Question

我有3张桌子

orders
-----------------
| id | order_data |
-----------------

codes
-----------------------------
| id | code_data | order_id |
-----------------------------

actions (please note, in the table the primary key doesn't exists)
--------------------------
| action_name | order_id |
--------------------------

如果操作行在表actions中没有唯一值，那么我可以按顺序计算操作数，因此DISTINCT中的COUNT()没有意义。我的问题：

SELECT `orders`.*, COUNT(DISTINCT `codes`.`id`), COUNT(`actions`.`action_name`) 
FROM `orders`
LEFT JOIN `codes` ON `codes`.`id` = `orders`.`id`
LEFT JOIN `actions` ON `actions`.`id` = `orders`.`id`
WHERE `actions`.`action_name` IN (/*some values*/)
GROUP BY `orders`.`id`

Answer 1

SELECT orders.*
     , COUNT(DISTINCT c.id)
     , COUNT(a.action_name) 
  FROM orders o
  LEFT 
  JOIN codes c
    ON c.id = o.id
  LEFT 
  JOIN actions a
    ON a.id = o.id
   AND a.action_name IN (/*some values*/)
 GROUP 
    BY o.id

Answer 2

在加入之前计算其他表格，如下所示：

SELECT
       `orders`.*
     , IFNULL(a.CountActs, 0) CountActs
     , IFNULL(c.CountCodes, 0) CountCodes
FROM `orders`
LEFT JOIN (
             SELECT `order_id`, count(*) CountCodes
             FROM `codes`
             GROUP BY `order_id`
            ) c ON `orders`.`id` = c.`order_id`
LEFT JOIN (
             SELECT `order_id`, count(*) CountActs
             FROM `actions`
             WHERE `actions`.`action_name` IN (/*some values*/)
             GROUP BY `order_id`
            ) a ON `orders`.`id` = a.`order_id`

在多LEFT JOIN中没有DISTINCT的COUNT（）

2 个答案: