排序Order by
order by 字段 asc/desc
-- asc从小到大排列，即升序
-- desc从大到小排序，即降序
-- 查询年龄在18到34岁之间的男性，按照年龄从小到到排序
select * from students where (age between 18 and 34) and gender=1;
select * from students where (age between 18 and 34) and gender=1 order by age;
select * from students where (age between 18 and 34) and gender=1 order by age asc;
-- 查询年龄在18到34岁之间的女性，身高从高到矮排序
select * from students where (age between 18 and 34) and gender=2 order by height desc;
order by 字段 asc/desc，字段 asc/desc……
-- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序
select * from students where (age between 18 and 34) and gender=2 order by height desc,age desc;
-- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序,
-- 如果年龄也相同那么按照id从大到小排序
select * from students where (age between 18 and 34) and gender=2 order by height desc,age asc,id desc;
-- 按照年龄从小到大、身高从高到矮的排序
select * from students order by age asc, height desc;
聚合函数
count (查询行数)
-- 查询男性有多少人，女性有多少人
select * from students where gender=1;
select count(*) from students where gender=1;
select count(*) as 男性人数 from students where gender=1;
select count(*) as 女性人数 from students where gender=2;
最大值
-- max
-- 查询最大的年龄
select age from students;
select max(age) from students;
-- 查询女性的最高 身高
select max(height) from students where gender=2;
最小值
-- min
求和
-- sum
-- 计算所有人的年龄总和
select sum(age) from students;
平均值
-- avg
-- 计算平均年龄
select avg(age) from students;
-- 计算平均年龄 sum(age)/count(*)
select sum(age)/count(*) from students;
round
-- 四舍五入 round(123.23 , 1) 保留1位小数
-- 计算所有人的平均年龄，保留2位小数
select round(sum(age)/count(*), 2) from students;
select round(sum(age)/count(*), 3) from students;
-- 计算男性的平均身高 保留2位小数
select round(avg(height), 2) from students where gender=1;
-- select name, round(avg(height), 2) from students where gender=1;
分组group by
l group by
-- 按照性别分组,查询所有的性别
select name from students group by gender;
select * from students group by gender;
select gender from students group by gender;
-- 计算每种性别中的人数
select gender,count(*) from students group by gender;
-- 计算男性的人数
select gender,count(*) from students where gender=1 group by gender;
l group_concat(...)
完整的语法如下：
group_concat([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符'])
select * from aa;
+------+------+
| id| name |
+------+------+
| 1 | 10|
| 1 | 20|
| 1 | 20|
| 2 | 20|
| 3 | 200 |
| 3 | 500 |
+------+------+
以id分组，把name字段的值打印在一行，逗号分隔(默认)
mysql> select id, group_concat(name) from aa group by id;
以id分组，把name字段的值打印在一行，分号分隔
mysql> select id, group_concat(name separator ';') from aa group by id;
+------+----------------------------------+
| id| group_concat(name separator ';') |
+------+----------------------------------+
|1 | 10;20;20 |
|2 | 20|
|3 | 200;500 |
+------+----------------------------------+
以id分组，把去冗余的name字段的值打印在一行， 逗号分隔
mysql> select id, group_concat(distinct name) from aa group by id;
+------+-----------------------------+
| id| group_concat(distinct name) |
+------+-----------------------------+
|1 | 10,20|
|2 | 20 |
|3 | 200,500 |
+------+-----------------------------+
以id分组，把name字段的值打印在一行，逗号分隔，以name排倒序
mysql> select id, group_concat(name order by name desc) from aa group by id;
>>>
+------+---------------------------------------+
| id| group_concat(name order by name desc) |
+------+---------------------------------------+
|1 | 20,20,10 |
|2 | 20|
|3 | 500,200|
+------+---------------------------------------+