Runtime Error

What This Error Means

This error occurs when a subquery is expected to return a single value, but it actually returns multiple rows. This can happen when a subquery is used with an aggregate function, but the GROUP BY clause is missing or incorrect.

Why It Happens

This error usually happens when a subquery is not properly correlated with the outer query. For example, if a subquery is used to retrieve a list of IDs, but the outer query is trying to retrieve a single value based on that ID. It can also happen when a subquery is used with an aggregate function, but the GROUP BY clause is missing or incorrect.

How to Fix It

1. 1To fix this error, you need to ensure that the subquery returns a single value. This can be done by adding a GROUP BY clause to the subquery, or by using a different query structure that does not rely on a subquery. Here's an example:

Example Code Solution

SELECT * FROM customers
WHERE country = (SELECT country FROM customers WHERE id = 1);

SELECT c.* FROM customers c
JOIN (SELECT country FROM customers WHERE id = 1) AS subquery ON c.country = subquery.country;