4 rows in set (0.00 sec) mysql> alter table t43 add constraint uq_phone unique(phone); Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc t43; +---+---+---+---+---+---+ | Field | Type | Null | Key | Default | Extra | +---+--...
mysql>CREATETABLEtb_dept2->(->idINT(11)PRIMARYKEY,->nameVARCHAR(22)UNIQUE,->locationVARCHAR(50)->); Query OK,0rowsaffected (0.37sec) mysql>DESCtb_dept2;+---+---+---+---+---+---+|Field|Type|Null|Key|Default|Extra|+---+---+---+---+---+---+|id|int(11)|NO|PRI|N...
3 rows in set mysql> 提示:UNIQUE 和 PRIMARY KEY 的区别:一个表可以有多个字段声明为 UNIQUE,但只能有一个 PRIMARY KEY 声明;声明为 PRIMAY KEY 的列不允许有空值,但是声明为 UNIQUE 的字段允许空值的存在。 删除唯一约束 在MySQL 中删除唯一约束的语法格式如下: ALTER TABLE <表名> DROP INDEX <唯一约束...
mysql> ALTER TABLE np_pk -> PARTITION BY HASH(id) -> PARTITIONS 4; Query OK, 0 rows affected (0.11 sec) Records: 0 Duplicates: 0 Warnings: 0 In the case of np_pk, the only column that may be used as part of a partitioning expression is id; if you wish to partition this table...
mysql> alter table student add majorId int UNIQUE;Query OK, 2 rows affected (0.29 sec)Records: 2 Duplicates: 0 Warnings: 0 1. 依然成功。 所以,一张表,主键最多有一个,而唯一性键可以有多个。 是否允许组合 假设两个字段组合成主键,那定义时格式:PRIMARY KEY(字段1,字段2)。在表级约束添加。
mysql> ALTER TABLE np_pk -> PARTITION BY HASH(id) -> PARTITIONS 4; Query OK, 0 rows affected (0.11 sec) Records: 0 Duplicates: 0 Warnings: 0 In the case of np_pk, the only column that may be used as part of a partitioning expression is id; if you wish to partition this table...
-- session 2 session2 > set innodb_lock_wait_timeout=1; Query OK, 0 rows affected (0.00 sec) session2 > start transaction; Query OK, 0 rows affected (0.00 sec) session2 > INSERT INTO ti (session_ref_id, customer_id, client_id, app_id) VALUES (NULL, 8001, 10, 5); ERROR 1205...
mysql> insert into menus(label,url) values('Home','home.html'); Query OK, 1 row affected (0.06 sec) mysql> insert into menus(label,url) values('About us','aboutus.html'); Query OK, 1 row affected (0.05 sec) mysql> insert into menus(label,url) values('Services','services.html'...
索引优化:MySQL会自动为唯一键创建索引,提高查询效率。 外键约束:唯一键可以作为外键,用于建立表与表之间的关系。 类型 单列唯一键:只针对某一列设置唯一性约束。 复合唯一键:针对多列设置唯一性约束。 应用场景 用户表:确保每个用户的用户名或邮箱地址唯一。
ERROR1062(23000): Duplicate entry'basket'forkey'uint'mysql>insertintousersvalues('ti','FOOT'); Query OK,1row affected (0.00sec) mysql>select*fromusers;+---+---+|uname|uint|+---+---+|li|basket||ti|FOOT|+---+---+2rowsinset(0.00sec) Q CREATEtableusers2 ( unameVARCHAR(50)PRIMARY...