Question

我有下表学生：

id | status | school | name
----------------------------
0  | fail   | skool1 | dan
1  | fail   | skool1 | steve
2  | pass   | skool2 | joe
3  | fail   | skool2 | aaron

我想要一个给我的结果

school | fail | pass  
---------------------
skool1 | 2    | 0   
skool2 | 1    | 1

我有这个，但它很慢，

SELECT s.school, (

SELECT COUNT( * ) 
FROM school
WHERE name = s.name
AND status = 'fail'
) AS fail, (

SELECT COUNT( * ) 
FROM school
WHERE name = s.name
AND status = 'pass'
) AS pass,

FROM Students s
GROUP BY s.school

建议？

Answer 1

这样的事情应该有效：

SELECT 
    school,
    SUM(CASE WHEN status = 'fail' THEN 1 ELSE 0 END) as [fail],
    SUM(CASE WHEN status = 'pass' THEN 1 ELSE 0 END) as [pass]
FROM Students
GROUP BY school
ORDER BY school

修改
几乎忘记了，但你也可以用这种方式编写查询：

SELECT school, COUNT(CASE WHEN status = 'fail' THEN 1 END) as [fail], COUNT(CASE WHEN status = 'pass' THEN 1 END) as [pass] FROM Students GROUP BY school ORDER BY school

我不确定第二个查询是否有任何性能优势。我的猜测是，如果它可能非常小。我倾向于使用第一个查询，因为我认为它更清楚，但两者都应该有效。另外，我没有一个MySql实例可以方便地进行测试，但根据@Johan的说法，ORDER BY子句是不必要的。

Answer 2

SELECT q.school, q.fail, q.failpass-q.fail as pass
FROM
  (
  SELECT s.school, sum(if(status = 'fail',1,0)) as fail, count(*) as failpass
  FROM students s
  GROUP BY s.school
  ) q

这样可以节省一个条件和在MySQL中，GROUP BY已经对结果进行了排序，因此不需要单独的ORDER BY。

sql查询帮助多个计数列和分组依据

2 个答案: