如何从具有多个关联的两个 MySQL 表中检索数据

Question

我得到了这两个 table，其中一个 table 有多个指向第二个 table 的外键。

Table rankings

+----------------+--------------+------+-----+---------+----------------+
| Field          | Type         | Null | Key | Default | Extra          |
+----------------+--------------+------+-----+---------+----------------+
| id             | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| search_text    | varchar(255) | YES  | MUL | NULL    |                |
| first_item_id  | bigint(20)   | YES  | MUL | NULL    |                |
| second_item_id | bigint(20)   | YES  | MUL | NULL    |                |
| third_item_id  | bigint(20)   | YES  | MUL | NULL    |                |
| forth_item_id  | bigint(20)   | YES  | MUL | NULL    |                |
+----------------+--------------+------+-----+---------+----------------+

Table item

+---------------------------+--------------+------+-----+---------+----------------+
| Field                     | Type         | Null | Key | Default | Extra          |
+---------------------------+--------------+------+-----+---------+----------------+
| id                        | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| item_code                 | varchar(255) | YES  | MUL | NULL    |                |
+----------------+--------------+------+-----+---------+---------------------------+

一个 ranking 记录可能与 item table 有多个关联，使用 first_item_id、second_item_id、third_item_id 或 forth_item_id 字段。我想用相应的 item_code 而不是 item.id 检索 ranking 记录。如果我有大量数据，最有效的方法是什么？

PS：item.id 有 10 个关联，如 first_item_id ... tenth_item_id。我正在使用 Rails ActiveRecord ORM。任何解决方法也可以。

样本数据ranking

SELECT id,search_text,first_item_id as first,second_item_id as second,third_item_id as third,forth_item_id as forth from rankings limit 10;

+----+-------------+-------+--------+-------+-------+
| id | search_text | first | second | third | forth |
+----+-------------+-------+--------+-------+-------+
|  1 | test 1      |     1 |      2 |     3 |     4 |
|  2 | test 2      |     1 |      2 |     3 |     4 |
|  3 | test 3      |     1 |      2 |     3 |     4 |
|  4 | test 4      |     1 |      2 |     3 |     4 |
+----+-------------+-------+--------+-------+-------+

样本item数据

SELECT id,item_code from items limit 5;

+--------+------------+
| id     | item_code  |
+--------+------------+
|      1 | 125659     |
|      2 | 125660     |
|      3 | 125661     |
|      4 | 125662     |
+--------+------------+

预期数据

+----+-------------+-------+--------+-------+-------+
| id | search_text | first | second | third | forth |
+----+-------------+-------+--------+-------+-------+
|  1 | test 1      | 125659| 125660 | 125661| 125662|
|  2 | test 2      | 125659| 125660 | 125661| 125662|
|  3 | test 3      | 125659| 125660 | 125661| 125662|
|  4 | test 4      | 125659| 125660 | 125661| 125662|
+----+-------------+-------+--------+-------+-------+

Answer 1

加入 table 多次（甚至很多次）应该不是问题，因为您是在主键上加入，即您有一个将要使用的索引。

select 
  r.id,
  r.search_text,
  i1.item_code as item_code_1,
  i2.item_code as item_code_2,
  i3.item_code as item_code_3,
  i4.item_code as item_code_4
from rankings r
left join item i1 on i1.id = r.first_item_id
left join item i2 on i2.id = r.second_item_id
left join item i3 on i3.id = r.third_item_id
left join item i4 on i4.id = r.forth_item_id
order by r.id;

我在这里使用外连接，因为您的所有项目列都可以为空。

如何从具有多个关联的两个 MySQL 表中检索数据

How to retrieve data from two MySQL tables with multiple associations

mysql

sql

join

foreign-keys