mysql - 将表行转换为 mysql 中的列

Question

假设我有一个类似于以下的数据库:

表student_info:

id    name
111   jon
112   dan
113   david
...

和表分数:

item_id    student_id    score
01         111           37
02         111           45
01         112           55
02         112           44
01         113           66
02         113           45
...

是否可以通过 mysql 查询生成下表？:

Student_Name  ITEM_1_SCORE  ITEM_2_SCORE
jon           37            45
dan           55            44
david         66            45
...

如果是这样，语法是什么？不知道这是join操作还是别的？

谢谢。

Answer 1

如果 item_id 只有两个值，那么可以硬编码值。示例

SELECT  a.Name AS Student_Name,
        MAX(CASE WHEN item_id = '01' THEN b.score END) Item_1_Score,
        MAX(CASE WHEN item_id = '02' THEN b.score END) Item_2_Score
FROM    student_info a
        LEFT JOIN scores b
            ON a.id = b.student_ID
GROUP   BY a.Name

• SQLFiddle Demo

否则，当您有未知数量的分数时，动态 SQL 是首选。

SELECT  GROUP_CONCAT(DISTINCT
        CONCAT('MAX(CASE WHEN item_id = ''',
               item_id,
               ''' THEN Score END) AS ',
               CONCAT('`Item_', item_id, '_Score`')
               )) INTO @sql
FROM scores;

SET @sql = CONCAT('SELECT   a.Name AS Student_Name, ', @sql, ' 
                    FROM    student_info a
                            LEFT JOIN scores b
                                ON a.id = b.student_ID
                    GROUP   BY a.Name');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

• SQLFiddle Demo
• SQLFiddle Demo (with extra item_id)

两个查询将输出相同的内容

╔══════════════╦══════════════╦══════════════╗
║ STUDENT_NAME ║ ITEM_1_SCORE ║ ITEM_2_SCORE ║
╠══════════════╬══════════════╬══════════════╣
║ dan          ║           55 ║           44 ║
║ david        ║           66 ║           45 ║
║ jon          ║           37 ║           45 ║
╚══════════════╩══════════════╩══════════════╝