MySQL - 结合SELECT和UPDATE？

Question

I would like MySQL to compare the values of 2 columns in a table. If both values are the same, an UPDATE-statement must be executed on another table. These are my 2 tables:

TABLE : EMPLOYEES

EMPLOYEE_ID | NAME | CREDIT
---------------------------
     1      | John |   5
     2      | Bill |   10
     3      | Mark |   7

TABLE : BONUSES

BONUS_ID | EMPLOYEE_ID | A | B | AMOUNT
---------------------------------------
    1    |      1      | x | x |   6
    2    |      2      | x | y |   19
    3    |      2      | y | x |   4
    4    |      3      | y | y |   12
    5    |      3      | x | x |   15

If in the bonuses-table the value of column A is equal to the value of column B, the "amount" value of that row must be added to the employees credit in the employees-table. In SQL, it would be something like this:

SELECT * FROM bonuses WHERE A = B;

..and after that:

UPDATE employees SET credit = credit + bonuses.amount

-> For example:

In the "bonuses" table, the first row value A is euqal to value B. That means that the employee with EMPLOYEE_ID 1 (John) must have added 6 to their credit. The same goes for the 4th row where value A is equal to value B. In that case, the employee with EMPLOYEE_ID 3 (Mark) must have added 12 to their credit.

Does anyone know how to do this? Thanks!

Answer 1

You can use a multiple-table update to join the tables:

UPDATE EMPLOYEES JOIN BONUSES USING (EMPLOYEE_ID)
SET    EMPLOYEES.CREDIT = EMPLOYEES.CREDIT + BONUSES.AMOUNT
WHERE  BONUSES.A = BONUSES.B

However, you may wish to consider whether this logic would be better implemented in a trigger :

CREATE TRIGGER foo AFTER INSERT ON BONUSES FOR EACH ROW
  IF NEW.A = NEW.B THEN
    UPDATE EMPLOYEES
    SET    CREDIT = CREDIT + NEW.AMOUNT
    WHERE  EMPLOYEE_ID = NEW.EMPLOYEE_ID
  END IF
;

Answer 2

Try this query

update EMPLOYEE E 
inner join BONUSES B on
E.EMPLOYEE_ID = B.EMPLOYEE_ID AND B.A = B.B
set E.CREDIT = E.CREDIT + B.AMOUNT;