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Oracle SQL HAVING clause

Oracle Database Tips by Donald BurlesonJuly 7, 2015

Question:  I have a question about the HAVING CLAUSE.  In this expression part we use some normal columns(ex: HAVING deptno=20), Group function on some column (HAVING max(sal)>3000) or both combination of normal column and group function (HAVING max(sal)>3000 and deptno=20).

Now, the columns which we are using in HAVING CLAUSE (in above examples sal, deptno) do they need to be present in both SELECT CLAUSE and GROUP BY CLAUSE or in either of the clause or they need not present in SELECT and GROUP BY clause?

Answer:  The Oracle HAVING clause is used in conjunction with the Oracle GROUP BY clause to limit the returned rows after the grouping.  SQL statements can utilize both a WHERE clause and an Oracle HAVING clause.  The WHERE clause will filter rows as they are selected from the table, and before grouping, the Oracle HAVING clause will filter rows after the grouping. 

Also, queries with the HAVING clause can be convoluted and sometimes difficult to maintain, with sub-optimal execution plans.  See re-writing SQL to remove the HAVING clause.

Answer (by Laurent Schneider):  HAVING DEPTNO=20

this is bad code. you should have

WHERE DEPTNO=20

The big difference is that the WHERE clause is evaluated before the aggregation, so the later will be faster.

If you have a column in the HAVING clause, it must be a group by expression

SELECT 1 FROM DUAL HAVING DUMMY='X';
SELECT 1 FROM DUAL HAVING DUMMY='X'
                          *
Error at line 1
ORA-00979: not a GROUP BY expression


Mostly, the expression you will see in having are aggregate functions like MIN/MAX/COUNT.

   
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