count函數(shù)條件計數(shù)筆記
-- 2班和4班占總?cè)藬?shù)的百分比 【常規(guī)做法】
SELECT
(
SELECT
count(id)
FROM
`user` u
WHERE
u.clazz IN (2, 4)
)/(
SELECT
count(id)
FROM
`user` u
)
-- 求二班人數(shù)，通過or null ?并直接在select中做除法 【簡便寫法】
SELECT ?
count(u.clazz = 2 OR u.clazz = 4 OR NULL ) / count(1)
FROM `user` u
-- 班級計數(shù)
SELECT count(DISTINCT clazz) FROM `user` u
