具有可充当唯一行标识符的一列或多列的表以及未通过 PRIMARY KEY 约束创建的表将空的结果集返回到SQLPrimaryKeys。ODBC 函数SQLSpecialColumns为没有主键的表报告行标识符候选项。 无论对于 CatalogName、SchemaName 或 TableName 参数是否存在值,SQLPrimaryKeys都将返回 SQL_SUCCESS。SQLFetch在无效值用于这些参数中时...
在创建或修改表时,您可以通过定义 PRIMARY KEY 约束来创建主键。 一个表只能有一个 PRIMARY KEY 约束,并且 PRIMARY KEY 约束中的列不能接受空值。由于 PRIMARY KEY 约束可保证数据的唯一性,因此经常对标识列定义这种约束。 如果为表指定了 PRIMARY KEY 约束,则 数据库引擎将通过为主键列创建唯一索引来强制数据的...
You can define a primary key in the SQL Server Database Engine by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique clustered index. However, your primary key can be specified as a nonclustered index instead....
如果另一个表中的 FOREIGN KEY 约束引用了 PRIMARY KEY 约束,则必须先删除 FOREIGN KEY 约束。 表包含应用于自身的 PRIMARY XML 索引。 在创建表时创建 PRIMARY KEY 约束 CREATE TABLE (Transact-SQL) 为现有表创建或删除 PRIMARY KEY 约束 ALTER TABLE (Transact-SQL) ...
Create a primary key in a new table The following example creates a table and defines a primary key on the columnTransactionIDin theAdventureWorks2022database. SQL CREATETABLE[Production].[TransactionHistoryArchive1] ( TransactionIDINTIDENTITY(1,1)NOTNULL,CONSTRAINTPK_TransactionHistoryArchive1_Transact...
mysql的key和index多少有点令人迷惑,单独的key和其它关键词结合的key(primary key)实际表示的意义是不同,这实际上考察对数据库体系结构的了解的。 1 :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),
PRIMARY KEY (ID) ); SQL Server / Oracle / MS Access:CREATE TABLE Persons( ID int NOT NULL PRIMARY KEY, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int ); 要允许主键约束的命名,并在多列上定义主键约束,请使用以下SQL语法:MySQL...
You can delete (drop) a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. When the primary key is deleted, the corresponding index is deleted. This may be the clustered index of the table, causing the table to become a heap. For more information, seeHeaps...
Using Transact-SQL Applies to: SQL Server 2016 (13.x) and later Azure SQL Database Azure SQL Managed Instance You can modify a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. You can modify the primary key of a table by changing the column order, index...
Primary keys and foreign keys are two types of constraints that can be used to enforce data integrity in SQL Server tables. These are important database objects. Primary key constraints A table typically has a column or combination of columns that contain values that uniquely identify each row ...