CTE vs Temp Table vs Temp variables

Temp tables: they are just like any normal tables except that they are created in the TempDB database. They persist until dropped, or until the connection that created them disappears. They are visible in the procedure that created them and any procedures that it calls. Just like normal tables and have primary keys, constraints, indexes and column statistics which are kept for the table which makes query execution really fast in most cases.

Temp tables have space assigned to them in the TempDB database and are generally accessed only from memory, unless the server is under memory pressure or the amount of data in the table is large.

Table Variables: These tables behave very much like other variables in their scoping rules. They are created when they are declared and are dropped when they go out of scope. They cannot be explicitly dropped. Like with temp tables, table variables also reside in TempDB. They are also generally accessed only from memory, unless the server is under memory pressure or the amount of data in the table is large.

Table variables can have a primary key, but no indexes can be created on them. They don’t have any statistics maintained on the columns. This makes table variables less optimal for large numbers of rows, as the optimizer has no way of knowing the number of rows in the table variable.

CTE: CTE’s are more like temporary views than anything else. When you look at the execution plan, you’ll see that they are substituted/inlined into the query but not materialized and stored anywhere. With the exception of recursion, they’re more to make queries simpler to write than faster to run.

Hope this helps…

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