Home Database Mysql Tutorial Why Does My SQL Subquery Produce an 'Only One Expression Allowed' Error?

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

Jan 11, 2025 pm 01:17 PM

Why Does My SQL Subquery Produce an

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)
Copy after login

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)
Copy after login

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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