以下是SQL中几种常见的约束(CONSTRAINT): PRIMARY KEY: 这是一个或多个字段的组合,它唯一标识表中的每一行。在一个表中只能有一个主键约束,并且主键字段的值不能为NULL。 FOREIGN KEY: 它是一个或多个字段的组合,它引用另一个表的主键。外键约束用于确保引用的数据存在,并维护数据之间的引用完整性。 UNIQUE:...
In SQL, we will get an error If we try to insertNULLor duplicate values in the primary key column. NOT NULL Constraint Error We get this error when we supply the primary key with aNULLvalue. This is because primary keys need to obey theNOT NULLconstraint. For example, -- NOT NULL Co...
CONSTRAINTPK_PersonPRIMARYKEY(ID,LastName) ); Note:In the example above there is only ONEPRIMARY KEY(PK_Person). However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName). SQL PRIMARY KEY on ALTER TABLE
Constraints play a crucial role in maintaining the integrity and consistency of data in a database. For example, the PRIMARY KEY and FOREIGN KEY constraints prevent duplicate and NULL values in certain columns, ensuring that relationships between tables remain consistent. The NOT NULL and CHECK cons...
SQL支持域的概念,并可以用CREATE DOMAIN语句建立一个域以及该域应该满足的完整性约束条件。 [例14]建立一个性别域,并声明性别域的取值范围 CREATE DOMAIN GenderDomain CHAR(2) CHECK (VALUE IN ('男','女') ); 这样[例10]中对Ssex的说明可以改写为Ssex GenderDomain [例15]建立一个性别域GenderDomain,并...
);--建表时创建约束,没有指定约束名,则系统将自动命名约束名SQL>CREATETABLEtb_constraint_1 ( empnoNUMBERPRIMARYKEY,--主键约束enameVARCHAR2(20)NOTNULL,--非空约束emailVARCHAR2(60)UNIQUE,--唯一约束salNUMBER(5)CHECK(sal>1500),--核查约束deptnoNUMBER(4)REFERENCEStb_dept(deptno)--外键约束);--建表...
SQL複製 -- Create a table with a primary key>CREATETABLEpersons(first_nameSTRINGNOTNULL, last_nameSTRINGNOTNULL, nicknameSTRING,CONSTRAINTpersons_pk PRIMARYKEY(first_name, last_name));-- create a table with a foreign key>CREATETABLEpets(nameSTRING, owner_first_nameSTRING, ow...
而且指定索引文件位于主文件组on [primary] IGNORE_DUP_KEY 的意思是当往包含一个唯一约束中的列插入重复数据时SqlServer所作的反应,当选择此项时候SqlServer返回一个错误信息,跳过此行数据的插入,继续执行下面的数据插入操作,没有此项时候不仅会返回一个错误信息还将回滚这个insert语句。你上面的代码的意思建立一个名...
命名空間:Microsoft.Data.Schema.Sql.SchemaModel 組件:Microsoft.Data.Schema.Sql (在 Microsoft.Data.Schema.Sql.dll 中) 語法 C#複製 publicinterfaceISqlPrimaryKeyConstraint:ISqlConstraint,ISqlExtendedPropertyHost,ISqlSpecifiesIndex,ISqlSpecifiesStorage,ISqlModelElement,IScriptSourcedModelElement,IModelElement,IM...
Primary keys must contain UNIQUE values, and cannot contain NULL values.A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).PRIMARY KEY on CREATE TABLEThe following SQL creates a PRIMARY KEY on the "ID" column when...