1、primary key vs foreign key:主键用于唯一标识表中的每一行记录,不能包含NULL值。外键用于建立表与表之间的关系,可以包含NULL值。 2、unique vs foreign key:唯一约束确保表中的某一列或多列的值唯一,不能重复。外键用于建立表与表之间的关系,引用另一表中的主键。 3、check vs foreign key:检查约束用于确保...
Foreign Key Constraints Primary Key vs Foreign Key – Summarization Primary Keys In order for a table to qualify as a relational table, it must have a primary key. The primary key consists of one or more columns whose data contained within are used to uniquely identify each row in the table...
Re: JOIN vs. Foreign/Primary key Thomas Bloem July 28, 2008 07:44AM Re: JOIN vs. Foreign/Primary key Huu Da Tran July 28, 2008 07:49AM Re: JOIN vs. Foreign/Primary key Thomas Bloem July 28, 2008 08:04AM Re: JOIN vs. Foreign/Primary key ...
创建user 表: CREATETABLEuser(idINTNOTNULLAUTO_INCREMENT,usernameVARCHAR(30)NOTNULL,passwordVARCHAR(20)NOTNULL,emailVARCHAR(30)NOTNULL,cellphoneCHAR(11),ipCHAR(15)NOTNULL,PRIMARYKEY(id)); 创建order 表: CREATETABLE`order`(oidINTNOTNULLAUTO_INCREMENT,uidINTreferencesuser(id),order_noCHAR(11)NOTNUL...
官方帮助文档是这样说明的“FOREIGN KEY 约束并不仅仅可以与另一表的 PRIMARY KEY 约束相链接,它还可以定义为引用另一表的 UNIQUE 约束。FOREIGN KEY 约束可以包含空值,但是,如果任何组合 FOREIGN KEY 约束的列包含空值,则将跳过组成 FOREIGN KEY 约束的所有值的验证。” ...
A foreign key constraint works in conjunction with primary key or unique constraints to enforce referential integrity among specified tables. For example, you can place a foreign key constraint on the title_id column in the publishers table to ensure that a value entered in that column matches ...
Can a column act as both primary key and foreign key Can a web service return an HTML formatted file? Can I comment on web.config? Can I convert Windows Form to Web Form? can i do div display:none; from c# code behind? can I get all public variables in a class? can I get value...
Figure 2. The foreign keys in the Orders table correspond to primary keys in the Customers table, which is the parent table. The primary key in the Customers table is CustomerNo, and the primary key in the Orders table is OrderNo. The primary key uniquely identifies the table and contains...
https://www.mssqltips.com/sqlservertip/5431/surrogate-key-vs-natural-key-differences-and-when-to-use-in-sql-server/ Rule #1: Never, repeat NEVER, create a PK on a value the user could ever change. If you do this, and the value changes, you must rekey every FK to the new value. ...
对FOREIGN KEY 约束建立索引 由于以下原因,对外键创建索引通常是有用的: 对PRIMARY KEY 约束的更改可由相关表中的 FOREIGN KEY 约束检查。 当在查询中组合相关表中的数据时,经常在联接条件中使用外键列,方法是将一个表的 FOREIGN KEY 约束中的列与另一个表中的主键列或唯一键列匹配。索引使 数据库引擎可以在外...