SQL Common Table Expressions (CTEs) are a powerful feature in SQL that allows you to define a temporary named result set that can be referred to within a query. CTEs can simplify complex queries, improve readability and maintainability, and make queries more efficient. However, there are some things that are not allowed in CTEs in SQL.
One of the things that is not allowed in CTEs is the use of aggregate functions in the CTE definition. Aggregate functions such as COUNT, SUM, AVG, MAX, and MIN can only be used in the SELECT statement that references the CTE, not in the CTE definition itself. This is because CTEs are evaluated before the SELECT statement, and so the result of an aggregate function cannot be determined until after the CTE has been evaluated.
Another thing that is not allowed in CTEs is the use of ORDER BY or GROUP BY clauses in the CTE definition. These clauses can only be used in the SELECT statement that references the CTE. This is because CTEs are not materialized tables, and so they do not have a physical order or grouping.
Recursive CTEs are another type of CTE that has some restrictions. Recursive CTEs allow you to traverse hierarchical data structures such as trees or graphs, but they must have a base case and a recursive case. The base case is the initial query that defines the starting point of the traversal, and the recursive case is the subsequent query that continues the traversal. Recursive CTEs must also have a UNION ALL clause that combines the base case and the recursive case, and the recursive case must reference the CTE name.
CTEs are also not allowed in certain SQL statements, such as INSERT, UPDATE, and DELETE statements. CTEs can only be used in SELECT statements or as part of a subquery in another statement.
SQL Common Table Expressions are a powerful feature in SQL that can simplify complex queries and improve query performance. However, there are some things that are not allowed in CTEs, such as aggregate functions in the CTE definition, ORDER BY and GROUP BY clauses in the CTE definition, and the use of CTEs in certain SQL statements. Understanding these restrictions is important when working with CTEs in SQL.
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