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 ...
Multiple unique nonclustered indexes can be defined on a table. For more information, see CREATE INDEX (Transact-SQL). Indexed view To create an indexed view, a unique clustered index is defined on one or more view columns. The view is executed and the result set is stored in the leaf ...
You cannot create a unique index on a single column if that column contains NULL in more than one row. Similarly, you cannot create a unique index on multiple columns if the combination of columns contains NULL in more than one row. These are treated as duplicate values for indexing purposes...
(FirstName, LastName); -- 唯一聚集索引 -- CREATE UNIQUE CLUSTERED INDEX IX_EmployeeID ON Employees(EmployeeID); -- 删除索引 DROP INDEX IX_LastName ON Employees; -- 唯一约束 ALTER TABLE Employees ADD CONSTRAINT UQ_Email UNIQUE (Email); -- 检查约束 -- 限制工资必须大于 0 ALTER TABLE ...
-- add unique constraint to an existing columnALTERTABLECollegesADDUNIQUE(college_id); Here, the SQL command adds theUNIQUEconstraint to thecolleges_idcolumn in the existingCollegestable. For Multiple Columns -- add unique constraint to multiple columnsALTERTABLECollegesADDUNIQUEUnique_College (college...
ALTER TABLE Persons ADD UNIQUE (ID); To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:MySQL / SQL Server / Oracle / MS Access:ALTER TABLE Persons ADD CONSTRAINT UC_Person UNIQUE (ID,LastName); ...
CREATE INDEX index1 ON schema1.table1 (column1); 在表上创建聚集索引,并为表使用由 3 个部分组成的名称 SQL 复制 CREATE CLUSTERED INDEX index1 ON database1.schema1.table1 (column1); 使用唯一约束创建非聚集索引并指定排序顺序 SQL 复制 CREATE UNIQUE INDEX index1 ON schema1.table1 (column...
Cityvarchar(255),UNIQUE(P_Id) ) 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: CREATETABLEPersons ( P_IdintNOTNULL, ...
UNIQUE: 建立唯一索引。 CLUSTERED: 建立聚集索引。 NONCLUSTERED: 建立非聚集索引。 Index_property: 索引属性。 UNIQUE索引既可以采用聚集索引结构,也可以采用非聚集索引的结构,如果不指明采用的索引结构,则SQL Server系统默认为采用非聚集索引结构。 1.42 删除索引语法: ...
Multiple unique nonclustered indexes can be defined on a table. For more information, see CREATE INDEX (Transact-SQL). Indexed view To create an indexed view, a unique clustered index is defined on one or more view columns. The view is executed and the result set is stored in the leaf ...