(1) group by的含義:將查詢結果按照1個或多個字段進行分組，字段值相同的為一組
(2) group by可用于單個字段分組，也可用于多個字段分組
select * from employee;
+------+------+--------+------+------+-------------+ | num | d_id | name | age | sex | homeaddr | +------+------+--------+------+------+-------------+ | 1 | 1001 | 張三 | 26 | 男 | beijinghdq | | 2 | 1002 | 李四 | 24 | 女 | beijingcpq | | 3 | 1003 | 王五 | 25 | 男 | changshaylq | | 4 | 1004 | Aric | 15 | 男 | England | +------+------+--------+------+------+-------------+
select * from employee group by d_id,sex;
select * from employee group by sex; +------+------+--------+------+------+------------+ | num | d_id | name | age | sex | homeaddr | +------+------+--------+------+------+------------+ | 2 | 1002 | 李四 | 24 | 女 | beijingcpq | | 1 | 1001 | 張三 | 26 | 男 | beijinghdq | +------+------+--------+------+------+------------+ 根據sex字段來分組，sex字段的全部值只有兩個('男'和'女')，所以分為了兩組 當group by單獨使用時，只顯示出每組的第一條記錄 所以group by單獨使用時的實際意義不大 group by + group_concat()
(1) group_concat(字段名)可以作為一個輸出字段來使用，
(2) 表示分組之后，根據分組結果，使用group_concat()來放置每一組的某字段的值的集合
select sex from employee group by sex;
+------+ | sex | +------+ | 女 | | 男 | +------+ select sex,group_concat(name) from employee group by sex; +------+--------------------+ | sex | group_concat(name) | +------+--------------------+ | 女 | 李四 | | 男 | 張三,王五,Aric | +------+--------------------+ select sex,group_concat(d_id) from employee group by sex; +------+--------------------+ | sex | group_concat(d_id) | +------+--------------------+ | 女 | 1002 | | 男 | 1001,1003,1004 | +------+--------------------+ group by + 集合函數
(1) 通過group_concat()的啟發，我們既然可以統計出每個分組的某字段的值的集合，那么我們也可以通過集合函數來對這個"值的集合"做一些操作
select sex,group_concat(age) from employee group by sex;
+------+-------------------+ | sex | group_concat(age) | +------+-------------------+ | 女 | 24 | | 男 | 26,25,15 | +------+-------------------+
分別統計性別為男/女的人年齡平均值 select sex,avg(age) from employee group by sex; +------+----------+ | sex | avg(age) | +------+----------+ | 女 | 24.0000 | | 男 | 22.0000 | +------+----------+
分別統計性別為男/女的人的個數 select sex,count(sex) from employee group by sex; +------+------------+ | sex | count(sex) | +------+------------+ | 女 | 1 | | 男 | 3 | +------+------------+ group by + having
(1) having 條件表達式：用來分組查詢后指定一些條件來輸出查詢結果
(2) having作用和where一樣，但having只能用于group by
select sex,count(sex) from employee group by sex having count(sex)>2; +------+------------+ | sex | count(sex) | +------+------------+ | 男 | 3 | +------+------------+ group by + with rollup
(1) with rollup的作用是：在最后新增一行，來記錄當前列里所有記錄的總和
select sex,count(age) from employee group by sex with rollup; +------+------------+ | sex | count(age) | +------+------------+ | 女 | 1 | | 男 | 3 | | NULL | 4 | +------+------------+ select sex,group_concat(age) from employee group by sex with rollup; +------+-------------------+ | sex | group_concat(age) | +------+-------------------+ | 女 | 24 | | 男 | 26,25,15 | | NULL | 24,26,25,15 | +------+-------------------+ 
)
模板)
)
