在MySQL中使用不同条件连接来自同一列的两个值

为此，您可以使用带有聚合函数的group_concat（）。让我们首先创建一个表 −

mysql> create table DemoTable1869
     (
     Id int,
     Subject varchar(20 ),
     Name varchar(20)
     );
Query OK, 0 rows affected (0.00 sec)

使用insert命令向表中插入一些记录−

mysql> insert into DemoTable1869 values(100,'MySQL','John');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1869 values(100,'MongoDB','Smith');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1869 values(101,'MySQL','Chris');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1869 values(101,'MongoDB','Brown');
Query OK, 1 row affected (0.00 sec)

使用select语句显示来自表的所有记录−

mysql> select * from DemoTable1869;

这将产生以下输出−

+------+---------+-------+
| Id   | Subject | Name  |
+------+---------+-------+
|  100 | MySQL   |  John |
|  100 | MongoDB | Smith |
|  101 | MySQL   | Chris |
|  101 | MongoDB | Brown |
+------+---------+-------+
4 rows in set (0.00 sec)

以下是使用不同条件连接来自同一列的2个值的查询−

mysql> select Id,concat(StudentFirstName,'',StudentLastName)
   from
   (
      select Id,
      max(case when Subject='MySQL' then Name end) as StudentFirstName,
      max(case when Subject='MongoDB' then Name end) as StudentLastName
      from DemoTable1869
      group by Id
   )tbl;

这将产生以下输出−

+------+---------------------------------------------+
| Id   | concat(StudentFirstName,'',StudentLastName) |
+------+---------------------------------------------+
|  100 | JohnSmith                                   |
|  101 | ChrisBrown                                  |
+------+---------------------------------------------+
2 rows in set (0.00 sec)

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