显示来自不同不相关表的两个总和

Question

I have 2 tables. One contains expenses, the other contains earnings. my_costs:

my_earnings:

From these two tables, I want to get data:

1. Grouping a period by month and year
2. The amount of expenses in the period
3. Average expenses per day in the period
4. Amount of earnings in the period

I am using the following query:

SELECT DATE_FORMAT(my_costs.DATE, "%M-%Y") AS Period, SUM(my_costs.sum) AS Costs,
ROUND(SUM(my_costs.sum)/DAY(LAST_DAY(my_costs.date)), 0) AS Average,

SUM(my_earnings.sum)

FROM my_costs

LEFT JOIN my_earnings ON DATE_FORMAT(my_costs.DATE, "%M-%Y") = DATE_FORMAT(my_earnings.date, "%M-%Y")

GROUP BY DATE_FORMAT(my_costs.DATE, "%M-%Y")
ORDER BY Period DESC

This request successfully gives me the period, costs and earnings. But the amount of earnings shown is incorrect. The numbers are much higher than expected, should be no more than 10,000 per month

Answer 1

Aggregating after joins can be inaccurate, because rows are duplicated or lost. I would recommend using union all and then aggregating:

SELECT DATE_FORMAT(ce.DATE, '%M-%M') AS Period,
       SUM(ce.costs) AS Costs, SUM(ce.Expenses) as expenses,
        ROUND(SUM(ce.sum)/DAY(LAST_DAY(ce.date)), 0) AS daily_Average
FROM ((SELECT date, sum as costs, 0 as expenses
       FROM my_costs
      ) UNION ALL
      (SELECT date, 0, sum
       FROM my_earnings
      )
     ) ce
GROUP BY period
ORDER BY MIN(ce.DATE) DESC;

Note that I changed the ORDER BY so the results are in time order.