Although SQL is a declarative language, it provides a way of implementing the logic of the sequential execution of code or reusing code.
Common table expressions (CTEs) are parts of a SQL statement that produce result sets, defined once, with a view to reuse it, possibly several times, in other parts of the statement.
The simplified syntax diagram for a CTE is as follows:
WITH <subquery name> AS (<subquery code>) [, ...]
SELECT <Select list> FROM <subquery name>;
In the preceding syntax, subquery code is a query whose results will be used later in the primary query, as if it were a real table. The subquery in parentheses after the AS keyword is a CTE. It can also be called a sub-statement or an auxiliary statement. The query after the WITH block is the primary or main query. The whole statement itself is called a WITH query.
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