而unique key 只是用来限制列元素不重复,并不一定用来快速检索,比如:我们想保证一个表中“身份证”字段的每一条记录不重复 or “电话号”字段每一条记录不重复,就可以用unique进行约束,而此时这样的列跟索引查找并没有关系。 foreign key:一个表中的 foreign key 指向另一个表中的 unique key (当然也可以是...
foreign key:一个表中的 foreign key 指向另一个表中的 unique key (当然也可以是primary key,而且一般就应该是primary key吧,毕竟primary key没null的值)。就是一张表中,foreign标识的列,其中每一个元素,都能对应到另一张表中unique标识的列元素。 index / key :两者一般可以划等号,同等看待。其功能:如果要...
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。若要确保验证了组合 FOREIGN KEY 约束的所有值,请将所有参与列指定为 NO...
MySQL 中 key, primary key ,unique key,index的区别 2015-06-15 11:01 −一、key与primary key区别 CREATE TABLE wh_logrecord ( logrecord_id int(11) NOT NULL auto_increment, user_name varchar(100) default NULL, operation_time date... ...
百度试题 结果1 题目SQL语言中实现候选码约束的语句是 ( ) A. 用Candidate Key 指定 B. 用Primary Key 指定 C. 用UNIQUE NOT NULL 约束指定 D. 用UNIQUE约束指定 相关知识点: 试题来源: 解析 C 反馈 收藏
1. A Key which uniquely identifies each row in the table is known as? Primary Key Unique Key Composite Key Foreign Key Answer:A) Primary Key Explanation: A Key which uniquely identifies each row in the table is known as Primary Key. ...
It is possible for a table to have multiplecandidate keys, which effectively behave similar to aprimary keyin that acandidate keyis unique,NOT NULL, and is a singular representation of that table record. However, when it comes to selecting which one attribute will be assigned as theprimary ke...
1.Unique keyin a tablecan benull, at least one but theprimary keycannot be nullin any table in a relational database like MySQL, Oracle, etc. 2. The unique key is represented using a unique constraint while a primary key is created using a primary key constraint in any table and it'...
There are three fundamental demands on the candidate key that we should never deviate from, if it is to become the subject for a primary key: The candidate key must be unique within its domain (the entity it represents, and beyond, if you also intend to access external entities, with is...
b) Unique Key c) Primary Key d) None of the Mentioned View Answer 6. A Key which is a set of one or more columns that can identify a record uniquely is called? a) Natural key b) Candidate key c) Not Null key d) Alternate key ...