DBA技术分享-MySQL外键查询语句

一、概述

作为DBA分享几个工作中关于外键的常用查询。具体如下 ：

二、相关SQL

1、查询用户数据库（模式）中定义的外键约束

select concat(fks.constraint_schema, '.', fks.table_name) as foreign_table,
       '->' as rel,
       concat(fks.unique_constraint_schema, '.', fks.referenced_table_name)
              as primary_table,
       fks.constraint_name,
       group_concat(kcu.column_name
            order by position_in_unique_constraint separator ', ') 
             as fk_columns
from information_schema.referential_constraints fks
join information_schema.key_column_usage kcu
     on fks.constraint_schema = kcu.table_schema
     and fks.table_name = kcu.table_name
     and fks.constraint_name = kcu.constraint_name
-- where fks.constraint_schema = 'database name'
group by fks.constraint_schema,
         fks.table_name,
         fks.unique_constraint_schema,
         fks.referenced_table_name,
         fks.constraint_name
order by fks.constraint_schema,
         fks.table_name;

注意：如果您需要特定数据库（模式）的信息，请取消注释 where 子句并提供您的数据库名称。

2、查询所有引用具有外键的特定的表。

select distinct concat(table_schema, '.', table_name) as foreign_table,
       '>-' as rel,
       concat(referenced_table_schema, '.', referenced_table_name) 
       as primary_table
from information_schema.key_column_usage
where referenced_table_name = 'table name' -- provide table name here
--      and table_schema = 'database name'
order by foreign_table;

说明：

3、查询没有外键的表

select tab.table_schema as database_name,
       tab.table_name,
       '>- no FKs' as foreign_keys
from information_schema.tables tab
left join information_schema.table_constraints fks
          on fks.table_schema = tab.table_schema
          and fks.table_name = tab.table_name
          and fks.constraint_type = 'FOREIGN KEY'
where tab.table_type = 'BASE TABLE'
      and tab.table_schema not in ('mysql', 'information_schema',
                                   'performance_schema', 'sys')
      and fks.table_name is null
     -- and tab.table_schema = 'your database name'
order by tab.table_schema,
         tab.table_name;

说明：

4、查找没有关系的表 - Loner Tables

select 'No FKs >-' as refs,
       concat(tab.table_schema, '.', tab.table_name) as 'table',
       '>- no FKs' as fks
from information_schema.tables tab
left join information_schema.referential_constraints ref
          on tab.table_schema = ref.constraint_schema
          and tab.table_name = ref.table_name
left join information_schema.referential_constraints ref_by
          on tab.table_schema = ref_by.unique_constraint_schema
          and tab.table_name = ref_by.referenced_table_name
where ref.constraint_name is null
      and ref_by.constraint_name is null
      and tab.table_type = 'BASE TABLE'
      and tab.table_schema not in ('mysql', 'information_schema',
                                   'performance_schema', 'sys')
     -- and tab.table_schema = 'your database name'
order by tab.table_schema,
         tab.table_name;

说明：

5、MySQL 数据库中没有关系表的比率

select all_tables as table_count,
       no_rel as loner_tables,
       concat(cast(100.0*(no_rel/all_tables) as decimal(5,2)), '%')
              as loner_ratio
from 
    (select count(distinct concat(tab.table_schema, '.', tab.table_name)) 
            as all_tables,
            SUM(case when ref.constraint_name is null
                          and ref_by.constraint_name is null
                     then 1
                     else 0 end) as no_rel
    from information_schema.tables tab
    left join information_schema.referential_constraints ref
              on tab.table_schema = ref.constraint_schema
              and tab.table_name = ref.table_name
    left join information_schema.referential_constraints ref_by
              on tab.table_schema = ref_by.unique_constraint_schema
              and tab.table_name = ref_by.referenced_table_name
    where tab.table_type = 'BASE TABLE'
          and tab.table_schema not in ('mysql', 'information_schema',
                                       'sys', 'performance_schema')
    ) temp;

说明：