However, you can have manyUNIQUEconstraints per table, but only onePRIMARY KEYconstraint per table. SQL UNIQUE Constraint on CREATE TABLE The following SQL creates aUNIQUEconstraint on the "ID" column when the "Persons" table is created: ...
3 rows in set mysql> 在修改表时创建唯一约束 在修改表时添加唯一约束的语法格式为: ALTER TABLE <数据表名> ADD CONSTRAINT <唯一约束名> UNIQUE(<列名>); 【实例3】修改学生表student,指定学生的名子唯一,输入的 SQL 语句和运行结果如下所示。 mysql> alter table student add constraint name unique(name)...
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。 2.2在修改表时添加唯一约束 在修改表时添加唯一约束的语法格式为: ALTER TABLE ADD CONSTRAINT UNIQUE(); 实例2,修改数据表 demo_department,指定部门的名称唯一,输入的 SQL 语句和运行结果如下所示。 mysql> ALTER TABLE demo_depa...
You can create a unique constraint in SQL Server by using SQL Server Management Studio or Transact-SQL to ensure no duplicate values are entered in specific columns that don't participate in a primary key. Creating a unique constraint automatically creates a corresponding unique index....
Also,Unique_Collegeis a name given to theUNIQUEconstraint defined forcollege_idandcollege_codecolumns. Note:Our online SQL editor doesn't support this action as it is based on SQLite. Error When Inserting Duplicate Values We will get an error if we try to insert duplicate values in a column...
uniquekey constraint requires that every value in a column or set of columns beunique. No rows of a table may have duplicate values in a column (the uniquekey) or set of columns (the composite unique key) with a unique key constraint. ...
uniquekey constraint requires that every value in a column or set of columns beunique. No rows of a table may have duplicate values in a column (the uniquekey) or set of columns (the composite unique key) with a unique key constraint. ...
key 是数据库的物理结构,它包含两层意义和作用, 一是约束(偏重于约束和规范数据库的结构完整性), 二是索引(辅助查询用的)。 包括primary key, unique key, foreign key 等。 primary key有两个作用,一是约束作用(constraint),用来规范一个存储主键和唯一性,但同时也在此key上建立了一个主键索引; ...
并返回IntrgrityErrorEN/**添加流程模型并返回modelId * @param process_id //流程唯一标识key ...
You can create a unique constraint in SQL Server by using SQL Server Management Studio or Transact-SQL to ensure no duplicate values are entered in specific columns that don't participate in a primary key. Creating a unique constraint automatically creates a corresponding unique index. Note For ...