SQL查詢優化 - 替代方式

Question

我需要填補這一變量對給定action_to（包含用戶ID）

type_1_count = "select count(action_type) from action where action_type = 1 AND user_from = 13213" 

type_2_count = "select count(action_type) from action where action_type = 2 AND user_from = 13213" 

type_3_count = "select count(action_type) from action where action_type = 3 AND user_from = 13213" 

或

$type_counts = "three count sub queries in single query" 

通常我們，'三計數查詢'或'三國單個查詢'中的子查詢。

有沒有更好的方法，我可以在單個查詢中獲取動作類型的計數？

Answer 1

使用條件彙總：

select sum(action_type = 1) as type_1_count, 
     sum(action_type = 2) as type_2_count, 
     sum(action_type = 3) as type_3_count  
from action 
where user_from = 13213; 

或者，使用group by：

select action_type, count(*) as cnt 
from action 
where user_from = 13213; 

不同的是，第一個查詢生成一行與不同的計數。第二行在數據中每action_type產生一行，計數作爲第二列。

編輯：

表達sum(action_type = 3)計數其中action_type具有3.在MySQL的值的行數，布爾值被視爲整數在數值上下文，與真正的爲1，假爲0，所以， sum(action_type = 3)統計action_type在該值上的行數。