主键和外键的区别

首要的关键：
主键用于确保特定列中的数据是唯一的。它是一列不能有 NULL 值。它要么是现有的表列，要么是数据库根据定义的顺序专门生成的列。

例子：参考图——
STUD_NO 和 STUD_PHONE 都是关系 STUDENT 的候选键，但可以选择 STUD_NO 作为主键(只有一个候选键)。

外键：
外键是关系数据库表中的一列或一组列，它提供两个表中数据之间的链接。它是一列(或多列)引用另一个表的一列(通常是主键)。

例子：参考图——
STUDENT_COURSE 中的 STUD_NO 是 STUDENT 关系中 STUD_NO 的外键。

数字：

让我们看看主键和外键的区别：

S.NO.	PRIMARY KEY	FOREIGN KEY
1	A primary key is used to ensure data in the specific column is unique.	A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables.
2	It uniquely identifies a record in the relational database table.	It refers to the field in a table which is the primary key of another table.
3	Only one primary key is allowed in a table.	Whereas more than one foreign key are allowed in a table.
4	It is a combination of UNIQUE and Not Null constraints.	It can contain duplicate values and a table in a relational database.
5	It does not allow NULL values.	It can also contain NULL values.
6	Its value cannot be deleted from the parent table.	Its value can be deleted from the child table.
7	It constraint can be implicitly defined on the temporary tables.	It constraint cannot be defined on the local or global temporary tables.