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Description

MariaDB Error 1242 occurs when a subquery, which is expected to return only a single value or row, instead retrieves multiple rows. This typically happens in contexts where only one result is valid, such as in a `SELECT` list, a `WHERE` clause using single-value comparison operators like `=`, or in an `INSERT` statement's `VALUES` clause.

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Error Message

Subquery returns more than 1 row

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Known Causes

4 known causes

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Incorrect Single-Value Comparison

A subquery used with an operator expecting a single value (e.g., `=`, `<`, `>`) in a `WHERE` clause retrieves multiple rows, violating the operator's requirement.

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Scalar Subquery in SELECT List

A subquery placed in the `SELECT` clause is expected to return at most one row and one column (a scalar value), but its execution yields multiple rows.

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Missing or Ineffective LIMIT 1

The subquery logic intends to produce a single result but lacks an explicit `LIMIT 1` clause, allowing it to return multiple rows if the data matches multiple conditions.

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Unexpected Data Patterns

The underlying data contains duplicate or unanticipated entries that cause the subquery to return more rows than originally expected by the query's design.

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Solutions

4 solutions available

1. Limit Subquery Results to One Row easy

Modify the subquery to guarantee it returns at most one row using LIMIT 1.

1

Identify the subquery causing the error. This is usually enclosed in parentheses within your main query.

2

Append `LIMIT 1` to the end of the subquery. This will instruct MariaDB to stop processing the subquery as soon as it finds the first matching row.

SELECT column_name FROM another_table WHERE condition LIMIT 1

3

Replace the original subquery in your main query with the modified one.

SELECT main_column FROM main_table WHERE main_column = (SELECT column_name FROM another_table WHERE condition LIMIT 1);

2. Use `EXISTS` or `IN` for Row Existence Checks medium

Rewrite the query to check for the existence of rows rather than expecting a single value.

1

Determine if your intention is to check if *any* row matches the subquery's criteria or if a *specific* row matches.

2

If you only need to know if at least one row exists, use the `EXISTS` operator.

SELECT main_column FROM main_table WHERE EXISTS (SELECT 1 FROM another_table WHERE another_table.some_column = main_table.some_column);

3

If you need to match against a set of values from the subquery, use the `IN` operator. This is suitable if the subquery *can* legitimately return multiple rows, and you want to match against any of them.

SELECT main_column FROM main_table WHERE main_column IN (SELECT column_name FROM another_table WHERE condition);

3. Aggregate Subquery Results medium

Use an aggregate function (like MAX, MIN, AVG, COUNT) to ensure the subquery returns a single value.

1

Analyze the subquery and determine if there's a logical way to aggregate its results into a single value that fits your needs.

2

Wrap the subquery with an appropriate aggregate function. For example, if you want the maximum value from the subquery:

SELECT main_column FROM main_table WHERE main_column = (SELECT MAX(column_name) FROM another_table WHERE condition);

3

Consider other aggregate functions like `MIN()`, `AVG()`, or `COUNT()` based on the specific requirement of your query.

SELECT main_column FROM main_table WHERE main_column = (SELECT AVG(column_name) FROM another_table WHERE condition);

4. Refactor with a JOIN Operation advanced

Reconstruct the query using JOINs for better performance and clarity when multiple rows are expected.

1

Understand the relationship between the tables involved in your query and the subquery.

2

Rewrite the query using a `JOIN` clause. This is often the most robust and performant solution when the intent is to combine data from multiple tables based on matching criteria.

SELECT m.main_column FROM main_table m JOIN another_table a ON m.some_column = a.some_column WHERE a.condition;

3

If you need to select specific columns from either table, adjust the `SELECT` list accordingly. If you need to aggregate results after the join, add `GROUP BY` and aggregate functions.

SELECT m.main_column, COUNT(a.column_name) FROM main_table m JOIN another_table a ON m.some_column = a.some_column WHERE a.condition GROUP BY m.main_column;

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Related Errors

5 related errors

MariaDB Error 1242: Subquery Returns Multiple Rows