SUM is an aggregate function. It will calculate the total for each group. + is used for calculating two or more columns in a row.
Consider this example,
ID VALUE1 VALUE2
===================
1 1 2
1 2 2
2 3 4
2 4 5
SELECT ID, SUM(VALUE1), SUM(VALUE2)
FROM tableName
GROUP BY ID
will result
ID, SUM(VALUE1), SUM(VALUE2)
1 3 4
2 7 9
SELECT ID, VALUE1 + VALUE2
FROM TableName
will result
ID, VALUE1 + VALUE2
1 3
1 4
2 7
2 9
SELECT ID, SUM(VALUE1 + VALUE2)
FROM tableName
GROUP BY ID
will result
ID, SUM(VALUE1 + VALUE2)
1 7
2 16
Try the following:
SELECT *, (FieldA + FieldB) AS Sum
FROM Table