在MySQL中将0替换为null？

Replace 0 with null in MySQL?

您可以使用MySQL中的NULLIF()将0替换为NULL。语法如下：

1	SELECT *,NULLIF(yourColumnName,0) as anyVariableName from yourTableName;

为了理解上述语法，让我们创建一个表。创建表的查询如下：

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mysql> create table Replace0WithNULLDemo
-> (
-> Id int NOT NULL auto_increment,
-> Name varchar(20),
-> Marks int,
-> PRIMARY KEY(Id)
-> );
Query OK, 0 rows affected (0.53 sec)

现在，您可以使用insert命令在表中插入一些记录。查询如下：

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mysql> insert into Replace0WithNULLDemo(Name,Marks) values('John',76);
Query OK, 1 row affected (0.16 sec)

mysql> insert into Replace0WithNULLDemo(Name,Marks) values('Carol',86);
Query OK, 1 row affected (0.20 sec)

mysql> insert into Replace0WithNULLDemo(Name,Marks) values('Sam',0);
Query OK, 1 row affected (0.17 sec)

mysql> insert into Replace0WithNULLDemo(Name,Marks) values('Mike',0);
Query OK, 1 row affected (0.16 sec)

mysql> insert into Replace0WithNULLDemo(Name,Marks) values('Larry',98);
Query OK, 1 row affected (0.19 sec)

mysql> insert into Replace0WithNULLDemo(Name,Marks) values('Bob',0);
Query OK, 1 row affected (0.17 sec)

使用select语句显示表中的记录。查询如下：

1	mysql> select *from Replace0WithNULLDemo;

以下是输出：

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+----+-------+-------+
| Id | Name | Marks |
+----+-------+-------+
| 1 | John | 76 |
| 2 | Carol | 86 |
| 3 | Sam | 0 |
| 4 | Mike | 0 |
| 5 | Larry | 98 |
| 6 | Bob | 0 |
+----+-------+-------+
6 rows in set (0.00 sec)

现在让我们用NULL替换0。查询如下：

1	mysql> select *,NULLIF(Marks,0) as ReplaceZeroWithNULL from Replace0WithNULLDemo;

以下是显示新列的输出，其中new已用NULL替换了0：

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+----+-------+-------+---------------------+
| Id | Name | Marks | ReplaceZeroWithNULL |
+----+-------+-------+---------------------+
| 1 | John | 76 | 76 |
| 2 | Carol | 86 | 86 |
| 3 | Sam | 0 | NULL |
| 4 | Mike | 0 | NULL |
| 5 | Larry | 98 | 98 |
| 6 | Bob | 0 | NULL |
+----+-------+-------+---------------------+
6 rows in set (0.00 sec)