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SQL 约束

• NOT NULL

• UNIQUE

• PRIMARY KEY

• FOREIGN KEY

• CHECK

• DEFAULT

        UNIQUE 和 PRIMARY KEY 约束均为列或列集合提供了唯一性的保证。

        区别：每个表可以有多个 UNIQUE 约束，但是每个表只能有一个 PRIMARY KEY 约束。

 unique  

sql  ?unique ?  Constraint   on   create   table（表未被创建）

MySql  

            CREATE TABLE Persons

            (

                Id_P int NOT NULL,

                LastName varchar(255) NOT NULL,

                FirstName varchar(255),

                Address varchar(255),

                City varchar(255),

                UNIQUE (Id_P)

            )

SQL Server / Oracle / MS Access:

            CREATE TABLE Persons

            (

                Id_P int NOT NULL UNIQUE,

                LastName varchar(255) NOT NULL,

                FirstName varchar(255),

                Address varchar(255),

                City varchar(255)

            )

MySQL / SQL Server / Oracle / MS Access（多个列定义 UNIQUE 约束）:

            CREATE TABLE Persons

            (

                Id_P int NOT NULL,

                LastName varchar(255) NOT NULL,

                FirstName varchar(255),

                Address varchar(255),

                City varchar(255),

                CONSTRAINT   uc_PersonID   UNIQUE (Id_P,LastName)

            )

表已被创建

             alter  table  Persons   add   unique(Id_P)

             alter  table  Persons   add   constraint   uc_PersonID   UNIQUE (Id_P,LastName) 

撤销 UNIQUE 约束

            MySql

                    alter   table  Persons   drop   index  uc_PersonID

          SQL Server / Oracle / MS Access:

                    alter    table  Persons   drop   constraint     uc_PersonID         

PRIMARY KEY            

        MySQL:

                CREATE TABLE Persons

                    (

                        Id_P int NOT NULL,

                        LastName varchar(255) NOT NULL,

                        FirstName varchar(255),

                        Address varchar(255),

                        City varchar(255),

                        PRIMARY KEY (Id_P)

                    )

        SQL Server / Oracle / MS Access:

                CREATE TABLE Persons

                (

                Id_P int NOT NULL PRIMARY KEY,

                LastName varchar(255) NOT NULL,

                FirstName varchar(255),

                Address varchar(255),

                City varchar(255)

                )

        MySQL / SQL Server / Oracle / MS Access(为多个列定义 PRIMARY KEY 约束):

                CREATE TABLE Persons

                (

                  Id_P int NOT NULL,

                  LastName varchar(255) NOT NULL,

                  FirstName varchar(255),

                  Address varchar(255),

                 City varchar(255),

                 CONSTRAINT pk_PersonID PRIMARY KEY (Id_P,LastName)

                )

表已被创建

    MySQL / SQL Server / Oracle / MS Access:

            ALTER   TABLE    Persons    ADD    PRIMARY   KEY (Id_P)

为多个列定义 PRIMARY KEY 约束 

        MySQL / SQL Server / Oracle / MS Access:

            ALTER    TABLE    Persons    ADD CONSTRAINT pk_PersonID PRIMARY KEY (Id_P,LastName)

撤销 PRIMARY KEY 约束

            MySQL:

            ALTER     TABLE     Persons     DROP  PRIMARY     KEY

            SQL Server / Oracle / MS Access:

            ALTER     TABLE     Persons      DROP     CONSTRAINT     pk_PersonID

FOREIGN KEY 约束   

        FOREIGN KEY 约束用于预防破坏表之间连接的动作。

        FOREIGN KEY 约束也能防止非法数据插入外键列，因为它必须是它指向的那个表中的值之一。

SQL  FOREIGN  KEY  Constraint  on  CREATE  TABLE

        MySQL:

            CREATE TABLE Orders

            (

                Id_O int NOT NULL,

                OrderNo int NOT NULL,

                Id_P int,

                primary   key (Id_O),

                foreign  key (Id_P) references Persons(Id_P)

            )

        SQL Server / Oracle / MS Access:

        CREATE TABLE Orders

        (

            Id_O int NOT NULL primary key,

            OrderNo int NOT NULL,

            Id_P int  foreign   key   references Persons(Id_P)

        )

为多个列定义 FOREIGN KEY 约束:

MySQL / SQL Server / Oracle / MS Access:

        CREATE TABLE Orders

        (

            Id_O int NOT NULL,

            OrderNo int NOT NULL,

            Id_P int,

            PRIMARY KEY (Id_O),

            constraint    fk_PerOrders   foreign key (Id_P)

            references    Persons(Id_P)

        )

表已存在

MySQL / SQL Server / Oracle / MS Access:

        ALTER TABLE Orders

        ADD foreign KEY (Id_P)

        references Persons(Id_P)

为多个列定义 FOREIGN KEY 约束

      MySQL / SQL Server / Oracle / MS Access:

        ALTER   TABLE   Orders

        Add    constraint     fk_PerOrders

        Foreign  Key (Id_P)

        References   Persons(Id_P)

撤销 FOREIGN KEY 约束

        MySQL:

        ALTER  TABLE  Orders

        DROP   foreign   key  fk_PerOrders

        SQL Server / Oracle / MS Access:

        ALTER  TABLE  Orders

        DROP  constraint    fk_PerOrders

CHECK 约束

                (CHECK 约束用于限制列中的值的范围。)

        如果对单个列定义 CHECK 约束，那么该列只允许特定的值。

        如果对一个表定义 CHECK 约束，那么此约束会在特定的列中对值进行限制。

SQL CHECK Constraint on CREATE TABLE

My SQL:

    CREATE TABLE Persons

    (

    Id_P int NOT NULL,

    LastName varchar(255) NOT NULL,

    FirstName varchar(255),

    Address varchar(255),

    City varchar(255),

    check (Id_P>0)

    )

SQL Server / Oracle / MS Access:

    CREATE TABLE Persons

    (

    Id_P int NOT NULL  check (Id_P>0),

    LastName varchar(255) NOT NULL,

    FirstName varchar(255),

    Address varchar(255),

    City varchar(255)

    )

为多个列定义 CHECK 约束:

   MySQL / SQL Server / Oracle / MS Access:

        create  table   Persons

    (

        Id_P int NOT NULL,

        LastName varchar(255) NOT NULL,

        FirstName varchar(255),

        Address varchar(255),

        City varchar(255),

        constraint   chk_Person check (Id_P>0  and  City=‘Sandnes‘)

    )

表已存在

MySQL / SQL Server / Oracle / MS Access:

    alter  table  Persons

    Add  check  (Id_P>0)

为多个列定义 CHECK 约束

MySQL / SQL Server / Oracle / MS Access:

    alter   table  Persons

    ADD   constraint   chk_Person CHECK (Id_P>0 AND City=‘Sandnes‘)

撤销 CHECK 约束

SQL Server / Oracle / MS Access:

        alter   table   Persons

        drop   constraint    chk_Person

MySQL:

        alter   table   Persons

        drop   check   chk_Person

SQL DEFAULT 约束

        DEFAULT 约束用于向列中插入默认值。

        如果没有规定其他的值，那么会将默认值添加到所有的新记录。

SQL DEFAULT Constraint on CREATE TABLE

My SQL / SQL Server / Oracle / MS Access:

        CREATE TABLE Persons

        (

        Id_P int NOT NULL,

        LastName varchar(255) NOT NULL,

        FirstName varchar(255),

        Address varchar(255),

        City varchar(255) DEFAULT ‘Sandnes‘

        )

        通过使用类似 GETDATE() 这样的函数，DEFAULT 约束也可以用于插入系统值：

    CREATE TABLE Orders

    (

        Id_O int NOT NULL,

        OrderNo int NOT NULL,

        Id_P int,

        OrderDate date DEFAULT GETDATE()

    )

表已存在

MySQL:

        alter  table   Persons

        alter  City   set   default ‘sandes‘

SQL Server / Oracle / MS Access:

        alter  table Persons

        alter   column   City   set   default  ‘sandnes‘

撤销 DEFAULT 约束

MySQL:

        alter   table  Persons

        alter  City drop default

SQL Server / Oracle / MS Access:

        alter   table  Persons

        alter  column city  drop  default

sql 高级 （四）（SQL 约束 (Constraints)）

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原文地址：http://my.oschina.net/u/2356966/blog/487772