 = ‘Value1’SELECT *FROM #IntermediateTableWHERE Name LIKE ‘%Value%’“`
In this example, we have created a local temporary table called #IntermediateTable to store the intermediate results of the SELECT statement. We then use this temporary table to further filter the results.
Data Manipulation
SQL Server temporary tables can also be used to manipulate data in multiple steps. For example:
“`CREATE TABLE #TempTable(ID INT,Name VARCHAR(50))INSERT INTO #TempTableSELECT ID, NameFROM Table1WHERE Column1 = ‘Value1’UPDATE #TempTableSET Name = ‘NewName’WHERE ID = 1DELETE FROM #TempTableWHERE Name LIKE ‘%Value%’“`
In this example, we have created a local temporary table called #TempTable and inserted some data into it. We then update and delete the data in multiple steps using the temporary table.
Replacing Cursor Operations
SQL Server temporary tables can also be used as an alternative to cursor operations. Cursors are often used to iterate through a result set and perform operations on each row. However, cursors can be slow and resource-intensive. By using temporary tables, you can often achieve the same result with better performance. Here’s an example:
“`CREATE TABLE #TempTable(ID INT,Name VARCHAR(50))INSERT INTO #TempTableSELECT ID, NameFROM Table1WHERE Column1 = ‘Value1’DECLARE @ID INTDECLARE @Name VARCHAR(50)WHILE EXISTS (SELECT * FROM #TempTable)BEGINSELECT TOP 1 @ID = ID, @Name = Name FROM #TempTable– Perform operations on @ID and @Name hereDELETE FROM #TempTable WHERE ID = @IDEND“`
In this example, we have created a local temporary table called #TempTable and inserted some data into it. We then use a WHILE loop to iterate through the result set and perform operations on each row. The temporary table is used to store the result set, eliminating the need for a cursor.
Optimizing SQL Server Temporary Tables
While SQL Server temporary tables can be a powerful tool, they can also be a source of performance issues if not used correctly. Here are some tips for optimizing temporary tables:
Minimize the Number of Temporary Tables
Try to minimize the number of temporary tables you create. Each temporary table creates overhead on the server, so if possible, use a single temporary table to store intermediate results.
Use Memory-Optimized Temporary Tables
SQL Server 2014 introduced memory-optimized temporary tables, which can significantly improve performance for temporary tables. These tables are stored in memory instead of disk, resulting in faster access times. To use memory-optimized temporary tables, you need to enable the In-Memory OLTP feature of SQL Server.
Distribute Data Across Multiple Temporary Tables
When dealing with large datasets, consider splitting the data across multiple temporary tables. This can help reduce the load on the server and improve performance. For example:
“`CREATE TABLE #TempTable1(ID INT,Name VARCHAR(50))INSERT INTO #TempTable1SELECT ID, NameFROM Table1WHERE Column1 = ‘Value1’ AND ID % 2 = 0CREATE TABLE #TempTable2(ID INT,Name VARCHAR(50))INSERT INTO #TempTable2SELECT ID, NameFROM Table1WHERE Column1 = ‘Value1’ AND ID % 2 = 1SELECT *FROM #TempTable1UNION ALLSELECT *FROM #TempTable2“`
In this example, we have split the data from Table1 into two temporary tables based on the ID column. We then combine the two temporary tables using a UNION ALL statement.
FAQs
Q. Can I use SQL Server temporary tables in stored procedures?
A. Yes, SQL Server temporary tables can be used in stored procedures, as well as in other types of database objects.
Q. Do temporary tables have indexes?
A. Yes, you can create indexes on temporary tables just like you would with permanent tables. However, keep in mind that creating too many indexes on temporary tables can reduce performance.
Q. Can I access temporary tables across sessions?
A. Only global temporary tables can be accessed across sessions. Local temporary tables are only visible to the session that created them.
Q. Do temporary tables support constraints?
A. Yes, you can create constraints on temporary tables just like you would with permanent tables.
Q. Can I use temporary tables in parallel execution plans?
A. Yes, temporary tables can be used in parallel execution plans. However, keep in mind that parallel execution can increase the load on the server, so use with caution.
Conclusion
In conclusion, SQL Server temporary tables are a powerful tool for database developers and administrators. By understanding their definition, usage, and optimization techniques, you can improve your database applications and reduce the load on the server. Keep these tips in mind the next time you are working with temporary tables in SQL Server.