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Description

This error indicates that a subquery, which was designed to return a single value or a single row, has unexpectedly returned more than one. It typically occurs when the main query expects a unique result from the subquery for comparison or assignment.

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Error Message

Subquery returns more than 1 row

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Known Causes

3 known causes

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Scalar Subquery Mismatch

A subquery used in a context expecting a single scalar value (e.g., in a WHERE clause with '=', or a SET statement) returns multiple rows.

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Subquery in SELECT List

A subquery embedded directly within the SELECT list of the main query returns multiple rows, while only a single row is permitted.

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Inadequate Filtering

The WHERE clause or JOIN conditions within the subquery are not specific enough, causing it to match and return more rows than intended.

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Solutions

4 solutions available

1. Use `LIMIT 1` for Single Value Expectation easy

If the subquery is expected to return at most one row, add `LIMIT 1` to enforce this.

1

Identify the subquery that is causing the error. This is typically a subquery used in a `WHERE` clause with comparison operators like `=`, `>`, `<`, `>=`, `<=`, or in an `INSERT` statement's `VALUES` clause.

2

Append `LIMIT 1` to the end of the subquery. This will ensure that even if the subquery *could* return multiple rows, only the first one is considered, preventing the error.

SELECT column_name FROM your_table WHERE condition = (SELECT subquery_column FROM another_table WHERE another_condition LIMIT 1);

2. Use `IN` or `ANY`/`ALL` for Multiple Values easy

If the subquery is intended to match against multiple values, use `IN` or `ANY`/`ALL` operators.

1

Examine the main query and the subquery. If the intention is to check if a value exists within a set of results from the subquery, change the comparison operator.

2

Replace `=` with `IN` if you want to check if the outer value is present in *any* of the rows returned by the subquery.

SELECT column_name FROM your_table WHERE column_name IN (SELECT subquery_column FROM another_table WHERE another_condition);

3

Alternatively, use `ANY` or `ALL` with comparison operators if you need to compare against a set of values. For example, `> ANY` means greater than at least one value in the set.

SELECT column_name FROM your_table WHERE column_name > ANY (SELECT subquery_column FROM another_table WHERE another_condition);

3. Refactor Subquery to Return a Single Row medium

Modify the subquery's logic to guarantee it returns only one row.

1

Analyze the subquery's `WHERE` clause and any `GROUP BY` or aggregate functions. The goal is to make the subquery's result set uniquely identifiable.

2

If the subquery involves grouping, ensure that the `GROUP BY` clause, combined with the `WHERE` clause, results in only one group. If not, consider adding more specific filtering conditions.

SELECT MAX(subquery_column) FROM another_table WHERE specific_condition GROUP BY group_column HAVING COUNT(*) = 1; -- Example to ensure one group

3

If the subquery is intended to fetch a specific record (e.g., the latest or highest value), use aggregate functions like `MAX()`, `MIN()`, `AVG()`, or add a `LIMIT 1` with an `ORDER BY` clause to deterministically select a single row.

SELECT subquery_column FROM another_table WHERE another_condition ORDER BY some_date_column DESC LIMIT 1;

4

If the subquery is part of an `INSERT` statement and you're trying to insert a single value derived from multiple potential matches, you might need to select a specific value based on a unique identifier or a business logic.

INSERT INTO target_table (column1) VALUES ((SELECT subquery_column FROM another_table WHERE unique_identifier = 'some_value' LIMIT 1));

4. Join Instead of Using a Correlated Subquery medium

Rewrite the query using a `JOIN` operation for better performance and clarity when dealing with relationships between tables.

1

Examine the subquery. If it's a correlated subquery (meaning it references columns from the outer query), consider if a `JOIN` can achieve the same result.

2

Identify the common columns between the outer query's table and the subquery's table that link them. These will be used in the `ON` clause of the `JOIN`.

3

Replace the subquery with a `JOIN` clause. If the subquery was in the `WHERE` clause, you might need to use `LEFT JOIN` and then filter in the `WHERE` clause, or use `INNER JOIN` if a match is required.

SELECT t1.column_name FROM your_table t1 JOIN another_table t2 ON t1.join_column = t2.join_column WHERE t2.another_condition;

4

If the original subquery was intended to return a single, aggregated value, you might need to perform the aggregation within the `JOIN` or use a subquery in the `FROM` clause (derived table) that is then joined.

SELECT t1.column_name FROM your_table t1 JOIN (SELECT subquery_column, join_column FROM another_table WHERE another_condition LIMIT 1) t2 ON t1.join_column = t2.join_column;

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Related Errors

5 related errors

MySQL Error 1242: Subquery Multi-Row Result