information_schema.key_column_usage 表可以查看索引列上的约束；
1、information_schema.key_column_usage 的常用列：
1、constraint_catalog ：约束类型这个永远是def
2、constraint_schema ：约束所在的数据库名
3、constraint_name ：约束名
4、table_catalog ：表类型这个永远是def
5、table_schema ：表所在的数据库名
6、column_name ：索引所在的列名
7、referenced_table_schema ：被引用表所在的数据库名
8、referenced_table_name ：被引用的表名
9、referenced_column_name ：被引用表的列名
2、例子：
通过information_schema.key_column_usage表来查看外键引用关系
1、创建有主外键关系的表
create table teacher(
id int not null auto_increment,
name varchar(16) not null,
primary key pk_teacher(id)
) engine=innodb default char set utf8; create table student(
id int not null auto_increment,
name varchar(16) not null,
teacher_id int not null,
constraint pk_student__id primary key(id),
constraint fk_stuent__teacher_id foreign key(teacher_id) references teacher(id)
) engine=innodb default char set utf8;
2、查看索引列上的约束
select * from KEY_COLUMN_USAGE where table_schema='tempdb' \G
*************************** 1. row ***************************
CONSTRAINT_CATALOG: def
CONSTRAINT_SCHEMA: tempdb
CONSTRAINT_NAME: PRIMARY
TABLE_CATALOG: def
TABLE_SCHEMA: tempdb
TABLE_NAME: student
COLUMN_NAME: id
ORDINAL_POSITION: 1
POSITION_IN_UNIQUE_CONSTRAINT: NULL
REFERENCED_TABLE_SCHEMA: NULL
REFERENCED_TABLE_NAME: NULL
REFERENCED_COLUMN_NAME: NULL
*************************** 2. row ***************************
CONSTRAINT_CATALOG: def
CONSTRAINT_SCHEMA: tempdb
CONSTRAINT_NAME: fk_stuent__teacher_id
TABLE_CATALOG: def
TABLE_SCHEMA: tempdb
TABLE_NAME: student
COLUMN_NAME: teacher_id
ORDINAL_POSITION: 1
POSITION_IN_UNIQUE_CONSTRAINT: 1
REFERENCED_TABLE_SCHEMA: tempdb
REFERENCED_TABLE_NAME: teacher
REFERENCED_COLUMN_NAME: id
*************************** 3. row ***************************
CONSTRAINT_CATALOG: def
CONSTRAINT_SCHEMA: tempdb
CONSTRAINT_NAME: PRIMARY
TABLE_CATALOG: def
TABLE_SCHEMA: tempdb
TABLE_NAME: teacher
COLUMN_NAME: id
ORDINAL_POSITION: 1
POSITION_IN_UNIQUE_CONSTRAINT: NULL
REFERENCED_TABLE_SCHEMA: NULL
REFERENCED_TABLE_NAME: NULL
REFERENCED_COLUMN_NAME: NULL