Question

First query

This query works fine and the engine does not complain about it

SELECT id 
FROM agencies
WHERE id IN ((SELECT id FROM agencies))
ORDER BY id;

Second query

This one does not work, the engine is complaining about Subquery returns more than 1 row, when -according to me- I am doing the exact same thing when @param_1 IS NULL

SELECT 
  @param_1 := NULL,

SELECT id 
FROM agencies
WHERE id IN (CASE WHEN @param_1 IS NULL THEN (SELECT id FROM agencies) ELSE 1 END )
ORDER BY id;

Does anybody see why the engine is complaining about the second query when it isnt for the first query ?

Thanks in advance,

Answer 1

CASE expects a scalar, single value. Not a record set.

SELECT id 
FROM agencies
WHERE id IN (
        SELECT id FROM agencies WHERE @param_1 IS NULL
        UNION ALL
        SELECT 1 WHERE @param_1 IS NOT NULL
        )
ORDER BY id;

OR

SELECT id 
FROM agencies
WHERE id IN (SELECT id FROM agencies)
     AND @param_1 IS NULL
UNION ALL
SELECT id 
FROM agencies
WHERE @param_1 IS NOT NULL AND id = 1
ORDER BY id;

Another choice is to use IF

IF @param_1 IS NULL
    SELECT id 
    FROM agencies
    WHERE id IN (SELECT id FROM agencies)
    ORDER BY id;
ELSE
    SELECT id 
    FROM agencies
    WHERE id = 1
    ORDER BY id;