Why Does My SQL Subquery Produce an 'Only One Expression Allowed' Error?

SQL Subquery Error: "Only One Expression Allowed"

This error, "Only one expression can be specified in the select list when the subquery is not introduced with EXISTS," arises when a subquery used within an IN clause returns multiple columns. The IN operator expects a single-column result set for comparison.

Problematic Query:

The following query attempts to use a subquery returning A_ID and a count (ud) within an IN clause:

select count(distinct dNum)
from myDB.dbo.AQ
where A_ID in
(SELECT DISTINCT TOP (0.1) PERCENT A_ID,
          COUNT(DISTINCT dNum) AS ud
FROM myDB.dbo.AQ
WHERE M > 1 and B = 0
GROUP BY A_ID ORDER BY ud DESC)

Root Cause:

The inner SELECT statement returns two columns: A_ID and ud. The IN operator cannot handle this; it requires a single column for comparison against the A_ID column in the outer query.

Corrected Query:

The solution is to modify the subquery to return only the A_ID column. The sorting by count can be done within the subquery using ORDER BY COUNT(DISTINCT dNum) DESC:

select count(distinct dNum)
from myDB.dbo.AQ
where A_ID in
(SELECT DISTINCT TOP (0.1) PERCENT A_ID
FROM myDB.dbo.AQ
WHERE M > 1 and B = 0
GROUP BY A_ID
ORDER BY COUNT(DISTINCT dNum) DESC)

This revised query correctly selects the top 10% of A_ID values based on the count of distinct dNum values and then uses those A_ID values to filter the outer query.

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