如何将 SQL 表中的所有值转换为具有多列的单行

Question

我需要创建下表的视图

学生	分数
詹姆士	500
杰克	200
吉米	100

变成这样

学生_1	标记_1	学生_2	标记_2	学生_3	Marks_3
詹姆士	500	杰克	200	吉米	100

我是 SQL 的新手，我正在集思广益如何做到这一点，但我无法弄清楚。 任何帮助将不胜感激，谢谢。

Answer 1

这是您案例的示例。 您可以使用PREPARE语句

SELECT CONCAT( "SELECT ",
     GROUP_CONCAT(
     CONCAT('"',Student,'"'),CONCAT(' AS "Student_',@Nr:=@Nr+1,'", ')
    ,CONCAT('"',Marks,'"'),CONCAT(' AS "Marks_',@Nr,'" ')  SEPARATOR ' , ')
    ," FROM DUAL;") INTO @myquery
FROM stud
CROSS JOIN (SELECT @Nr:=0) as Init
ORDER BY Marks DESC;


## ONLY FOR DEBUG
SELECT @myquery;

PREPARE stmt FROM @myquery;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

样本

MariaDB [Bernd]> SELECT * from stud;
+----+---------+-------+
| id | Student | Marks |
+----+---------+-------+
|  1 | James   |   500 |
|  2 | Jack    |   200 |
|  3 | Jimmy   |   100 |
+----+---------+-------+
3 rows in set (0.01 sec)

MariaDB [Bernd]> SELECT CONCAT( "SELECT ",
    ->  GROUP_CONCAT(
    ->  CONCAT('"',Student,'"'),CONCAT(' AS "Student_',@Nr:=@Nr+1,'", ')
    -> ,CONCAT('"',Marks,'"'),CONCAT(' AS "Marks_',@Nr,'" ')  SEPARATOR ' , ')
    -> ," FROM DUAL;") INTO @myquery
    -> FROM stud
    -> CROSS JOIN (SELECT @Nr:=0) as Init
    -> ORDER BY Marks DESC;
Query OK, 1 row affected (0.00 sec)
    
MariaDB [Bernd]> ## ONLY FOR DEBUG
MariaDB [Bernd]> SELECT @myquery;
+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| @myquery                                                                                                                                                 |
+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| SELECT "James" AS "Student_1", "500" AS "Marks_1"  , "Jack" AS "Student_2", "200" AS "Marks_2"  , "Jimmy" AS "Student_3", "100" AS "Marks_3"  FROM DUAL; |
+----------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
    
MariaDB [Bernd]> PREPARE stmt FROM @myquery;
Query OK, 0 rows affected (0.00 sec)
Statement prepared

MariaDB [Bernd]> EXECUTE stmt;
+-----------+---------+-----------+---------+-----------+---------+
| Student_1 | Marks_1 | Student_2 | Marks_2 | Student_3 | Marks_3 |
+-----------+---------+-----------+---------+-----------+---------+
| James     | 500     | Jack      | 200     | Jimmy     | 100     |
+-----------+---------+-----------+---------+-----------+---------+
1 row in set (0.00 sec)

MariaDB [Bernd]> DEALLOCATE PREPARE stmt;
Query OK, 0 rows affected (0.00 sec)

MariaDB [Bernd]>