MySQL：使用另一个表中的值在一个查询中多次更新同一行

Question

Given the following table structures:

T1:

id a
1  1
2  2
3  3

T2:

id t1_id b
1  1     1
2  1     2
3  2     3

I need to add the value of t2.b to the value of t1.a where t2.t1_id = t1.id .

A simple update with a join like the following isn't working:

UPDATE t1
  JOIN t2 
    ON t2.t1_id = t1.id
   SET t1.a = t1.a + t2.b
 WHERE t2.id IN(1,2)

Expected T1 result (adds 1 and 2 to t1.a = 1; 3 to t1.a = 2):

id a
1  4
2  5
3  3

Actual T1 result (only adds 1 to t1.a = 1; 3 to t1.a = 2):

id a
1  2
2  5
3  3

At the moment I'm looking at a select that computes the full sum of values to be added using a group by , and then that result set is joined to the update... This seems like overkill for something so simple though! Does anyone have an elegant and efficient solution?

Answer 1

Came up with this real quickly.

UPDATE T1 AS t
JOIN (
    SELECT
        t1_id, SUM(b) AS sum_total
    FROM
        T2
    WHERE
        t1_id IN (1, 2)
    GROUP BY
        T2.t1_id
) AS t2 ON t.id = t2.t1_id
SET  t.a =  t.a + t2.sum_total;