How Can I Fix the 'Subquery Returns More Than 1 Row' Error in SQL?

Overcoming the "Subquery Returns More Than 1 Row" Error

Setting Criteria with Multiple Subquery Values

Encountering the "subquery returns more than 1 row" error when attempting to set criteria based on the results of a subquery can be frustrating. This error occurs when your subquery returns multiple rows, but your main query expects only one.

To resolve this issue, it's crucial to understand the difference between the equality operator (=) and the IN operator. The equality operator can only compare two values, and it expects the subquery to return a single value. On the other hand, the IN operator allows you to compare a value to multiple values or a set of values returned by a subquery.

Using the IN Operator

When your subquery returns multiple rows, you should utilize the IN operator to compare your main query's value against those multiple values. The syntax is as follows:

SELECT *
FROM table
WHERE id IN (subquery)

In this example, the subquery will return multiple rows, and the main query will check if the id column matches any of those row values.

Example Usage

Consider the following scenario:

• Query 1: SELECT id FROM table1 returns multiple rows with values 1, 2, and 3.
• Query 2: SELECT * FROM table2 WHERE id = (SELECT id FROM table1) will result in the error because the subquery returns multiple rows.

To resolve this error, modify Query 2 to use the IN operator:

SELECT * 
FROM table2 
WHERE id IN (SELECT id FROM table1)

Now, Query 2 will correctly retrieve the records from table2 where the id column matches any of the values returned by the subquery.

Conclusion

Understanding the distinction between the equality operator and the IN operator is essential when working with subqueries. By employing the IN operator, you can overcome the "subquery returns more than 1 row" error and effectively set criteria based on multiple values obtained from a subquery.

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