mysql行列互相转换

列转行：

mysql> select * from test;                                                                                                         +------+----------+-------+
| id   | subject  | score |
+------+----------+-------+
|    1 | chinese  |    98 |
|    2 | math     |    95 |
|    2 | politics |    87 |
|    5 | chinese  |    97 |
|    5 | math     |   100 |
|    5 | politics |    92 |
+------+----------+-------+
6 rows in set (0.00 sec)

mysql> select (case id when 1 then 'first' when 2 then 'second' when 5 then 'fifth' end) grade,sum(case subject when 'chinese' then score else 0 end) chinese,sum(case subject when 'math' then score else 0 end) math,sum(case subject when 'politics' then score else 0 end) politics from test group by id;
+--------+---------+------+----------+
| grade  | chinese | math | politics |
+--------+---------+------+----------+
| first  |      98 |    0 |        0 |
| second |       0 |   95 |       87 |
| fifth  |      97 |  100 |       92 |
+--------+---------+------+----------+
3 rows in set (0.00 sec)

行转列：

复制上面的结果到一个新表test2

mysql> create table test2 as select (case id when 1 then 'first' when 2 then 'second' when 5 then 'fifth' end) grade,sum(case subject when 'chinese' then score else 0 end) chinese,sum(case subject when 'math' then score else 0 end) math,sum(case subject when 'politics' then score else 0 end) politics from test group by id;
Query OK, 3 rows affected (0.12 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from test2;
+--------+---------+------+----------+
| grade  | chinese | math | politics |
+--------+---------+------+----------+
| first  |      98 |    0 |        0 |
| second |       0 |   95 |       87 |
| fifth  |      97 |  100 |       92 |
+--------+---------+------+----------+
3 rows in set (0.00 sec)

mysql> select * from (select (case grade when 'first' then 1 when 'second' then 2 when 'fifth' then 5 end) id,'chinese' subject,chinese as score from test2 union all select (case grade when 'first' then 1 when 'second' then 2 when 'fifth' then 5 end) id,'math' subject,math as score from test2 union all select (case grade when 'first' then 1 when 'second' then 2 when 'fifth' then 5 end) id,'politics' subject,politics as score from test2 order by id) a where a.score<>0;
+------+----------+-------+
| id   | subject  | score |
+------+----------+-------+
|    1 | chinese  |    98 |
|    2 | math     |    95 |
|    2 | politics |    87 |
|    5 | math     |   100 |
|    5 | politics |    92 |
|    5 | chinese  |    97 |
+------+----------+-------+
6 rows in set (0.00 sec)

mysql> select * from test;
+------+----------+-------+
| id   | subject  | score |
+------+----------+-------+
|    1 | chinese  |    98 |
|    2 | math     |    95 |
|    2 | politics |    87 |
|    5 | chinese  |    97 |
|    5 | math     |   100 |
|    5 | politics |    92 |
+------+----------+-------+
6 rows in set (0.00 sec)