Question

我有一个带有列（a，b）的表A和一个带有列（b，c）的表B.我想做以下事情：

STEP 1: do a   A LEFT OUTER JOIN B ON A.b = B.b
STEP 2: convert all c = NULL in the Joined table to c = <default_value>
STEP 3: SUM(A.a * B.c) over the table I get after STEP 2

在一个复合语句中有快速的方法吗？

谢谢！

Answer 1

您可以在一个查询中执行这三个步骤，首先LEFT OUTER JOIN并使用IFNULL将NULL值设置为默认值，然后使用SUM：

SELECT
  SUM(A.a + IFNULL(B.c, <default_value>))
FROM A 
LEFT OUTER JOIN B ON A.b = B.b;

SQL小提琴演示：

Answer 2

select
  sum(A.a + IFNULL(B.c, some_default)) as some_sum
FROM
  A
  LEFT JOIN B
    ON
      B.b = A.b

Answer 3

 select sum(A.a * (case when B.c = null then <default> else B.c end)) as joined_sum
 from A left outer join B on (A.b = B.b);

复合mysql选择After outer join

3 个答案: