A question regarding aggregate function

[LarryChen]

Suppose I have a table A and a table B.

Table A:

Usage Expense Name

Food 100 Jason
Food 150 Mike
Rent 1000 Jenny
Gas 50 Jason
Fun 60 John
Rent 1200 Jason
Gas 30 Jenny
Fun 70 Mike
Rent 900 Mike
Food 200 John
Fun 40 Jenny
Gas 80 Mike
Food 180 Jenny
Rent 800 John
Gas 60 John
Fun 30 Jason

Table B:

Usage Expense Name
Food 100 Bob
Fun 40 Bob
Gas 50 Bob
Rent 1100 Bob

Now if I want to use a query to display total expense in Food from John, Jenny, Mike, Jason and Bob, how can I do that? Thanks.

[jcaccia]

Use UNION to combine results from both tables. Do you need to get the total expenses by Name or the combined total? If you need the combined total use SUM(Expense) (you will have to use it anyway if there might be more than one Food record with the same Name). If you need individual totals by Name use GROUP BY.

[LarryChen]

Use UNION to combine results from both tables. Do you need to get the total expenses by Name or the combined total? If you need the combined total use SUM(Expense) (you will have to use it anyway if there might be more than one Food record with the same Name). If you need individual totals by Name use GROUP BY.

Thanks for your reply. It works! But I still don't understand how UNION adds the results together from two queries?