mysql数据查询之五子句(where、group by、having、order by和limit)

概念了解:
select 语句中包含一个查询子句 (五子句,包括 where group by order by  、 having limit


1.where 
经常用到,就不细说了,可以搭配一些比较运算符>,<…;逻辑运算符&&,||,!


2.group by 

(1) 将表中的数据按sex进行分组,并用统计函数进行统计
select sex,count(*),max(age),min(age),avg(age),sum(age) from student group by sex;
cout():统计分组后,每组的总记录数;

(2)按sex进行分组之后并排序
select sex,count(*) from student group by sex asc;


(3)先按grade进行分组,再按sex进行分组
select *,count(*) from student group by grade,sex;

(4)group_connect(字段名) 可以对分组的结果的某个字段值进行字符串连接,保留该组某个字段的所有值
select grade,count(*),group_concat(name)  from student group by grade;
+-------+----------+--------------------+
| grade | count(*) | group_concat(name) |
+-------+----------+--------------------+
|     3 |        4 | qiao,zhang,li,chen |
|     4 |        3 | li,zhao,qian       |
+-------+----------+--------------------+

(5)with rollup 关键字,可以在每次分组之后,根据当前分组的字段进行统计,并向上一级进行汇报,可以和ifnull()混用

select grade,count(*) from student group by grade with rollup;
+-------+----------+
| grade | count(*) |
+-------+----------+
|     3 |        4 |
|     4 |        3 |
NULL |        7 |
+-------+----------+
3 rows in set (0.00 sec)

select ifnull(grade,'总计'),count(*) from student group by grade with   llup;
+------------------------+----------+
| ifnull(grade,'总计')   | count(*) |
+------------------------+----------+
| 3                      |        4 |
| 4                      |        3 |
| 总计                   |        7 |
+------------------------+----------+


3.having
可以把where看作where的子集,having有一些where之外的能力

(1)统计函数只能having使用
select grade,count(*) from student group by grade having count(*) >= 2;


(2)having可以使用字段别名,where不能
select grade,count(*) as total from student group by grade having total >= 2


4.order by +  [asc/desc]

将表中的数据先按age升序排序(默认),再按grade降序排序
select * from student order by age , grade desc;


5.limit

(1)用来限制长度,查询表中的指定条数据
select * from student limit 3;

(2)限制起始值,限制指定长度
select * from student limit 2,2;

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转载自blog.csdn.net/dangbai01_/article/details/79892768
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