Mysql從單個列中選擇計數值

Question

A有一個名為key的列 - 1,1,2,2,2,2,3現在我用3個查詢來做：

SELECT count(key) as k FROM `test` WHERE key=1
SELECT count(key) as k FROM `test` WHERE key=2
SELECT count(key) as k FROM `test` WHERE key=3

如何在一個查詢中計算多少1,2,3？

Answer 1

使用分組：

SELECT `key`, COUNT(*) FROM `test` GROUP BY `key`;

Answer 2

你可以這樣做

select count(key) as K FROM test where key in (1,2,3)

Answer 3

這是另一種選擇：

SELECT sum(key=1) as k1, sum(key=2) as k2,sum(key=3) as k3 FROM `test`

如果key的值是某個其他組的一部分，您還可以按列添加組。

Answer 4

嘗試這個

SELECT  COUNT(Key) as K
        FROM 
        test 
        GROUP BY Key