SQL：将记录与相同的ReferenceID组合在一起

Question

我的表格中有以下记录

ID   StudentID   Semester   Grade     
1    1           First      100
2    1           Second     90
3    2           First      90
4    1           Third      85

我想将学生证1的所有记录合并到一个记录中

StudentID   First   Second   Third
1           100     90       85 
2           90      null     null

有什么想法吗？

Answer 1

由于你没有提到你正在使用的RDBMS，试试这个。

SELECT  StudentID,
        MAX(CASE WHEN Semester = 'First' THEN Grade ELSE NULL END) AS First,
        MAX(CASE WHEN Semester = 'Second' THEN Grade ELSE NULL END) AS Second,
        MAX(CASE WHEN Semester = 'Third' THEN Grade ELSE NULL END) AS Third
FROM    tableName
GROUP BY StudentID

• SQLFiddle Demo