多列和條件的左連接

Question

我有兩張桌子，一張用於訂單，一張用於產品

ORDERS TABLE 

+----------+-----------+--------+---------------------+----------+----------+---------------------+--------+
| order_id | order_by  | status | created_at          | product1 | product2 | delivery_date       | pickup |
+----------+-----------+--------+---------------------+----------+----------+---------------------+--------+
|        1 | Maria     |      0 | 2020-07-19 00:00:00 |        1 |        3 | 2020-07-17 00:00:00 |      0 |
|        2 | Joao      |      0 | 2020-07-20 00:00:00 |        2 |        0 | 2020-07-20 00:00:00 |      0 |
|        3 | Jose      |      1 | 2020-07-20 00:00:00 |        3 |        0 | 2020-07-20 00:00:00 |      0 |
|        4 | Matias    |      0 | 2020-07-19 23:50:40 |        1 |        3 | 2020-07-19 00:00:00 |      0 |
|        5 | Matias    |      0 | 2020-07-19 23:50:40 |        1 |        3 | 2020-07-19 00:00:00 |      0 |
|        6 | Joaozinho |      0 | 2020-07-19 00:00:00 |        1 |        3 | 2020-07-22 00:00:00 |      0 |
+----------+-----------+--------+---------------------+----------+----------+---------------------+--------+


PRODUCTS TABLE

+----+-----------+-------+
| id | name      | price |
+----+-----------+-------+
|  1 | Produto 1 |    11 |
|  2 | Produto 2 |    22 |
|  3 | Produto 3 |    33 |
|  4 | Produto 4 |    44 |
|  5 | Produto 5 |    55 |
+----+-----------+-------+


EXPECTED OUTPUT

+----------+-----------+--------+---------------------+----------+----------+---------------------+--------+-----------+----------+-----------+----------+
| order_id | order_by  | status | created_at          | product1 | product2 | delivery_date       | pickup | p1_name   | p1_price | p2_name   | p2_price |
+----------+-----------+--------+---------------------+----------+----------+---------------------+--------+-----------+----------+-----------+----------+
|        1 | Maria     |      0 | 2020-07-19 00:00:00 |        1 |        3 | 2020-07-17 00:00:00 |      0 | Produto 1 |       11 | Produto 3 |      33  |
|        2 | Joao      |      0 | 2020-07-20 00:00:00 |        2 |        0 | 2020-07-20 00:00:00 |      0 | Produto 2 |       22 |   null    |     null |
|        3 | Jose      |      1 | 2020-07-20 00:00:00 |        3 |        0 | 2020-07-20 00:00:00 |      0 | Produto 3 |       33 |   null    |     null |
|        4 | Matias    |      0 | 2020-07-19 23:50:40 |        1 |        3 | 2020-07-19 00:00:00 |      0 | Produto 1 |       11 | Produto 3 |       33 |
|        5 | Matias    |      0 | 2020-07-19 23:50:40 |        1 |        3 | 2020-07-19 00:00:00 |      0 | Produto 1 |       11 | Produto 3 |       33 |
|        6 | Joaozinho |      0 | 2020-07-19 00:00:00 |        1 |        3 | 2020-07-22 00:00:00 |      0 | Produto 1 |       11 | Produto 3 |       22 |
+----------+-----------+--------+---------------------+----------+----------+---------------------+--------+------+-----------+-------+------------------+

我需要在訂單表中的每個產品（product1、product2..）和具有不同名稱（product1_name、product2_name..）的價格列上左連接

Answer 1

select o.*, p1.name, p1.price
from orders o
inner join products p1 on p1.id=o.product1

添加 product2 應該不是那么難，並且留作練習......

Answer 2

您需要兩個join products表：

select
    o.*,
    p1.name  p1_name,
    p1.price p1_price,
    p2.name  p2_name,
    p2.price p2_price
from orders o
left join products p1 on p1.id = o.product1
left join products p2 on p2.id = o.product2

如果product1始終可用，您可以將第一個left join轉換為inner join ，這會使查詢稍微更有效。