-811 THE RESULT OF AN EMBEDDED SELECT STATEMENT OR A SUBSELECT IN THE SET CLAUSE OF AN UPDATE STATEMENT IS A TABLE OF MORE THAN ONE ROW, OR THE RESULT OF A SUBQUERY OF A BASIC PREDICATE IS MORE THAN ONE VALUE

Explanation
 Execution of an embedded SELECT statement or
a subselect in the SET clause of an UPDATE statement has resulted
in a result table that contains more than one row.  Alternatively,
a subquery contained in a basic predicate has produced more than one
value.

System action
 The statement cannot be processed.

Programmer response
 Examine the syntax of the statement to
ensure that it contains the proper condition specifications. If the
statement syntax is correct, there might be a problem with the data
that is causing more than one row or value to be returned when you
do not expect it.

SQLSTATE
 21000