Question

我有以下表格：

 Table2       
 StudentNumbers  ExamType
 ------------------------
           1234         1
           2343         2   
           3345         5
           3454         1
           5465         2
            ...

 Table1   
 StudentNumbers  ExamType   ExamDate  School  Area             Info
 ------------------------------------------------------------------
           1234         1       0825  warren    ny    0x504B03042D0
           1234         1       0829   north    nj    0x63D86E1FFFF
           1233         2       0921   north    nj    0xA001400646F
           2343         1       0922  warren    ny    0x01400646174
           2343         1       0925   north    ny    0x100100070se
            ...

我正在尝试编写查询以获得以下结果：

StudentNumbers  ExamType   ExamDate  School  Area            Info
-----------------------------------------------------------------
          1234         1       0829   north    nj   0x63D86E1FFFF
          2343         1       0925   north    ny   0x100100070se

我写了以下查询：

Select t1.StudentNumbers, t1.ExamDate, t1.School, t1.Info, t1.ExamType        
from Table1 as t1
  Join(
       Select ts.StudentNumbers, max(ts.ExamDate) as ExamDate
       from Table2 as ts
         Join Table1 as pl on 
           ts.StudentNumbers = pl.StudentNumbers where ts.ExamType = pl.ExamType
       group by ts.StudentNumbers
       ) as t2 on t1.StudentNumbers = t2.StudentNumbers
                    and t1.ExamDate = t2.ExamDate

上面的查询工作，并给我我希望的结果是基于Table2的特定ExamType的最大ExamDate。但是在这种聚合方法中使用多个连接是最好的方法吗？或者是否有更清洁的选择。

Answer 1

您的查询很好，但我认为使用子查询外的Table2表会是更好的选择。

SELECT *
FROM (
      SELECT StudentNumbers, ExamType, MAX(ExamDate) AS ExamDate
      FROM Table1
      GROUP BY StudentNumbers, ExamType
      ) t1 JOIN Table1 t2 ON t1.StudentNumbers = t2.StudentNumbers
             AND t1.ExamType = t2.ExamType AND t1.ExamDate = t2.ExamDate
WHERE EXISTS(
             SELECT 1
             FROM Table2 t3
             WHERE t1.StudentNumbers = t3.StudentNumbers
               AND t1.ExamType = t3.ExamType
             )

Answer 2

SELECT
    t1.*
FROM Table1 t1
INNER JOIN Table2 t2
ON t1.StudentNumbers = t2.StudentNumbers
WHERE t1.ExamDate = (SELECT MAX(ExamDate) FROM Table1 WHERE StudentNumbers = t1.StudentNumbers)
ORDER BY t1.StudentNumbers ASC

虽然可能不是最有效的解决方案。

编写sql join脚本的任何更清洁或替代方法

2 个答案: