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法律如何防止 MySQL GROUP BY 将 NULL 值折叠到单行中？
要实现这一目标，可将 IFNULL() 与 ORDER BY 子句结合使用。我们先创建一个名为表 table 的表 -
mysql> create table DemoTable1511 -> ( -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> FirstName varchar(20) -> ); Query OK, 0 rows affected (1.97 sec)
使用 insert 命令将一些记录插入表中 -
mysql> insert into DemoTable1511(FirstName) values('John');
Query OK, 1 row affected (0.20 sec)
mysql> insert into DemoTable1511(FirstName) values('Robert');
Query OK, 1 row affected (0.29 sec)
mysql> insert into DemoTable1511(FirstName) values('Mike');
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable1511(FirstName) values('Robert');
Query OK, 1 row affected (1.08 sec)
mysql> insert into DemoTable1511(FirstName) values(NULL);
Query OK, 1 row affected (0.68 sec)
mysql> insert into DemoTable1511(FirstName) values(NULL);
Query OK, 1 row affected (1.91 sec)
mysql> insert into DemoTable1511(FirstName) values('Mike');
Query OK, 1 row affected (0.51 sec)使用 select 语句显示表中的所有记录 -
mysql> select * from DemoTable1511;
这将生成以下输出 -
+----+-----------+ | Id | FirstName | +----+-----------+ | 1 | John | | 2 | Robert | | 3 | Mike | | 4 | Robert | | 5 | NULL | | 6 | NULL | | 7 | Mike | +----+-----------+ 7 rows in set (0.00 sec)
以下是对 MySQL GROUP BY 进行查询以防止将 NULL 值折叠到单行的方法 -
mysql> select min(Id) as Id,FirstName from DemoTable1511 group by FirstName,ifnull(FirstName,Id) order by Id;
这将生成以下输出 -
+------+-----------+ | Id | FirstName | +------+-----------+ | 1 | John | | 2 | Robert | | 3 | Mike | | 5 | NULL | | 6 | NULL | +------+-----------+ 5 rows in set (0.00 sec)
更新日期：2019-12-11
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