SQL update query and 'subquery returned more than one value?

Your subquery is not correlated at all. The identifier "TableA. Key" in the subquery refers to the TableA in the subquery's FROM clause, not the target table of the update (which happens also to be TableA).

You don't want to update TableA. Field with the result set of a two-table join. You simply want this.

You said that column returned by the query (TableB. Field) is the primary key. But the issue occurred because of duplicate TableB.

Key values. Make sure that TableB. Key is not duplicate for any value.

Here you will have to write some login to return only one record in case of subquery returns more than 1 record for a value. For example.

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