QueryBuilder: Add support for HAVING

[blackcoder87]

The HAVING clause, like the WHERE clause, specifies selection conditions. The WHERE clause specifies conditions on columns in the select list, but cannot refer to aggregate functions. The HAVING clause specifies conditions on groups, typically formed by the GROUP BY clause. The query result includes only groups satisfying the HAVING conditions. (If no GROUP BY is present, all rows implicitly form a single aggregate group.)

The HAVING clause is applied nearly last, just before items are sent to the client, with no optimization. (LIMIT is applied after HAVING.)

The SQL standard requires that HAVING must reference only columns in the GROUP BY clause or columns used in aggregate functions. However, MySQL supports an extension to this behavior, and permits HAVING to refer to columns in the SELECT list and columns in outer subqueries as well.

SELECT COUNT(col1) AS col2 FROM t GROUP BY col2 HAVING col2 = 2;

SELECT user, MAX(salary) FROM users GROUP BY user HAVING MAX(salary) > 10;

Additional context
https://dev.mysql.com/doc/refman/5.7/en/select.html
https://dev.mysql.com/doc/refman/8.1/en/select.html
https://mariadb.com/kb/en/select/