组合查询

Question

MYSQL的新手，但是如何结合这两个查询以显示类型='i'的SUM（总计）减去类型='p'的SUM（总计）。

SELECT userid, SUM(total)
FROM invoices
WHERE type = 'i'
GROUP BY 1

SELECT userid, SUM(total)
FROM invoices
WHERE type = 'p'
GROUP BY 1

Answer 1

您可以使用case表达式

SELECT userid, 
    SUM(case when type = 'i' then total else 0 end) typei,
    SUM(case when type = 'p' then total else 0 end) typep,
    SUM(case when type = 'i' then total else 0 end) - SUM(case when type = 'p' then total else 0 end) as diff
FROM invoices
GROUP BY userid

或者您可以将其写为

SUM(case when type = 'i' then total when type = 'p' then -1 * total else 0 end) as diff

编辑：要过滤聚合产生的结果，可以使用having子句

SELECT userid, 
    SUM(case when type = 'i' then total else 0 end) typei,
    SUM(case when type = 'p' then total else 0 end) typep,
    SUM(case when type = 'i' then total else 0 end) - SUM(case when type = 'p' then total else 0 end) as diff
FROM invoices
GROUP BY userid
HAVING diff > 0