SQL Server 2005 - 如何比较字段值，并在每次出现不同时返回计数

Question

DECLARE @CURRENTSCHOOL TABLE (STUDENT VARCHAR(8), COURSE VARCHAR(8), SCHOOL VARCHAR(2))
INSERT INTO @CURRENTSCHOOL VALUES ('10000000','MCR1010','11')
INSERT INTO @CURRENTSCHOOL VALUES ('12000000','MCR6080','11')
INSERT INTO @CURRENTSCHOOL VALUES ('13000000','MCR6090','15')

DECLARE @OTHERSCHOOLS TABLE (STUDENT VARCHAR(8), COURSE VARCHAR(8), SCHOOL VARCHAR(2))
INSERT INTO @OTHERSCHOOLS VALUES ('10000000','MCR1010','11')
INSERT INTO @OTHERSCHOOLS VALUES ('10000000','MCR1011','14')
INSERT INTO @OTHERSCHOOLS VALUES ('10000000','MCR1012','15')
INSERT INTO @OTHERSCHOOLS VALUES ('12000000','MCR6080','19')
INSERT INTO @OTHERSCHOOLS VALUES ('13000000','MCR6090','15')

对于上面的样本数据。两张桌子。当前学校和其他学校。

Currentschool 是学生当前正在学习的课程，包括学校代码， 并且是主表。

其他学校可能是学生可以在不同学校继续学习的其他课程。

我需要将当前学校表与使用学生 ID 号匹配的其他学校表进行比较，并且对于其他学校中的每个不同学校代码，它需要返回一个计数。

例如：

Student:    OtherSchoolCount:
10000000    2        (because of 2 different school codes than than the current school)
12000000    1        (because of 1 different school code than than the current school)
13000000    blank    (because not a different school code)

这可能吗？

非常感谢

M。

Answer 1

SELECT cs.student, 
       COUNT(os.course) 
FROM   @CURRENTSCHOOL cs 
       LEFT JOIN @OTHERSCHOOLS os 
         ON cs.student = os.student 
            AND cs.school <> os.school 
GROUP  BY cs.student 

输出

STUDENT  
-------- -----------
10000000 2
12000000 1
13000000 0

如果 Null 确实优于 Zero，那么您可以执行此操作（或使用等效的 CTE）

SELECT student, 
       CASE 
         WHEN coursecount = 0 THEN NULL 
         ELSE coursecount 
       END coursecount 
FROM   (SELECT cs.student, 
               COUNT(os.course) coursecount 
        FROM   @CURRENTSCHOOL cs 
               LEFT JOIN @OTHERSCHOOLS os 
                 ON cs.student = os.student 
                    AND cs.school <> os.school 
        GROUP  BY cs.student) t 

哪个输出

student  courseCount
-------- -----------
10000000 2
12000000 1
13000000 NULL

更新：NullIF 可以用作 Case 语句的替代方案，请参阅 NULLIF() 有哪些应用？

Answer 2

select o.Student, count(*) as Count
from CURRENTSCHOOL c1 
inner join OTHERSCHOOLS o on c1.Student = o.Student --this join is to ensure student exists in both tables 
left outer join CURRENTSCHOOL c on o.Student = c.Student
    and o.School= c.School
where c.Student is null
group by o.Student