Using = with a subquery that can return multiple rows — should use IN instead. A subquery assumed to return a unique value actually has duplicates in the data. NOT IN / = ANY subquery returns multiple rows unexpectedly

Step 1: Replace `=` with `IN` for multi-row subqueries: `WHERE id IN (SELECT id FROM other_table WHERE condition)`. Step 2: Or add a LIMIT 1 if only one row is needed: `WHERE id = (SELECT id FROM other_table WHERE condition LIMIT 1)`. Step 3: Add DISTINCT or aggregation to the subquery to ensure a single result: `WHERE id = (SELECT MAX(id) FROM other_table WHERE condition)`

Medium severityquery

Power BI Refresh Error:
1242

What does this error mean?

A scalar subquery used in a comparison returned multiple rows when exactly one was expected.

Common causes

1Using = with a subquery that can return multiple rows — should use IN instead
2A subquery assumed to return a unique value actually has duplicates in the data
3NOT IN / = ANY subquery returns multiple rows unexpectedly

How to fix it

1Step 1: Replace `=` with `IN` for multi-row subqueries: `WHERE id IN (SELECT id FROM other_table WHERE condition)`
2Step 2: Or add a LIMIT 1 if only one row is needed: `WHERE id = (SELECT id FROM other_table WHERE condition LIMIT 1)`
3Step 3: Add DISTINCT or aggregation to the subquery to ensure a single result: `WHERE id = (SELECT MAX(id) FROM other_table WHERE condition)`

Frequently asked questions

When is it safe to use a subquery with = in MySQL?

Only when you are certain the subquery returns exactly one row — e.g., a subquery on a primary key column with an equality filter. Otherwise use IN.

How is MySQL error 1242 different from using IN?

`= (subquery)` requires exactly one row. `IN (subquery)` accepts any number of rows. Use IN for flexibility, or ensure uniqueness with aggregation (MAX, MIN) when using =.

Can MetricSign detect when a subquery error breaks a pipeline?

Yes — MetricSign captures ADF pipeline failures with the MySQL error code, letting you trace query logic errors back to specific Lookup or Script activities.

Official documentation: https://dev.mysql.com/doc/mysql-errors/8.0/en/server-error-reference.html