Скачать или смотреть Tutorial 18 - GROUP BY And HAVING Clause

Introduction to PostgreSQL HAVING clause
The HAVING clause specifies a search condition for a group or an aggregate. The HAVING clause is often used with the GROUP BY clause to filter groups or aggregates based on a specified condition.

The following statement illustrates the basic syntax of the HAVINGclause:

SELECT
column1,
aggregate_function (column2)
FROM
table_name
GROUP BY
column1
HAVING
condition;
In this syntax, the group by clause returns rows grouped by the column1. The HAVING clause specifies a condition to filter the groups.

It’s possible to add other clauses of the SELECT statement such as JOIN, LIMIT, FETCH etc.

PostgreSQL evaluates the HAVING clause after the FROM, WHERE, GROUP BY, and before the SELECT, DISTINCT, ORDER BY and LIMIT clauses.


Since the HAVING clause is evaluated before the SELECT clause, you cannot use column aliases in the HAVING clause. Because at the time of evaluating the HAVING clause, the column aliases specified in the SELECT clause are not available.

HAVING vs. WHERE
The WHERE clause allows you to filter rows based on a specified condition. However, the HAVING clause allows you to filter groups of rows according to a specified condition.

In other words, the WHERE clause is applied to rows while the HAVING clause is applied to groups of rows.

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