我有两个表，分别是“ db_product”和“ db_trans_detail”。
db_product
+---------+-------------+---------------+--------+
| number | product_id | product_name | price |
+---------+-------------+---------------+--------+
| 1 | prod-01 | aaaaaa | 20 |
| 2 | prod-02 | bbbbbb | 30 |
| 3 | prod-03 | cccccc | 45 |
| 4 | prod-04 | dddddd | 10 |
| 5 | prod-05 | eeeeee | 25 |
+---------+-------------+---------------+--------+
db_trans_detail
+----------------+-------------+------+
| trans_detail | product_id | qty |
+----------------+-------------+------+
| tr-0001 | prod-04 | 3 |
| tr-0001 | prod-02 | 1 |
| tr-0002 | prod-03 | 2 |
| tr-0003 | prod-04 | 1 |
| tr-0004 | prod-01 | 1 |
| tr-0005 | prod-05 | 1 |
+----------------+-------------+------+
我试图实现的是一个显示“ product_id”，“ product_id”和“ total_qty”列的表。我使用
过滤了每个表SELECT product_id, COUNT(qty) AS total_qty FROM db_trans_detail
GROUP BY product_id;
和
SELECT product_id, product_name FROM db_product;
他们工作正常。
当我尝试使用UNION模拟FULL JOIN时，缺少“ product_name”列。
SELECT a.product_id, COUNT(a.qty) AS total_qty FROM db_trans_detail a
LEFT JOIN (SELECT product_id, product_name FROM db_product) b
ON a.product_id = b.product_id
GROUP BY product_id
UNION
SELECT a.product_id, COUNT(a.qty) AS total_qty FROM db_trans_detail a
RIGHT JOIN (SELECT product_id, product_name FROM db_product) b
ON a.product_id = b.product_id
GROUP BY product_id
我得到的输出
+-------------+------------+
| product_id | total_qty |
+-------------+------------+
| prod-01 | 1 |
| prod-02 | 1 |
| prod-03 | 2 |
| prod-04 | 5 |
| prod-05 | 1 |
| NULL | 0 |
+-------------+------------+
虽然我希望是
+-------------+---------------+---------+
| product_id | product_name |total_qty|
+-------------+---------------+---------+
| prod-01 | aaaaaa | 1 |
| prod-02 | bbbbbb | 1 |
| prod-03 | cccccc | 2 |
| prod-04 | dddddd | 5 |
| prod-05 | eeeeee | 1 |
+-------------+---------------+---------+
感谢您的帮助。