Why Does My SQL Subquery Produce 'Only One Expression Can Be Specified in the Select List' Error?

Troubleshooting SQL Subquery Errors: "Only One Expression Can Be Specified..."

This error, "Only one expression can be specified in the select list when the subquery is not introduced with EXISTS," arises when your SQL query uses a subquery within an IN clause, and the subquery attempts to return multiple columns. The IN operator only accepts a single column for comparison.

Let's examine the problematic query:

<code class="language-sql">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)</code>

The inner SELECT statement incorrectly returns both A_ID and COUNT(DISTINCT dNum) AS ud. This violates the rule for subqueries used with IN.

The Corrected Query:

The solution involves restructuring the subquery to return only the A_ID column:

<code class="language-sql">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)</code>

This revised query now correctly selects only A_ID in the subquery, resolving the error. The ordering by COUNT(DISTINCT dNum) is handled within the subquery itself, ensuring the top 10% of A_ID values (based on the COUNT(DISTINCT dNum) are selected.

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