3. 创建表之后,添加外键 ALTER TABLE 表名 ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段名称) REFERENCES 主表名称(主表列名称); 4. 级联操作 1. 添加级联操作 语法:ALTER TABLE 表名 ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段名称) REFERENCES 主表名称(主表列名称) ON UPDATE CASCADE ON DELETE CA...
UNIQUE 约束和 CHECK 约束是可用于在 SQL Server 表中强制实施数据完整性的两种约束类型。 这些是重要的数据库对象。本文包含以下各节。UNIQUE 约束 CHECK 约束 相关任务 UNIQUE 约束约束是 SQL Server 数据库引擎为你强制执行的规则。 例如,您可以使用 UNIQUE 约束确保在非主键的特定列中不会输入重...
ALTERTABLEemployeesADDCONSTRAINTuq_employees_SBMCUNIQUE(SBMC);===employees=== 在SQL Server 中,UNIQUE和PRIMARY KEY都用于确保列中的数据唯一性,但它们有一些关键的区别。下面是详细解释: 1.PRIMARY KEY 唯一性:PRIMARY KEY强制列中的值是唯一的,即在表中每行的数据在该列上不能重复。 不能为空:PRIMARY KE...
Applies to: SQL Server 2016 (13.x) and later Azure SQL Database Azure SQL Managed InstanceYou 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...
Applies to: SQL Server 2016 (13.x) and later Azure SQL Database Azure SQL Managed InstanceYou 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...
add constraint fk_colname foreign key(colname) references 删除约束: alter table drop constraint 使用SSMS界面创建和删除约束 SQL Server中有五种约束类型,分别是 PRIMARY KEY约束、FOREIGN KEY约束、UNIQUE约束、DEFAULT约束、和CHECK约束。查看或者创建约束都要使用到 Microsoft SQL Server Managment Studio。
Constraints are rules that the SQL Server Database Engine enforces for you. For example, you can use UNIQUE constraints to make sure that no duplicate values are entered in specific columns that don't participate in a primary key. Although both a UNIQUE constraint and a PRIMARY KEY constraint...
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....
CONSTRAINTUC_PersonUNIQUE(ID,LastName) ); SQL UNIQUE Constraint on ALTER TABLE To create aUNIQUEconstraint on the "ID" column when the table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTERTABLEPersons ...
如需命名 UNIQUE 约束,并定义多个列的 UNIQUE 约束,请使用下面的 SQL 语法: MySQL / SQL Server / Oracle / MS Access: ALTER TABLEPersonsADD CONSTRAINT uc_PersonID UNIQUE(P_Id,LastName); 删除UNIQUE 约束 如果需要移除一个 UNIQUE 约束,可以使用以下 SQL 语句: ...