What is a SQL Server CTE?
CTE stands for Common Table Expression.
A CTE is basically a subquery. So why is it special? This is why they are good:
- Separate a subquery from the body of the main query.
- Cleaner code – CTEs enable you to separate your logically congruent code segments.
- Can be used for improving the execution plan of your query.
- Can be used for recursive, hierarchical queries.
When aren’t they good? Almost never. Basically, if you don’t need a subquery don’t use a CTE. Also don’t bother using a CTE when you are performing a query with resources outside of SQL Server, there are negative performance issues in SQL Server 2005, but is fixed in SQL Server 2008. Here is everything you will need to know about CTEs.