将列值设置为 MySQL 查询结果中的列名?
mysqlmysqli database
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) -> ); Query OK, 0 rows affected (0.75 sec)
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+'); Query OK, 1 row affected (0.18 sec) mysql> insert into valueAsColumn values(0,'Carol','B'); Query OK, 1 row affected (0.17 sec) mysql> insert into valueAsColumn values(0,'Sam','C'); Query OK, 1 row affected (0.17 sec) mysql> insert into valueAsColumn values(1,'John','D'); Query OK, 1 row affected (0.20 sec) mysql> insert into valueAsColumn values(1,'Carol','A'); Query OK, 1 row affected (0.20 sec) mysql> insert into valueAsColumn values(1,'Carol','C'); Query OK, 1 row affected (0.15 sec)
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)
