将列值设置为MySQL查询结果中的列名？
要将列值设置为查询结果中的列名，您需要使用CASE语句。
语法如下-
select yourIdColumnName,max(case when (yourColumnName1='yourValue1') then yourColumnName2 else NULL
end) as 'yourValue1',
max(case when (yourColumnName1='yourValue2') then yourColumnName2 else NULL
end) as 'yourValue2',
max(case when yourColumnName1='yourValue3') then yourColumnName2 else NULL
end) as 'yourValue3’,
.
.
N
from valueAsColumn
group by yourIdColumnName
order by yourIdColumnName;
为了理解上述语法，让我们创建一个表。创建表的查询如下-
mysql> create table valueAsColumn-> (
-> UserId int,
-> UserColumn1 varchar(10),
-> UserColumn2 varchar(10)
-> );
现在，您可以使用insert命令在表中插入一些记录。查询如下-
mysql> insert into valueAsColumn values(0,'John','A+');mysql> insert into valueAsColumn values(0,'Carol','B');
mysql> insert into valueAsColumn values(0,'Sam','C');
mysql> insert into valueAsColumn values(1,'John','D');
mysql> insert into valueAsColumn values(1,'Carol','A');
mysql> insert into valueAsColumn values(1,'Carol','C');
使用select语句显示表中的所有记录。查询如下-
mysql> select *from valueAsColumn;
以下是输出-
+--------+-------------+-------------+| UserId | UserColumn1 | UserColumn2 |
+--------+-------------+-------------+
| 0 | John | A+ |
| 0 | Carol | B |
| 0 | Sam | C |
| 1 | John | D |
| 1 | Carol | A |
| 1 | Carol | C |
+--------+-------------+-------------+
6 rows in set (0.00 sec)
这是将列值设置为列名的查询-
mysql> select UserId,-> max(case when (UserColumn1='John') then UserColumn2 else NULL end) as 'John',
-> max(case when (UserColumn1='Carol') then UserColumn2 else NULL end) as 'Carol',
-> max(case when (UserColumn1='Sam') then UserColumn2 else NULL end) as 'Sam'
-> from valueAsColumn
-> group by UserId
-> order by UserId;
以下是输出-
+--------+------+-------+------+| UserId | John | Carol | Sam |
+--------+------+-------+------+
| 0 | A+ | B | C |
| 1 | D | C | NULL |
+--------+------+-------+------+
2 rows in set (0.00 sec)
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