sql - SQL Server 中求和函数的工作方式

Question

鉴于此表

CREATE TABLE Table1
(
    [Classroom] int,
    [CourseName] varchar(8),
    [Lesson] varchar(9),
    [StartTime] char(4),
    [EndTime] char(4)
);
    
INSERT INTO Table1
    ([Classroom], [CourseName], [Lesson], [StartTime], [EndTime])
VALUES
    (1001, 'Course 1', 'Lesson 1', '0800', '0900'),
    (1001, 'Course 1', 'Lesson 2', '0900', '1000'),
    (1001, 'Course 1', 'Lesson 3', '1000', '1100'),
    (1001, 'Course 1', 'Lesson 6', '1100', '1200'),
    (1001, 'Course 2', 'Lesson 10', '1100', '1200'),
    (1001, 'Course 2', 'Lesson 11', '1200', '1300'),
    (1001, 'Course 1', 'Lesson 4', '1300', '1400'),
    (1001, 'Course 1', 'Lesson 5', '1400', '1500');

还有我的查询

WITH A AS 
(
    SELECT 
        ClassRoom,
        CourseName,
        StartTime,
        EndTime,
        PrevCourse = LAG(CourseName, 1, CourseName) OVER (ORDER BY StartTime)
    FROM   
        Table1
), B AS 
(
    SELECT 
        ClassRoom,
        CourseName,
        StartTime, EndTime,
        Ranker = SUM(CASE WHEN CourseName = PrevCourse THEN 0 ELSE 1 END) 
                    OVER (ORDER BY StartTime, CourseName)
    FROM   
        A
)
SELECT B.* 
FROM B;

我得到这个结果:

ClassRoom CourseName StartTime EndTime Ranker
---------------------------------------------
1001      Course 1   0800   0900    0
1001      Course 1   0900   1000    0
1001      Course 1   1000   1100    0
1001      Course 1   1100   1200    0
1001      Course 2   1100   1200    1
1001      Course 2   1200   1300    1
1001      Course 1   1300   1400    2
1001      Course 1   1400   1500    2

请关注栏排名。如果我没有误解的话，在当前类(class)与之前类(class)不同的第一行，则 sum(1);接下来的行，其中当前类(class) == 上一类(class)，然后求和(0)，所以我对排名器的期望应该是:(0,0,0,0)，(1,1)，(1,1)但它给我 (0,0,0,0), (1,1), (2,2)。

为什么最后我得到 (2, 2) ？我错过了什么吗？

Answer 1

表达式:

CASE WHEN CourseName = PrevCourse THEN 0 ELSE 1 END

当 CourseName 与 PrevCourse 不同时返回 1。

如果在 B 中再添加一列，您会看得更清楚:

B AS (
  SELECT ClassRoom
       , CourseName
       , StartTime
       , EndTime
       , CASE WHEN CourseName = PrevCourse THEN 0 ELSE 1 END flag -- check this
       , Ranker = SUM(CASE WHEN CourseName = PrevCourse THEN 0 ELSE 1 END)
                OVER (ORDER BY StartTime, CourseName)
  FROM   A
)

如果您想获得 (0,0,0,0), (1,1), (1,1) 形式的结果，您应该在 内添加一个 PARTITION BY 子句Ranker 列的 OVER:

Ranker = SUM(CASE WHEN CourseName = PrevCourse THEN 0 ELSE 1 END)
         OVER (PARTITION BY CourseName ORDER BY StartTime)

请参阅demo .