the difference between primary key and unique key primary key:主键约束 unique key:唯一键约束 相同点:唯一,既值都不能重复 不同点: 1.一个表中只能有一个主键约束,但是唯一键约束可以有多个; 2.主键约束可以由一个或多个键组成,而唯一键约束只作用在一个键上; 3.主键作用的字段值不能为空,而唯一键作...
In relational database design, the concepts of Primary Key and Unique Key are crucial for ensuring data integrity and establishing the relationships between tables. Although they may seem similar, they have distinct roles and characteristics. This article will explain the differences between the Primary...
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'...
- 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...
Unique: unique it allows null value only once. Primary key: It is unique but it does't allow null value. 0 Feb, 2011 3 in simple words...primary key is always unique and not null.but unique key is only unique it can be null. 0 Oct, 2010 12 From Microsoft SQL serv...
What is the basic difference between primary key and (unique key along with Not NULL). If we define a unique key as not null then it also take a unique value as well as not null value that means one can not insert Null value in that column. Thanks Was this answer useful? Yes ...
Unique key can have nulls Primary key cannot have nulls. In a single table we can create multiple unique keys.In a single table we can have only one primary key. === Unqiue key creates a non-clustered index by default.Primary Key
• primary key (字段1,字段2...):联合主键索引 • unique(字段1,字段2..):联合唯一索引 • index(字段1,字段2..):联合普通索引 1. 2. 3. 应用场景 • 编号:int • 姓名:varchar(255) • 身份号:char(18) • 电话:char(11) ...
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Primary key is used as a unique record identity in a table, primary key usually also be indexed, to support quick search based on the column used as a primary key. A primary key does not depend on any other key, be it in the same table, or a different one. A foreign key ...