Temporary tables in SQL are special types of tables that are created to hold data temporarily during a session or transaction. Unlike regular tables that are stored in the database until they are explicitly deleted, temporary tables only exist for the duration of a user session, or for as long as the scope in which they were created applies. They are particularly useful for storing intermediate results when performing complex queries, allowing developers to simplify their SQL code and improve performance.
There are two primary types of temporary tables: local and global. Local temporary tables, prefixed with a single hash symbol (e.g., #TempTable), are visible only to the session that created them and are automatically dropped once that session ends. On the other hand, global temporary tables, prefixed with double hash symbols (e.g., ##GlobalTempTable), can be accessed by any user and remain in the database until all sessions referencing them are closed. This feature makes global temporary tables useful for sharing data between multiple sessions without the risk of affecting permanent tables.
Using temporary tables can significantly enhance the efficiency of complex queries. For example, if a developer needs to join several large tables to calculate aggregated statistics, they might first insert the relevant data into a temporary table. Then, subsequent queries can operate on this smaller dataset rather than on the original larger tables, speeding up query execution and reducing resource consumption. This approach not only improves performance but also makes it easier to manage and debug queries. Temporary tables serve as an excellent tool for developers seeking to optimize their SQL operations.