(1)在字段级以key方式建立, 如 create table t (id int not null primary key); (2)在表级以constraint方式建立,如create table t(id int, CONSTRAINT pk_t_id PRIMARY key (id)); (3)在表级以key方式建立,如create table t(id int, primary key (id)); 其它key创建类似,但不管那种方式,既建立了...
•primary key 有两个作用,一是约束作用(constraint),用来规范一个存储主键和唯一性,但同时也在此key上建立了一个index; •unique key 也有两个作用,一是约束作用(constraint),规范数据的唯一性,但同时也在这个key上建立了一个index; •foreign key也有两个作用,一是约束作用(constraint),规范数据的引用完整...
主键约束(primarykeyConstraints)的含义是,要求表中在指定的列上没有两行具有相同的值,且不允许为空值。()A.正确B.错误
美 英 un.主关键字;主码 网络主键;主表的主键信息;主键约束 英汉 网络释义 un. 1. 主关键字 2. 主码 例句 更多例句筛选
Primary Key The primary key is a column or collection of columns that uniquely identify each row in the table. ThePrimaryKeyproperty accesses one or moreDataColumnobjects that define the primary key of theDataTable. The primary key acts both as a unique constraint for the table and allows ...
This column, or columns, is called the primary key (PK) of the table and enforces the entity integrity of the table. You can create a primary key by defining a PRIMARY KEY constraint when you create or modify a table. A table can have only one PRIMARY KEY constraint, and a column ...
KEY KEY 是数据库的物理结构,包含两层含义,一是约束,偏重于约束和规范数据库的结构完整性,二是索引,辅助查询。 •primary key 有两个作用,一是约束作用(constraint),用来规范一个存储主键和唯一性,但同时也在此key上建立了一个index; •unique key 也有两个作用,一是约束作用(constraint),规范数据的唯一性...
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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). PRIMARY KEY on ALTER TABLE To create aPRIMARY KEYconstraint on the "ID" column when the...
You can change the primary key for the table that you are creating. A primary key is a column or set of columns that can be used to identify or access a particular row or rows. A primary key is a unique key, that is, it is a key that is constrained so that no two of its valu...