首先primary key跟unique都是Constraints,属于logical object,而index则是physical object,会占用index page并被保存在磁盘上。 Primary key Constraints和unique Constraints都需要保证列是唯一的,不能有重复值,但是一张表只能有一个Primary key Constraints,但是可以有多个unique Constraints。 主键约束比唯一索引约束严格,当...
- A primary key can’t be created on a null able column wherein a unique key can be created on a null able column. - There can be only one primary key on a table wherein a table can have multiple unique key defined on it. Let’s create a table with a primary key and a unique...
主键（sno,cno）为非聚集索引，唯一键（sno）为聚集索引。clustered 和nonclustered 指定为 PRIMARY KEY 或 UNIQUE 约束创建聚集或非聚集索引。PRIMARY KEY 约束默认为 CLUSTERED；UNIQUE 约束默认为 NONCLUSTERED。聚集索引的区别 聚集索引:物理存储按照索引排序 非聚集索引:物理存储不按照索引排序 优势与缺点...
答案:SQL是一种所见既所得到的语言,在执行SQL语句中如果写好了SQL语句,就能按照需求写出或者查询出相应的数据信息,例如INSERT、DELETE、UPDATE决定了去做什么,一定会实现SQL的需求,这种特性在交叉平台和交叉语言开发环境中占有非常重要的优势。 2. 下列语句所返回的结果是否相同? SELECT * FROM CHECKS; select * fro...
id int not null primary key auto_increment, title varchar(16) not null )default charset=utf8; create table info( id int not null primary key auto_increment, name varchar(16) not null, email varchar(32) not null, age int, depart_id int ...
Order和Taco表的定义中将id identity替换为id bigint generated by default as identity primary key，还...
百度试题 题目SQL数据定义语言中,表示外键约束的关键字是()。 A.CHECKB.FOREIGN KEYC.PRIMARY KEYD.UNIQUE相关知识点: 试题来源: 解析 B 反馈 收藏
SQL NOT NULL UNIQUE PRIMARY KEY FOREIGN KEY CHECK DEFAULT In this article, we will go through the first three constraints; SQL NOT NULL, UNIQUE and SQL PRIMARY KEY, and we will complete the rest three constraints in the next article. Let us start discussing each one of these SQL...
SQL Server: CREATE TABLE Customer (SID integer PRIMARY KEY, Last_Name nvarchar(30), First_Name nvarchar(30)); In the examples above, SID column is specified as the primary key. Below is an example of specifying the primary key on a single column usingALTER TABLE. The same SQL statement ...
Unique indexes are implemented in the following ways:PRIMARY KEY or UNIQUE constraint When you create a PRIMARY KEY constraint, a unique clustered index on the column or columns is automatically created if a clustered index on the table does not already exist and you do not specify a unique ...