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https://issues.apache.org/jira/browse/DERBY-7164?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17862557#comment-17862557
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Richard N. Hillegas commented on DERBY-7164:
--------------------------------------------
Agreed. That is my understanding of the EXISTS predicate. Other variations of
this mistake are:
{noformat}
CONNECT 'jdbc:derby:memory:db;create=true';
-- correct
SELECT COUNT(*)
FROM SYSIBM.SYSDUMMY1
WHERE FALSE
;
-- incorrect. should be true since the query returns one row
VALUES
(
EXISTS
(
SELECT COUNT(*)
FROM SYSIBM.SYSDUMMY1
WHERE FALSE
)
)
;
-- incorrect for the same reason
SELECT EXISTS
(
SELECT COUNT(*)
FROM SYSIBM.SYSDUMMY1
WHERE FALSE
)
FROM SYSIBM.SYSDUMMY1;
{noformat}
> Wrong result from EXISTS aggregate subquery that counts zero rows
> -----------------------------------------------------------------
>
> Key: DERBY-7164
> URL: https://issues.apache.org/jira/browse/DERBY-7164
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.17.1.0
> Reporter: Lukas Eder
> Priority: Major
>
> This query should return true as the aggregate subquery always produces
> exactly 1 row, so the EXISTS predicate is trivially always true:
> {code:sql}
> select exists (
> select count(*)
> from SYSIBM.SYSDUMMY1
> where false
> )
> from SYSIBM.SYSDUMMY1;
> {code}
> But it returns false, probably due to a wrong optimisation
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