Seaquel
SQL Server error code

SQL Server error 512

SQL Server raises error 512 at severity 16, which marks a problem with the query or data that you can correct.

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
Msg512
Severity16
Group0–999

How to fix it

In other databases

Source: Database Engine events and errors