sql subtract subquery sum multiple tables

Question

How can I subtract the sum of a subquery table from another table?

SELECT i.column1 * i.column2 AS Expr1
    , i.column1 * i.column2 - (SELECT SUM(table2.column1) AS Expr1 
                               FROM table2 
                               WHERE (table2.column3 = table1.column3)) AS derivedExpression
FROM table1

Only the derivedExpression in the first row is correct. The rest rows returns null for derivedExpression. For Expr1, everything is fine. Any help?

Answer 1

SELECT i.column1 * i.column2 AS Expr1
    , i.column1 * i.column2 - (SELECT COALESCE(SUM(table2.column1), 0) AS Expr1 
                               FROM table2 
                               WHERE (table2.column3 = table1.column3)) AS derivedExpression
FROM table1