Question

我在SQL server中有一个表（T1）：

id     subject     student1      student2     student3     score1      score2     score3
1      Maths       Peter         Leo          John         11          12         13
2      Phy         Mary          May          Leo          21          22         23

是否可以使用查询来产生以下结果：

id    subject   name      score
1     Maths     Peter     11
1     Maths     Leo       12
1     Maths     John      13
2     Phy       Mary      21
2     Phy       May       22
2     Phy       Leo       23

Answer 1

虽然这是一个univot查询，但实际上我发现用显式逻辑更容易做到这一点：

select t1.id, t1.subject,
       (case when n.n = 1 then student1
             when n.n = 2 then student2
             when n.n = 3 then student3
        end) as student,
       (case when n.n = 1 then score1
             when n.n = 2 then score2
             when n.n = 3 then score3
        end) as score
from t1 cross join
     (select 1 as n union all select 2 union all select 3) n;

这应该具有与unpivot相当的性能。并且，它应该具有比三个union all操作更好的性能（因为这需要扫描表三次）。

Answer 2

尝试此查询

SELECT a.id, a.subject, b.*
FROM dbo.Table1 a
CROSS APPLY (
    SELECT 1, a.student1, a.score1 
    UNION ALL
    SELECT 2, a.student2, a.score2 
    UNION ALL
    SELECT 3, a.student3, a.score3 
) b(num, student, score)

Answer 3

假设该表被称为类，您可以将它与自身联合起来：

(SELECT student1 as student, subject from Classes)
UNION
(SELECT student2 as student, subject from Classes)
UNION
(SELECT student3 as student, subject from Classes)

查询表以使每行成为3行

3 个答案: