MySql 选择资格到期日期

Question

我有三个 MySql 表：

Users：UserId、UserName

Tests：TestId、TestName

Passes：PassId、UserId、TestId、DateTaken

我想返回一个表格，显示每个用户通过的最新测试的日期，例如，

|--------|---------|---------|---------|---------|---------|
|User    |Test A   |Test B   |Test C   |Test D   |Test E   |
|--------|---------|---------|---------|---------|---------|
|James   |Null     |6/3/11   |Null     |Null     |4/3/11   |
|Mark    |Null     |1/4/11   |8/5/11   |23/5/10  |Null     |
|--------|---------|---------|---------|---------|---------|

在此示例中，James 从未通过测试 A、C 或 D。他可能已多次参加测试 B，但最近一次是在 2011 年 6 月 3 日。

我计划在 ASP.NET GridView 中显示此数据。最好的方法是什么 - 可以在 SELECT 语句中完成吗？请帮忙！

非常感谢。

Answer 1

select
u.username,
max(case when testid = 1 then datetaken else null end) as A,
max(case when testid = 2 then datetaken else null end) as B,
max(case when testid = 3 then datetaken else null end) as C,
max(case when testid = 4 then datetaken else null end) as D,
max(case when testid = 5 then datetaken else null end) as E
from users as u
left join passes as p on u.userid = p.userid
group by u.userid

Answer 2

为了实现这一点，您需要对每个测试结果使用子查询
试试这个：

SELECT User,
    (SELECT MAX(DateTaken) FROM Passes p INNER JOIN Tests t ON p.TestId=t.TestId WHERE p.UserId=u.UserId AND TestName='Test A') AS 'Test A',
    (SELECT MAX(DateTaken) FROM Passes p INNER JOIN Tests t ON p.TestId=t.TestId WHERE p.UserId=u.UserId AND TestName='Test B') AS 'Test B',
    (SELECT MAX(DateTaken) FROM Passes p INNER JOIN Tests t ON p.TestId=t.TestId WHERE p.UserId=u.UserId AND TestName='Test C') AS 'Test C',
    (SELECT MAX(DateTaken) FROM Passes p INNER JOIN Tests t ON p.TestId=t.TestId WHERE p.UserId=u.UserId AND TestName='Test D') AS 'Test D',
    (SELECT MAX(DateTaken) FROM Passes p INNER JOIN Tests t ON p.TestId=t.TestId WHERE p.UserId=u.UserId AND TestName='Test E') AS 'Test E'
FROM Users u

Answer 3

这是相当通用的解决方案。 MySQL 确实支持我写的所有内容，但是可能有一个 MySQL 特定的解决方案可能更好。

Select
    UserName,
    Max(TestA) testa,
    Max(TestB) testb,
    Max(TestC) testc,
    Max(TestD) testd,
    Max(TestE) teste
FROM
(



    SELECT
        u.UserName,
        Case When TestName = "Test A" then p.DateTaken END TestA,
        Case When TestName = "Test B" then p.DateTaken END TestB,
        Case When TestName = "Test C" then p.DateTaken END TestC,
        Case When TestName = "Test D" then p.DateTaken END TestD,
        Case When TestName = "Test E" then p.DateTaken END TestE
    FROM
        Users u
        LEFT JOIN tests t
        ON u.UserId = t.userid
        LEFT JOIN (SELECT
                Max(DateTaken) DateTaken,
                userId,
                TestId,
            FROM 
                Passes
            GROUP BY 
                userId,
                TestId) p
        ON t.testId = p.TestId) t
Group BY
    UserName