我无法在 mysql 中没有重复的情况下加入两个表

Question

I have two tables:

1. Members (member_id, member_gender)
2. Orders (order_id, member_id, order_amount) I have to retrieve data about the amount of members, amount of buyers (members with at least 1 order), amount of orders But unfortunately I have a hard time because when i try to join these two tables i recieve dublicates and I am unable to count distinctive members So my initial code was: SELECT count(m.member_id) AS Amount_of_members ,count(o.order_id) ,sum(o.order_amount) FROM tbl_member m LEFT JOIN tbl_order o ON m.member_id = o.member_id

Answer 1

You can ask the db to count only unique occurrences of the member id

SELECT count(DISTINCT m.member_id) AS Amount_of_members

You can also run a subquery to group the orders up so there is only one row per member before you join to the members table, which means the members data won't be doubled up if a member has 2 orders, tripled up if they have 3 etc

SELECT 
  count(m.member_id) AS Amount_of_members ,
  sum(x.count_orders) as total_count_of_orders,
  sum(x.sum_orders) as total_sum_of_orders
FROM 
  tbl_member m 
  LEFT JOIN (
    SELECT 
      o.member_id,
      count(o.order_id) as count_orders ,
      sum(o.order_amount) as sum_orders
    FROM
      tbl_order o
    GROUP BY o.member_id
 )x ON m.member_id = x.member_id

Generally the "squash the many side of a 1:M relationship down to one row before the join is done" is a helpful way to manage the data, especially if there are multiple joins that need to be made. Getting everything 1:1 means no duplicates pop up