How can I sum columns across multiple tables in MySQL?

Question

In MySQL I have two tables:

Table MC:
----------------
|TransNo | Qty |
|--------|-----|
|  xxx1  |  4  | 
|  xxx3  |  3  |

and

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Answer 1

SELECT SUM(t.Qty) AS total_qty
    FROM (SELECT Qty FROM MC
          UNION ALL
          SELECT Qty FROM Amex) t