"Subquery returns more than 1 row" when use select to set value of parameter

Question

I have an issue when I try to set the value of a variable with a subquery.

This is my SQL code:

SELECT @V_SOURCE = (SELECT ITEM_SOURCE
                    FROM TABLE1
                    WHERE OPP_CODE = @V_OPP_CODE
                      AND PDGROUPNO = @V_PRD_GROUP_NO
                      AND DELETE_FLAG IS NULL
                      AND CONTRACTOR = @V_CONTRACTOR
                      AND OPP_ITEM_NO = @_OPP_ITEM_NO)

When I run this code with an assumed variable that is used in WHERE condition, it returns only 1 row and 1 col that is correct but if I run this code with store procedure it will return the error:

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

Answer 1

I find an issue in my query because of I query data to use for this query some CONTRACTOR is NULL then it makes an error when using this query

SELECT @V_SOURCE = (SELECT ITEM_SOURCE
                FROM TABLE1
                WHERE OPP_CODE = @V_OPP_CODE
                  AND PDGROUPNO = @V_PRD_GROUP_NO
                  AND DELETE_FLAG IS NULL
                  AND CONTRACTOR = @V_CONTRACTOR
                  AND OPP_ITEM_NO = @_OPP_ITEM_NO)

When I filter data that CONTRACTOR is NULL out, I don't get any error now.