In order to use HAVING in SQL queries , must there be a GROUP BY to aggregate the column names?

Are there any special cases where it is possible to use HAVING without a GROUP BY in SQL queries?

Must they co-exist at the same time?



They don't have to coexist, as proved by the fact that the following query in Oracle works:

select * from dual having 1 = 1;

Similarly, in PostgreSQL the following query works:

select 1 having 1 = 1;

So having doesn't require group by.

Having is applied after the aggregation phase and must be used if you want to filter aggregate results. So the reverse isn't true, and the following won't work:

select a, count(*) as c
from mytable
group by a
where c > 1;

You need to replace where with having in this case, as follows:

select a, count(*) as c
from mytable
group by a
having c > 1;

NB The following query form will also work:

select *
from (
  select a, count(*) as c
  from mytable
  group by a
where c > 1;

You can see that using having is simply a shorthand version of this last query.

In summary, having is applied after the group by phase whereas where is applied before the group by phase.

  • 2
    Another example: SELECT MIN(a) AS mina, MAX(a) As maxa FROM mytable HAVING MIN(a) < MAX(a); Jan 24 '14 at 9:16
  • 1
    Yes. And PostgreSQL allows the following construct select 1 having count(*) = 1; which I have yet to grasp. Jan 24 '14 at 9:22
  • SQL Server allows too, SELECT 1 AS id, 'Colin' AS name; while others like Oracle have a special dual table. I don't think that either of these syntaxes is ANSI/ISO SQL (which requires FROM). Jan 24 '14 at 9:23
  • I didn't mean the lack of from but the reference to count(*) in the having clause without any indication as to over which columns this is being aggregated. Presumably it aggregates over all columns in the select clause. Jan 24 '14 at 9:25
  • Ah ok. Yes, I agree that that's what it does (aggregate over all rows you mean - over all rows of an empty table). Jan 24 '14 at 9:26

Having is used to filter groups .

where clause is used to filter rows.


In the absence of GROUP BY clause the query considers the whole relation as one group.


     select count(*)
     from dual
     having count(*) > 5;

HAVING is filtering the groups. If you have not GROUP BY cause, all rows presents one group. So, if predicate in HAVING evaluates as true, you get one row, otherwise no rows.

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