我們都知道SQL中適用case when then來轉(zhuǎn)化數(shù)據(jù)庫中的信息 比如 select (case sex when 0 then ‘男’ else ‘女’ end) AS sex from studentInfo
那么在集合函數(shù)中它有什么用呢 ？
假設(shè)數(shù)據(jù)庫有一張表名為student的表。
如果現(xiàn)在要你根據(jù)這張表，查出江西省男女個數(shù)，廣東省男生個數(shù)，浙江省男女個數(shù) 怎么寫SQL語句？即要生成下結(jié)果表
答案是:select sex ,count ( case province when ‘廣東省’ then ‘廣東省’ end )as 廣東省 ,count ( case province when ‘江西省’ then ‘江西省’ end )as 江西省 ,count ( case province when ‘浙江省’ then ‘浙江省’ end )as 浙江省 from student group by sex
count()函數(shù)即根據(jù)給定的范圍和group by(統(tǒng)計方式) 而統(tǒng)計行數(shù)據(jù)的條數(shù)
我們一步步來理解上面語句
1. select sex from student (查詢數(shù)據(jù)表中的存在的男女條數(shù))
2.select sex, count (*) as num from student group by sex (查詢表中男女?dāng)?shù)量)
3.select sex ,province, count (*)as num from student group by sex,province (查詢各省男女?dāng)?shù)量)
重點來了，如果我把count(*) 中的 *號換成任一列名呢？ 如count(province) 會怎樣？
4.select sex ,province, count (province)as num from student group by sex,province (查詢各省男女?dāng)?shù)量)
結(jié)果跟上圖一樣：這說明換不換都一樣。又有count (province)等價于 count(case province when ‘浙江省’ then ‘浙江省’ else province end )
但是如果我們縮小范圍呢即count(case province when ‘浙江省’ then ‘浙江省’ end ) 那么請看下面
5.select sex ,province, count ( case province when ‘浙江省’ then ‘浙江省’ end )as num from student group by sex,province
即統(tǒng)計男女?dāng)?shù)量范圍限定在浙江省 再精簡一下即下面
6.select sex, count ( case province when ‘浙江省’ then ‘浙江省’ end ) as 浙江省 from student group by sex
已經(jīng)接近我們的要求了，現(xiàn)在只要加上另幾個字段就是了
7.select sex ,count ( case province when ‘廣東省’ then ‘廣東省’ end )as 廣東省 ,count ( case province when ‘江西省’ then ‘江西省’ end )as 江西省 ,count ( case province when ‘浙江省’ then ‘浙江省’ end )as 浙江省 from student group by sex
小結(jié)：當(dāng)然實現(xiàn)有很多種方法 可以多個子查詢拼接起來也不無可厚非。我這只是一種思路
補(bǔ)充：case when then 知識點
(1) select (case province when ‘浙江省’ then ‘浙江’ when ‘江西省’ then ‘江西’ end ) as 省份 from student
如果默認(rèn)范圍如果沒全包含則為空 像上圖的廣東省為空
(2)select (case province when ‘浙江省’ then ‘浙江’ when ‘江西省’ then ‘江西’ else province end ) as 省份 from student
.png)
