mysql，子查詢或按條件加入組

Question

+------------+------+-----------------------+ 
| invoice | amount | paid | date  | 
+------------+--------+--------+------------+ 
| 1001  | 55 | 1  | 2015-01-01 | 
+------------+--------+--------+------------+ 
| 1002  | 30 | 0  | 2015-02-01 | 
+------------+--------+--------+------------+ 
| 1003  | 59 | 1  | 2015-02-01 | 
+------------+--------+--------+------------+ 
| 1004  | 78 | 0  | 2015-03-01 | 
+------------+--------+--------+------------+ 
| 1005  | 65 | 1  | 2015-03-01 | 
+------------+--------+--------+------------+ 
| 1006  | 107 | 0  | 2015-04-01 | 
+------------+--------+--------+------------+ 

例如，如果我有上面的表， 我想獲取總髮票金額和付款金額。

總髮票金額

select sum(amount) from invoice group by DATE_FORMAT(date,"%Y-%m") 

來支付發票

select sum(amount) from invoice where paid=1 group by DATE_FORMAT(date,"%Y-%m") 

我怎樣才能結合成單一的查詢？

Answer 1

使用IF對paid = 1的行進行求和。

SELECT DATE_FORMAT(date, '%Y-%m') AS month, 
     SUM(amount) AS total, 
     SUM(IF(paid = 1, amount, 0)) AS total_paid 
FROM invoice 
GROUP BY month;