mysql - 将值拆分为其他几列

Question

代码:

CREATE TABLE table1 (
  day DATE,
  name VARCHAR(40)
  );
INSERT INTO table1 (day, name)
VALUES 
('2018-01-01', 'test1'),
('2018-01-01', 'test2'),
('2018-01-01', 'example'),
('2018-01-01', 'somevalue'),
('2018-01-02', 'test3'),
('2018-01-03', 'test4');

我想将结果拆分成如下内容:

day - name1 - name2 - namex  
DATE - value - value - value 

而不是在我选择数据时复制日期。

预期结果:

day - name - name - name - name ...
2018-01-01 - test1 - test2 - example - somevalue
2018-01-02 - NULL - NULL - NULL - NULL - test3

SQL FIDDLE

Answer 1

例如，您可以通过动态 sql 执行此操作，首先找到不同的名称值，然后围绕它们构建其余代码

给予

MariaDB [sandbox]> select * from t;
+------------+-----------+
| day        | name      |
+------------+-----------+
| 2018-01-01 | test      |
| 2018-01-01 | test      |
| 2018-01-01 | example   |
| 2018-01-01 | somevalue |
| 2018-01-02 | test      |
| 2018-01-03 | test      |
+------------+-----------+
6 rows in set (0.00 sec)

set @sql = concat('select day, ',
(select group_concat(maxstr)
from
(select concat('max(case when name = ', char(39),name,char(39),' then  name else null end) as ', concat('name',@rn:=@rn+1)) maxstr
from  
(select distinct name from t) t,(select @rn:=0) rn
) s
)
,
' from t group by day')
;

构建此代码

select day, max(case when name = 'test' then  name else null end) as name1,
        max(case when name = 'example' then  name else null end) as name2,
        max(case when name = 'somevalue' then  name else null end) as name3 
from t group by day;

运行时产生这个结果

+------------+-------+---------+-----------+
| day        | name1 | name2   | name3     |
+------------+-------+---------+-----------+
| 2018-01-01 | test  | example | somevalue |
| 2018-01-02 | test  | NULL    | NULL      |
| 2018-01-03 | test  | NULL    | NULL      |
+------------+-------+---------+-----------+
3 rows in set (0.00 sec)

使用动态 sql 的优点是它非常火并且忘记代码将捕获任何新值。不过要注意 group_concat 的限制。

像这样执行动态sql-

prepare sqlstmt from @sql;
execute sqlstmt;
deallocate prepare sqlstmt;