How do temporary tables and table expressions work in SQL and when should each be used?

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Temporary tables and common table expressions, known as CTEs, are used to organize intermediate data storage within complex SQL scripts. Temporary tables are physically created in the system database tempdb or local session space, persist for the duration of the client connection, and can contain indexes, triggers, and constraints. Common table expressions are virtual result sets that exist only for the execution of a single specific SQL query and are not stored on disk.

The choice between temporary tables and CTEs depends on the nature of the problem being solved and the volume of data being processed. If you need to repeatedly access the same set of intermediate data across multiple sequential queries or need to build a complex index to speed up further filtering, a temporary table is the optimal choice. If the query is built hierarchically using recursion or represents a logical separation of a monolithic SQL expression into readable blocks, it is better to use CTE.

When properly designing complex scripts, consider the following recommendations.

1. Avoid abusing temporary tables in high-load transactions, as they put a strain on DBMS service subsystems.
2. Use recursive CTEs exclusively for traversing tree-like structures and graphs, controlling the depth of nesting.
3. Remember that the query optimizer cannot always effectively estimate the amount of data in a temporary table, which sometimes leads to performance degradation.
4. Monitor code readability by breaking long queries into logical named blocks using WITH.

Understanding the features of both mechanisms allows you to write cleaner, more maintainable, and higher-performing code for working with databases of any complexity.

How do temporary tables and table expressions work in SQL and when should each be used?

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