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?
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4 Answers
No.
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.
answered Jan 24, 2014 at 8:42
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2Another example:
SELECT MIN(a) AS mina, MAX(a) As maxa FROM mytable HAVING MIN(a) < MAX(a);Jan 24, 2014 at 9:16 -
1Yes. And PostgreSQL allows the following construct
select 1 having count(*) = 1;which I have yet to grasp. Jan 24, 2014 at 9:22 -
SQL Server allows too,
SELECT 1 AS id, 'Colin' AS name;while others like Oracle have a specialdualtable. I don't think that either of these syntaxes is ANSI/ISO SQL (which requiresFROM). Jan 24, 2014 at 9:23 -
I didn't mean the lack of
frombut the reference tocount(*)in thehavingclause without any indication as to over which columns this is being aggregated. Presumably it aggregates over all columns in theselectclause. Jan 24, 2014 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, 2014 at 9:26
Having is used to filter groups .
where clause is used to filter rows.
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1Your first statement is not correct.
havingis applied after the aggregation phase so can be used to filter groups. Jan 24, 2014 at 8:52
In the absence of GROUP BY clause the query considers the whole relation as one group.
e.g.
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.
