怎么删除check约束,定义check约束
如何修改或删除mysql CHECK约束 constraint
mysql是不支持check约束的。如果你创建表的时候加上了check约束也是不起作用的。所以,你不用更改或删除之前的check约束。
MySQL 关于去除约束的问题
现在要说的是在列这一层次过滤的基于表定义之前就规范好的 CHECK 约束。（MySQL 版本 = 8.0.16）
mysql create table f1 (r1 int constraint tb_f1_r1_chk1 check (mod(r1,3)=0));
Query OK, 0 rows affected (0.03 sec)
mysql create table f2 (r1 int constraint tb_f2_r1_chk1 check (mod(r1,3)=0) not enforced);
Query OK, 0 rows affected (0.02 sec)
这里 CHECK 约束的相关限制如下：
1. constraint 名字在每个数据库中唯一。
也就是说单个数据库里不存在相同的两个 constraint，如果不定义，系统自动生成一个唯一的约束名字。
2. check 约束针对语句 insert/update/replace/load data/load xml 生效；针对对应的 ignore 语句失效。
3. 并非每个函数都可以使用，比如函数结果不确定的：NOW()，CONNECTION_ID()，CURRENT_USER()。
4. 不适用于存储过程和存储函数。
5. 系统变量不适用。
6. 子查询不适用。
7. 外键动作（比如 ON UPDATE, ON DELETE） 不适用。
8. enforced 默认启用，如果单独加上 not enforced ，check 约束失效。
怎样删除表中字段studentid的check约束
drop constraint 约束名 只要你知道check约束的约束名
你的那个约束名不是那个约束名。名称可能是'CK_表名_字段名_随机产生的'，你找找看
MYSQL如何撤销check约束？ 不要W3C里面的 ALTER TABLE Persons DROP CONSTRAINT chk_Person
对于非unique索引和外键可以可以设置禁用，其他的，暂时还没发现。
想要禁用非unique索引的话可以用：ALTER TABLE ... DISABLE KEYS，这个语句需要是MyISAM的表才行。
想要禁用外键的话可以用类似下面的语句：
mysql SET foreign_key_checks = 0;
mysql SOURCE dump_file_name;
mysql SET foreign_key_checks = 1;
详细的说明，可以参考mysql文档，这里贴两段摘自MySql 5.6版的英文说明，帮助理解。
If you use ALTER TABLE on a MyISAM table, all nonunique indexes are created in a separate batch (as for REPAIR TABLE). This should make ALTER TABLE much faster when you have many indexes.This feature can be activated explicitly for a MyISAM table. ALTER TABLE ... DISABLE KEYS tells MySQL to stop updating nonunique indexes. ALTER TABLE ... ENABLE KEYS then should be used to re-create missing indexes. MySQL does this with a special algorithm that is much faster than inserting keys one by one, so disabling keys before performing bulk insert operations should give a considerable speedup. Using ALTER TABLE ... DISABLE KEYS requires the INDEX privilege in addition to the privileges mentioned earlier.While the nonunique indexes are disabled, they are ignored for statements such as
SELECT and EXPLAIN that otherwise would use them.
To make it easier to reload dump files for tables that have foreign key relationships,
mysqldump automatically includes a statement in the dump output to set
foreign_key_checks to 0. This avoids problems with tables having to be reloaded in a particular order when the dump is reloaded. It is also possible to set this variable manually:mysql SET foreign_key_checks = 0;
mysql SOURCE dump_file_name;
mysql SET foreign_key_checks = 1;
This enables you to import the tables in any order if the dump file contains tables that are not correctly ordered for foreign keys. It also speeds up the import operation. Setting
foreign_key_checks to 0 can also be useful for ignoring foreign key constraints during LOAD DATA and ALTER TABLE operations. However, even ifforeign_key_checks = 0
, InnoDB does not permit the creation of a foreign key constraint where a column references a nonmatching column type. Also, if an InnoDB table has foreign key constraints,
ALTER TABLE cannot be used to change the table to use another storage engine. To alter the storage engine, drop any foreign key constraints first.
想删除sql数据库里的字段 但是有check约束 请问怎么才能删掉
1.首先要把约束删掉,一般你删除时,都会报错,并把约束显示出来的，或者把表的创建脚本导出来,
就可以看到对应栏位的约束了.
2.删除对应栏位;
例如:
alter
table
mp_report
drop
CONSTRAINT
[DF__mp_report__test1__0CBAE877]
alter
table
mp_report
drop
column
[test1]
SQL SERVER中如何更改CHECK约束？
好像没有修改约束，你可以先删除约束，再添加约束\x0d\x0a添加约束\x0d\x0a\x0d\x0aALTER\x0d\x0aTABLE employees\x0d\x0a\x0d\x0aADD\x0d\x0aCONSTRAINT emp_manager_fk\x0d\x0a\x0d\x0aFOREIGN\x0d\x0aKEY(manager_id)\x0d\x0a\x0d\x0aREFERENCES\x0d\x0aemployees(employee_id);\x0d\x0a\x0d\x0a删除约束\x0d\x0a\x0d\x0aALTER\x0d\x0aTABLE employees\x0d\x0a\x0d\x0aDROP\x0d\x0aCONSTRAINT emp_manager_fk;