MySQL外键的基础知识？

外键是一种可用于维护表之间一致性的约束类型。如果我们为表创建一个外键，则该表称为子表，而第二个表将称为父表。

在父表中，外键将充当主键。现在让我们创建一个表。

创建子表。

mysql> create table StudentEnrollment
   -> (
   -> StudentId int,
   -> StudentName varchar(200),
   -> StudentFKPK int
   -> );
Query OK，0 rows affected (0.91 sec)

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创建父表

mysql> create table College
   -> (
   -> StudentFKPK int,
   -> CourseId int,
   -> CourseName varchar(200),
   -> CollegeName varchar(200),
   -> primary key(StudentFKPK)
   -> );
Query OK，0 rows affected (0.46 sec)

在父表中，列’StudentFKPK’是主键。我们将使用ALTER命令添加外键。

以下是添加外键的语法。

ALTER table yourChildTableName add constraint anyConstraintName
foreign key(primary key column name for parent table) 
references College(primary key column name for parent table);

将上述语法实现在以下查询中。

mysql> alter table StudentEnrollment add constraint StudCollegeConst foreign key(StudentFKPK) references College(StudentFKPK);
Query OK，0 rows affected (1.78 sec)
Records: 0，Duplicates: 0，Warnings: 0

检查外键约束是否存在的语法。

SELECT  TABLE_NAME, COLUMN_NAME,CONSTRAINT_NAME,REFERENCED_TABLE_NAME,
REFERENCED_COLUMN_NA FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHER  REFERENCED_TABLE_NAME = 'yourParentTableName or your referencedTableName';

将上述语法实现在以下查询中。

mysql> SELECT
   ->   TABLE_NAME,
   ->   COLUMN_NAME,
   ->   CONSTRAINT_NAME,
   ->   REFERENCED_TABLE_NAME,
   ->   REFERENCED_COLUMN_NAME
   ->   FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
   ->   WHERE
   ->   REFERENCED_TABLE_NAME = 'College';

以下是输出结果。

+-------------------+-------------+------------------+-----------------------+------------------------+
| TABLE_NAME        | COLUMN_NAME | CONSTRAINT_NAME  | REFERENCED_TABLE_NAME | REFERENCED_COLUMN_NAME |
+-------------------+-------------+------------------+-----------------------+------------------------+
| StudentEnrollment | StudentFKPK | StudCollegeConst | College               | StudentFKPK            |
+-------------------+-------------+------------------+-----------------------+------------------------+
1 row in set, 2 warnings (0.03 sec)