本文介绍如何使用 SQL Server Management Studio 或 Transact-SQL 在 SQL Server 中创建外键关系。 当希望将一个表的行与另一个表的行相关联时,您可在这两个表之间创建关系。 权限 使用外键创建新表需要在数据库中具有CREATE TABLE权限,并对在其中创建表的架构具有ALTER SCHEMA权限。
1altertableTwoTable--修改某一张表2add3constraintPK_TwoTable_idprimarykey(id) ,4constraintUQ_TwoTable_nameunique(name),5constraintCK_TwoTable_agecheck(age>=0andage<=150),6constraintCK_TwoTable_gendercheck(gender='m'orgender='f'),7constraintDF_TwoTable_genderdefault('m')forgender,8constraint...
在非 NULL 的資料行上主索引鍵等同 UNIQUE 條件約束,而且 SQL Server 會實作 UNIQUE 條件約束來當作非叢集索引。 結合這些事實,以下範例在非 NULL 資料行 accountkey 上定義 UNIQUE 條件約束。 結果是一個將主索引鍵條件約束強制做為非 NULL 資料行上之 UNIQUE 條件約束的非叢集索引。 接下來,該資料表會轉換為...
1altertableTwoTable2drop3constraintFK_TwoTable_OneTable_fid,4constraintCF_TwoTable_gender; 修改表格结构 增加一列 语法: alter table 表名 add 列名 类型 [约束条件] 1alter table TableName add 列名 类型2--增加多列的时候,用逗号隔开,不要要多余的add3alter table TwoTable4add stuOneintnotnulldefault...
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....
-- 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...
UNIQUE constraints Constraints are rules that the SQL Server Database Engine enforces for you. For example, you can useUNIQUEconstraints to make sure that no duplicate values are entered in specific columns that don't participate in a primary key. Although both aUNIQUEconstraint and aPRIMARY KEY...
To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTERTABLEPersonsADDUNIQUE(P_Id) To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use ...
如果表变量是在 EXEC 语句或 sp_executesql 存储过程外部创建的,则不能使用 EXEC 语句或 sp_executesql 存储过程来运行引用表变量的动态 SQL Server 查询。 由于表变量只能在其本地范围内引用,因此 EXEC 语句和 sp_executesql 存储过程将超出表变量的范围。 但是,可以创建表变量并在 EXEC 语句或 sp_executesql...
Before getting started, let me briefly describe SQL Server unique indexes vs. unique constraints: A unique index ensures that the values in the index key columns are unique. A unique constraint also guarantees that no duplicate values can be inserted into the column(s) on which the c...