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HAVING Clause

The optional HAVING clause has the general form

where condition is the same as specified for the WHERE clause.

HAVING eliminates group rows that do not satisfy the condition. HAVING is different from WHERE: WHERE filters individual rows before the application of GROUP BY, while HAVING filters group rows created by GROUP BY. Each column referenced in condition must unambiguously reference a grouping column, unless the reference appears within an aggregate function or the ungrouped column is functionally dependent on the grouping columns.

The presence of HAVING turns a query into a grouped query even if there is no GROUP BY clause. This is the same as what happens when the query contains aggregate functions but no GROUP BY clause. All the selected rows are considered to form a single group, and the SELECT list and HAVING clause can only reference table columns from within aggregate functions. Such a query will emit a single row if the HAVING condition is true, zero rows if it is not true.

Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE cannot be specified with HAVING.

FROM Clause

The FROM clause specifies one or more source tables for the SELECT. If multiple sources are specified, the result is the Cartesian product (cross join) of all the sources. But usually qualification conditions are added (via WHERE) to restrict the returned rows to a small subset of the Cartesian product.

One of

• [ INNER ] JOIN

• LEFT [ OUTER ] JOIN

• RIGHT [ OUTER ] JOIN

• FULL [ OUTER ] JOIN

• CROSS JOIN

Join 종류

 설명

 Cross Join

 Outer Node 의 각 row 에 대해 Inner Node 의 모든 row가 결합되어 반환

 Inner Join

 Outer Node 의 각 row 에 대해 Inner Node 의 모든 row 중 Join 조건을 만족하는 row 에 대해서만 결합되어 반환

 Equi Join