查詢合併兩個結果取決於條件

Question

Select SUM(Breakfast) FROM `expense` WHERE MONTHNAME(Date)='February' 

Select (BreakFast+lunch+dinner) as Total FROM `expense` WHERE Date='2016-01-01' 

合併結果取決於條件。

1. SUM(Breakfast)如果是2月份，應該選擇。

如果不是意味着

(BreakFast+lunch+dinner) as Total必須加油特定日期。

我嘗試了聯盟，但我沒有得到期望的結果

Table expense 
-------------- 
Date  Breakfast Lunch Dinner 
2016-02-02 20 20 30 
2016-02-03 30 30 20 
2016-01-02 40 30 20      90(If It is a month Of Not a Feb)   




      50(If It is a month Of Feb) 
      -------------------------- 

Answer 1

使用UNION ALL：

Select SUM(Breakfast) as Total FROM `expense` WHERE month(Date)=2 
union all 
Select Sum(BreakFast)+sum(lunch)+sum(dinner) as Total FROM `expense` WHERE month(Date)<>2 

使用案例（最快的選項）：

SELECT CASE 
WHEN MONTH(DATE)=2 THEN SUM(Breakfast) 
WHEN MONTH(DATE)<>2 THEN SUM(BreakFast)+SUM(lunch)+SUM(dinner) 
ELSE 0 END 
AS Total FROM `expense`