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I found myself writing the following:

select 'yes' 
where exists(select * from foo where val=1)
and not exists(select * from foo where val<>1);

and wondering if there is a more concise way without sacrificing too much readability.

I found one way which I am posting as an answer but I'm not entirely happy with it and would be very interested in alternatives

In this case val is unique within foo - there are no duplicates

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Do I understand correctly that you want exactly one row in the result of the subquery? – Erwin Brandstetter Sep 4 '12 at 16:03
Which subquery? – Jack Douglas Sep 4 '12 at 17:40
The one you mention in the title. I wasn't sure whether it should be one result after or before "distinct". – Erwin Brandstetter Sep 4 '12 at 18:00
Ah yes, that one :) I was rather confusingly referring to the sub-query in my answer - yours is far more specific and flexible, eg you can also use count(distinct val), though in my real-world case it makes no difference – Jack Douglas Sep 4 '12 at 21:05
up vote 8 down vote accepted

Concise, fast (especially with many rows), my favorite concerning readability and would work with dupes, too:

SELECT count(*) = 1 AND min(val) = 1 FROM foo;

Returns TRUE / FALSE .. or NULL - only in the case of exactly one row with val IS NULL, because count() never returns NULL or no row.

The second 1 in the example just happens to be the same as the first, because of your example.

The query in the question fails with NULL values. Consider the simple demo:

CREATE TABLE foo (id int, val int);
INSERT INTO foo VALUES (1, 1),(2, NULL);

SELECT 'yes' 

IS DISTINCT FROM would fix this, but it could still fail with duplicates in val - which you have ruled out for this case.

Your answer works fine.
Returns 'yes' / no row.

I would prefer this shorter form, though. Don't forget that PostgreSQL (unlike Oracle) has a proper boolean type.

SELECT array_agg(val) = array[1] FROM foo;

Returns TRUE / FALSE / NULL.

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excellent, thanks, I knew there would be a better way :) – Jack Douglas Sep 4 '12 at 17:38

A variation on @Erwin's answer. No COUNT() at all, only MIN() and MAX(). It may be slightly more efficient with big table and (not in your case) duplicate val:

SELECT MIN(val) = 1 AND MAX(val) = 1 FROM foo;
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+1 thanks. It handles nulls and duplicates differently of course (if there were any) – Jack Douglas Sep 5 '12 at 12:07
@Jack: Yes. Does your table have nulls? Or you want answers for both cases (with and without)? – ypercubeᵀᴹ Sep 5 '12 at 12:12
no mine doesn't - I can use either :) – Jack Douglas Sep 5 '12 at 14:14
Would be much faster on bigger tables with a matching index, but performs identically in the absence of such an index - like when testing query results. – Erwin Brandstetter Sep 5 '12 at 18:47
select 'yes' where (select array_agg(val) from foo)=array[1];
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This one returns true, false or an empty result:

 select j.val is null 
 from foo left join foo as j on j.val <> foo.val 
 where foo.val = 1 limit 1;
share|improve this answer
at first glance, this doesn't seem to return false if there are values in foo where val<>1? – Jack Douglas Sep 5 '12 at 7:42
@JackDouglas Oh, sorry. I understood the task wrong the first time. Fixed. – grayhemp Sep 5 '12 at 8:51
Works - except with NULL as value which has not been ruled out in this case. – Erwin Brandstetter Sep 5 '12 at 12:31
@ErwinBrandstetter NULL can be worked out by using IS [NOT] DISTINCT FROM I think. – grayhemp Sep 8 '12 at 11:05
@grayhemp: Not in this case. LEFT JOIN foo j ON j.val <> foo.val fails to detect a row with j.val IS NULL to begin with. If you'd include it with ON j.val IS DISTINCT FROM foo.val you'd then need to check on another column of j defined NOT NULL to tell the two cases apart. But no additional column is defined. – Erwin Brandstetter Sep 8 '12 at 13:30

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