This error may occur when a SINGLE SELECT statement is executed and more than one row complies with the search condition.

This error can also occur if a subquery specified in a comparison predicate or a SET UPDATE clause of an UPDATE statement is executed and more than one row complies with the search condition.

The SINGLE SELECT statement can be replaced with a QUERY statement and a sequence of FETCH statements; or it can be ensured that at most one row complies with the conditions by expanding the search condition.

You can replace the comparison predicate with a quantity comparison predicate. You can try to change the subquery in such a way that it contains at most one row as the result by specifying DISTINCT or by expanding the search condition.