For Multiple Columns -- add unique constraint to multiple columnsALTERTABLECollegesADDUNIQUEUnique_College (college_id, college_code); Here, the SQL command adds theUNIQUEconstraint tocollege_idandcollege_codecolumns in the existingCollegestable. Also,Unique_Collegeis a name given to theUNIQUEconstrain...
To name aUNIQUEconstraint, and to define aUNIQUEconstraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: ALTERTABLEPersons ADDCONSTRAINTUC_PersonUNIQUE(ID,LastName); DROP a UNIQUE Constraint
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: ALTERTABLEPersonsADDCONSTRAINTuc_PersonIDUNIQUE(P_Id,LastName) To DROP a UNIQUE Constraint To drop a UNIQUE constraint, us...
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within ...
USE AdventureWorks2022; GO ALTER TABLE Person.Password ADD CONSTRAINT AK_Password UNIQUE (PasswordHash, PasswordSalt); GO 在新表中创建唯一约束 在“对象资源管理器” 中,连接到 数据库引擎的实例。 在“标准”栏上,选择“新建查询”。 将以下示例复制并粘贴到查询窗口中,然后选择“执行”。 该示例创建一...
有关详细信息,请参阅 Unique 约束和 check 约束。 备注 若要查询现有的检查约束,请使用 sys.check_constraints 系统目录视图。 权限 需要具有表的 ALTER 权限。 使用SQL Server Management Studio 在“对象资源管理器”中,展开要为其添加检查约束的表,右键单击“约束”,然后选择“新建约束” 。 在“检查约束”对话...
SQL_ISV_COLUMNS = 标识可由给定用户访问的永久性表的列。 (FIPS 过渡级别)SQL_ISV_CONSTRAINT_COLUMN_USAGE = 类似于CONSTRAINT_TABLE_USAGE视图,为给定用户拥有的各种约束标识列。 (中级)SQL_ISV_CONSTRAINT_TABLE_USAGE = 标识约束(引用、唯一和断言)使用的表,并且由给定用户拥有。 (中级)SQL_ISV_DOMAIN_...
3.1:Creating a Unique Constraint 3.2:Using CHECK Constraints 1:Keys A primary keyis aspecial type of constraint, which identifies asingle column or set of columns, which in turn uniquely identifies all rows in the table. Constraintsplace limitations on the data that can be entered into a colum...
To create aCHECKconstraint on the "Age" column when the table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTERTABLEPersons ADDCHECK(Age>=18); To allow naming of aCHECKconstraint, and for defining aCHECKconstraint on multiple columns, use the follo...
CONSTRAINT constraint_name UNIQUE(column1, column2, . column_n) ); NOT NULL Constraint This constraint makes sure that no matter what, a column cannot have a NULL value. By default, columns can hold NULL values. A sample of using NOT NULL in modifying an existing tables rules, is below...