having 은 조건에 만족하지 않는 그룹 rows를 제거한다. HAVING은 WHERE과는 다르다: WHERE는 GROUP BY의 적용 전에 개별 rows를 필터하는 대신 HAVING는 GROUP BY에 의해 생성된 group rows를 필터한다.

(해석은 하는데 무슨 뜻이여...

일단은 생성하고 나서 하나하나 생성하냐, 아니면 그룹으로 묶이는 애들 레벨에서 미리 처리하냐 차이인가?)

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.

[링크 : https://www.postgresql.org/docs/9.5/sql-select.html]

Posted by 구차니

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