Is it possible/economical to perform a SELF JOIN of a table (for this example, my table called myTable
has two columns pk
and fk
), and return a record if there is only one resulting record? I am thinking of something like the following, however, only_one_row()
is a fictional function that would need to be replaced with something real:
SELECT fk
FROM myTable as t1
INNER JOIN myTable AS t2 ON t2.fk=t1.fk
WHERE t1.pk=1
AND only_one_row();
For instance, if myTable(id,fk) had the following records, only one record is produced, and I which to select the record:
1 1
2 1
3 2
However, if myTable(id,fk) had the following records, two '1' records are produced, and the select should not return any rows:
1 1
2 1
3 2
4 1
I could use PHP to do so, but would rather just use SQL if feasible.
Use a HAVING
clause that counts the results.
SELECT fk
FROM myTable as t1
INNER JOIN myTable AS t2 ON t2.fk=t1.fk
WHERE t1.pk=1
HAVING COUNT(*) = 1
How about this:
SELECT fk
FROM myTable as t1
INNER JOIN myTable AS t2 ON t2.fk=t1.fk
WHERE t1.pk=1
GROUP BY fk
HAVING COUNT(fk) = 1
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