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In SQL Server, developers often face the decision of whether to use temporary tables or table variables when performing joins. Both methods come with their own advantages and disadvantages, and understanding these can help improve the performance and efficiency of your SQL queries. In this post, we will explore the differences between the two and provide some examples using SQL Server 2016, SQL Server 2017, MySQL 5.7, DB2 11.1, and Azure SQL.

Temporary tables are database tables that exist temporarily on the disk. They are useful for storing data that you need to work with during a single session. They can be indexed, have constraints, and trigger events, just like any other table. This makes them versatile and powerful, but also resource-intensive. On the other hand, table variables exist only in memory and are considered to be a better choice for smaller, less complex operations due to their lower resource requirements. However, they lack some of the features of temporary tables, such as the ability to have indexes (other than primary key indexes) or statistics.

One of the key differences between temporary tables and table variables is their transaction log activity. Temporary tables behave like regular tables and write a full log of their activities to the transaction log. This can slow down your queries in high-volume transactions. Table variables, on the other hand, have minimal logging, which can lead to improved performance.

Here's an example of how you would create a temporary table and a table variable in SQL Server:

-- Creating a temporary table
CREATE TABLE #TempTable (ID int, Name varchar(50))

-- Creating a table variable
DECLARE @TableVariable table (ID int, Name varchar(50))

In MySQL, the syntax for creating a temporary table is similar, but there is no direct equivalent for table variables. Instead, developers often use user-defined variables or derived tables to achieve similar results.

-- Creating a temporary table in MySQL
CREATE TEMPORARY TABLE TempTable (ID int, Name varchar(50))

In DB2, both temporary tables and declared temporary tables (the equivalent of table variables) are supported. Here is how you would create them:

-- Creating a global temporary table in DB2
DECLARE GLOBAL TEMPORARY TABLE session.TempTable (ID int, Name varchar(50))

-- Creating a declared temporary table in DB2
DECLARE LOCAL TEMPORARY TABLE TempTable (ID int, Name varchar(50))

In conclusion, when deciding whether to use a temporary table or a table variable, consider the complexity and size of the data you're dealing with, as well as the specific requirements of your query. If you need to perform complex operations or work with large amounts of data, a temporary table might be the better choice. For simpler operations or smaller datasets, a table variable can often provide better performance. As always, it's best to test different methods to determine which is best for your specific use case.

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