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如何防止MySQL GROUP BY将NULL值折叠成一行？

2024-04-26 23:50:08 435

为此，您可以将其IFNULL()与ORDERBY子句一起使用。让我们首先创建一个表table-

mysql> create table DemoTable1511
   -> (
   -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> FirstName varchar(20)
   -> );

使用插入命令在表中插入一些记录-

mysql> insert into DemoTable1511(FirstName) values('John');
mysql> insert into DemoTable1511(FirstName) values('Robert');
mysql> insert into DemoTable1511(FirstName) values('Mike');
mysql> insert into DemoTable1511(FirstName) values('Robert');
mysql> insert into DemoTable1511(FirstName) values(NULL);
mysql> insert into DemoTable1511(FirstName) values(NULL);
mysql> insert into DemoTable1511(FirstName) values('Mike');

使用select语句显示表中的所有记录-

mysql> select * from DemoTable1511;

这将产生以下输出-

+----+-----------+
| Id | FirstName |
+----+-----------+
|  1 | John      |
|  2 | Robert    |
|  3 | Mike      |
|  4 | Robert    |
|  5 | NULL      |
|  6 | NULL      |
|  7 | Mike      |
+----+-----------+
7 rows in set (0.00 sec)

这是防止MySQLGROUPBY将NULL值折叠为单行的查询-

mysql> select min(Id) as Id,FirstName from DemoTable1511 group by FirstName,ifnull(FirstName,Id) order by Id;

这将产生以下输出-

+------+-----------+
| Id   | FirstName |
+------+-----------+
|    1 | John      |
|    2 | Robert    |
|    3 | Mike      |
|    5 | NULL      |
|    6 | NULL      |
+------+-----------+
5 rows in set (0.00 sec)

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如何防止MySQL GROUP BY将NULL值折叠成一行？

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