SQL查询按月分组日期

Question

I have a database with 3 tables:

1. mothlist([number],[name])
2. temp_subs([sub_id],[date_created],[expiry_date])
3. temp_orders([order_id],[sub_id],[order_date])

I am trying to write a query to get the total counts expiry date and order for the current year grouped by month.

The query I have now is:

SELECT 
    monthlist.name 'Month',
    count(temp_subs.expiry_date) 'Expiry Date',
    count(temp_orders.order_date) 'Order Date'    
FROM 
    monthlist  
    FULL JOIN temp_subs 
    on 
        monthlist.number = datepart(month, temp_subs.expiry_date) and 
        datepart(year, temp_subs.expiry_date) = year(getdate())
    FULL JOIN temp_orders 
    on 
        monthlist.number = datepart(month, temp_orders.order_date) and 
        datepart(year, temp_orders.order_date) = year(getdate())
GROUP BY monthlist.number ,monthlist.name      
ORDER BY monthlist.number

If someone could tell me what am I dong wrong here I would be very grateful.

Answer 1

A double join means that every temp_sub is repeat for every temp_order. One way to avoid double-counting is adding a distinct on a unique column. If the tables have a primary key called id , that could look like:

SELECT monthlist.name 'Month'
  ,count(distinct temp_subs.id) 'Expiry Date'
  ,count(distinct temp_orders.id) 'Order Date'