A UNIQUE Constraint makes it impossible to COMMIT any operation that would cause the unique key to contain any non-null duplicate values. (Multiple null values are allowed, since the null value is never equal to anything, even another null value.) A UNIQUE Constraint is violated if its condit...
TheNULLvalue means “no data.”NULLcan be written in any lettercase(大小写不敏感). -- 引用自MySQL手册 NULL的长度问题 mysql> select length(''), length(null), length(0), length('0'); +---+---+---+---+ | length('') | length(null) | length(0) | length('0') | +---+-...
mysql中也是如此,null代表的是一种不确定性,所以通常用is null 或者not null来判定一个实例数据是否为不确定的,而不是直接==来进行值比较。 官方手册中中的定义:The NULL value means “no data.” NULL can be written in any lettercase也就产生了下面的关系操作符: IS NULL\IS NOT NULL:能正常比较 <=...
mysql> use test Database changed mysql> create table date_test( -> datecol DATE -> ); Query OK, 0 rows affected (0.48 sec) mysql> show create table date_test\G *** 1. row *** Table date_test Create Table: CREATE TABLE `date_test` ( `datecol` date DEFAULT NULL ) ENGINE=...
mysql> show variables like '%character%'; +---+---+ | Variable_name | Value | +---+---+ | character_set_client | latin1 | | character_set_connection | latin1 | | character_set_database | latin1 | | character_set_filesystem |...
The operation fails because the specified parent_id value does not exist in the parent table. Try to delete the previously inserted row from the parent table:--删除父表中的数据,但是这笔数据,子表有参照,就会报错 mysql>DELETEFROMparentWHEREidVALUES=1; ...
Null means “a missing and unknown value”.Let’s see details below. NULL 代表一个不确定的值,就算是两个 NULL,它俩也不一定相等。(像不像 C 中未初始化的局部变量) (root@localhost mysql3306.sock)[zlm]>SELECT 0 IS NULL, 0 IS NOT NULL, '' IS NULL, '' IS NOT NULL; ...
Null means “a missing and unknown value”.Let’s see details below. NULL代表一个不确定的值,就算是两个NULL,它俩也不一定相等.(像不像C中未初始化的局部变量) 代码语言:javascript 代码运行次数:0 运行 AI代码解释 (root@localhost mysql3306.sock)[zlm]>SELECT0ISNULL,0ISNOTNULL,''ISNULL,''ISNOT...
insert into tb_test(one,two) value ('',''); Query OK, 1 row affected #这也是刚刚讲过not null约束测试insert语句的时候, 插入('')空字符串会成功的原因! 3、在查询方式上的区别对比 #创建一个表,tb_test2 create table tb_test2( id int unsigned primary key auto_increment, ...
These are in fact values, whereas NULL means “not having a value.” You can test this easily enough by using IS [NOT] NULL as shown: mysql> SELECT 0 IS NULL, 0 IS NOT NULL, '' IS NULL, '' IS NOT NULL; +---+---+---+---+ | 0 IS NULL | 0 IS NOT NULL | '' IS...