MySQL单表查询实例详解

1、准备数据

以下操作将在该表中进行

create table student (
id int unsigned primary key auto_increment,
name char(12) not null,
gender enum("male","famale") default "male",
age tinyint unsigned not null,
hoc_group char(12) not null,
html tinyint unsigned not null,
css tinyint unsigned not null,
js tinyint unsigned not null,
sanction enum("大处分","小处分","无")
);

insert into student(name,gender,age,hoc_group,html,css,js,sanction) values
("Yunya","male",18,"first",88,93,76,"无"),
("Jack","male",17,"second",92,81,88,"无"),
("Bella","famale",17,"first",72,68,91,"小处分"),
("Dairis","famale",18,"third",89,54,43,"大处分"),
("Kyle","famale",19,"fifth",31,24,60,"大处分"),
("Alice","famale",16,"second",49,23,58,"无"),
("Ken","male",16,"third",33,62,17,"大处分"),
("Jason","male",21,"fourth",91,92,90,"无"),
("Tom","male",20,"fifth",88,72,91,"无"),
("Fiona","famale",19,"fourth",60,71,45,"无");

2、查询语法

SELECT DISTINCT(字段名1,字段名2...) FROM 表名
 WHERE 条件
 GROUP BY 字段名
 HAVING 筛选
 ORDER BY 字段名 asc/desc
 LIMIT 限制条数;

3、执行顺序

虽然查询的书写语法是上面那样的，但是其内部执行顺序却有些不太一样。

1.通过from找到将要查询的表

2.where规定查询条件，在表记录中逐行进行查询并筛选出符合规则的记录

3.将查到的记录进行字段分组group by，如果没有进行分组，则默认为一组

4.将分组得到的结果进行having筛选，可使用聚和函数（where时不可使用聚合函数）

5.执行select准备打印

6.执行distinct对打印结果进行去重

7.执行ordery by对结果进行排序

8.执行limit对打印结果的条数进行限制

4、select

select主要复负责打印相关的工作

4.1 全部查询

使用select * from 表名可拿到该表下全部的数据

以下示例将展示使用全部查询拿到student表中所有记录

select * from student;

+----+--------+--------+-----+-----------+------+-----+----+-----------+
| id | name | gender | age | hoc_group | html | css | js | sanction |
+----+--------+--------+-----+-----------+------+-----+----+-----------+
| 1 | Yunya | male | 18 | first | 88 | 93 | 76 | 无 |
| 2 | Jack | male | 17 | second | 92 | 81 | 88 | 无 |
| 3 | Bella | famale | 17 | first | 72 | 68 | 91 | 小处分 |
| 4 | Dairis | famale | 18 | third | 89 | 54 | 43 | 大处分 |
| 5 | Kyle | famale | 19 | fifth | 31 | 24 | 60 | 大处分 |
| 6 | Alice | famale | 16 | second | 49 | 23 | 58 | 无 |
| 7 | Ken | male | 16 | third | 33 | 62 | 17 | 大处分 |
| 8 | Jason | male | 21 | fourth | 91 | 92 | 90 | 无 |
| 9 | Tom | male | 20 | fifth | 88 | 72 | 91 | 无 |
| 10 | Fiona | famale | 19 | fourth | 60 | 71 | 45 | 无 |
+----+--------+--------+-----+-----------+------+-----+----+-----------+

4.2 字段查询

使用select 字段名1，字段名2 from 表名可拿到特定字段下相应的数据

以下示例将展示使用字段查询拿到每个学生的HTML\CSS\JS成绩

select name,html,css,js from student;

+--------+------+-----+----+
| name | html | css | js |
+--------+------+-----+----+
| Yunya | 88 | 93 | 76 |
| Jack | 92 | 81 | 88 |
| Bella | 72 | 68 | 91 |
| Dairis | 89 | 54 | 43 |
| Kyle | 31 | 24 | 60 |
| Alice | 49 | 23 | 58 |
| Ken | 33 | 62 | 17 |
| Jason | 91 | 92 | 90 |
| Tom | 88 | 72 | 91 |
| Fiona | 60 | 71 | 45 |
+--------+------+-----+----+

4.3 as 别名

使用select 字段名1 as 别名1, 字段名2 as 别名2 from 表名可将查询到的记录字段修改一个别名

以下示例将展示修改name字段为姓名，修改gender字段为性别，修改age字段为年龄的操作

select name as "姓名", gender as "性别", age as "年龄" from student;

+--------+--------+--------+
| 姓名 | 性别 | 年龄 |
+--------+--------+--------+
| Yunya | male | 18 |
| Jack | male | 17 |
| Bella | famale | 17 |
| Dairis | famale | 18 |
| Kyle | famale | 19 |
| Alice | famale | 16 |
| Ken | male | 16 |
| Jason | male | 21 |
| Tom | male | 20 |
| Fiona | famale | 19 |
+--------+--------+--------+

4.4 distinct

使用select distinct(字段名1, 字段名2) from 表名可将查询到的记录做一个取消重复的操作

以下示例将展示使用去重功能来看有多少个小组

select distinct(hoc_group) from student;

+-----------+
| hoc_group |
+-----------+
| first |
| second |
| third |
| fifth |
| fourth |
+-----------+

4.5 四则运算

查询结果可进行四则运算，以下示例将展示拿到每个同学三科总分的操作

select name, html+css+js as 总成绩 from student;
+--------+-----------+
| name | 总成绩 |
+--------+-----------+
| Yunya | 257 |
| Jack | 261 |
| Bella | 231 |
| Dairis | 186 |
| Kyle | 115 |
| Alice | 130 |
| Ken | 112 |
| Jason | 273 |
| Tom | 251 |
| Fiona | 176 |
+--------+-----------+

4.6 显示格式

使用concat()可将查询结果与任意字符串进行拼接

使用concat_ws()可指定连接符进行拼接，第一个参数是连接符

select concat("姓名->",name," ","性别->",gender) from student; # 合并成了一个字符串，注意用的空格分隔开的，不然会黏在一起

+--------------------------------------------------+
| concat("姓名->",name," ","性别->",gender) |
+--------------------------------------------------+
| 姓名->Yunya 性别->male  |
| 姓名->Jack 性别->male  |
| 姓名->Bella 性别->famale  |
| 姓名->Dairis 性别->famale  |
| 姓名->Kyle 性别->famale  |
| 姓名->Alice 性别->famale  |
| 姓名->Ken 性别->male  |
| 姓名->Jason 性别->male  |
| 姓名->Tom 性别->male  |
| 姓名->Fiona 性别->famale  |
+--------------------------------------------------+

select concat_ws("|||",name,gender,age) from student; # 使用|||为每个字段进行分割

+----------------------------------+
| concat_ws("|||",name,gender,age) |
+----------------------------------+
| Yunya|||male|||18 |
| Jack|||male|||17  |
| Bella|||famale|||17 |
| Dairis|||famale|||18 |
| Kyle|||famale|||19 |
| Alice|||famale|||16 |
| Ken|||male|||16  |
| Jason|||male|||21 |
| Tom|||male|||20  |
| Fiona|||famale|||19 |
+----------------------------------+

5、where

where条件是查询的第一道坎，能有效过滤出我们想要的任意数据

5.1、比较运算

使用比较运算符> < >= <= !=进行查询

以下示例将展示使用where过滤出js成绩大于80分的同学

select name, js from student where js > 80;

+-------+----+
| name | js |
+-------+----+
| Jack | 88 |
| Bella | 91 |
| Jason | 90 |
| Tom | 91 |
+-------+----+

5.2、逻辑运算

使用and or not可进行逻辑运算与多条件查询

以下示例将展示使用where多条件查询过滤出各科成绩都大于80分的同学

select name, html, css, js from student where html > 80 and css > 80 and js > 80;

+-------+------+-----+----+
| name | html | css | js |
+-------+------+-----+----+
| Jack | 92 | 81 | 88 |
| Jason | 91 | 92 | 90 |
+-------+------+-----+----+

5.3、成员运算

in可以在特定的值中进行获取，如in(80,90,100)则代表只取80或者90或者100的这几条记录。

以下示例将展示只取第一组first以及第二组second学生的个人信息

select name, gender, age, hoc_group from student where hoc_group in ("first","second");

+-------+--------+-----+-----------+
| name | gender | age | hoc_group |
+-------+--------+-----+-----------+
| Yunya | male | 18 | first |
| Jack | male | 17 | second |
| Bella | famale | 17 | first |
| Alice | famale | 16 | second |
+-------+--------+-----+-----------+

5.4、between and

between and也是取区间的意思，

以下示例将展示使用between and过滤出Js成绩大于等于60并且小于80的同学

select name, js from student where js between 60 and 80;

+-------+----+
| name | js |
+-------+----+
| Yunya | 76 |
| Kyle | 60 |
+-------+----+

5.5、like

like是模糊查询，其中％代表任意多个字符（类似于贪婪匹配的通配符.*），_代表任意一个字符（类似于非贪婪匹配的通配符.*?）。

以下示例将展示使用like/％匹配出姓名以k开头的所有同学的名字

select name from student where name like "k％";

+------+
| name |
+------+
| Kyle |
| Ken |
+------+

以下示例将展示使用like/_匹配出姓名以k开头并整体长度为3的同学的名字

select name from student where name like "k__";

+------+
| name |
+------+
| Ken |
+------+

5.6、正则匹配

使用RegExp可进行正则匹配，以下示例将展示使用正则匹配出名字中带有k的所有同学姓名

select name from student where name REGEXP "k+";

+------+
| name |
+------+
| Jack |
| Kyle |
| Ken |
+------+

6、group by

分组行为发生在where条件之后，我们可以将查询到的记录按照某个相同字段进行归类，一般分组都会配合聚合函数进行使用。

需要注意的是select语句是排在group by条件之后的，因此聚合函数也能在select语句中使用。

6.1、基本使用

以下示例将展示对hoc_group字段进行分组。

我们按照hoc_group字段进行分组，那么select查询的字段只能是hoc_group字段，想要获取组内的其他字段相关信息，需要借助函数来完成

select hoc_group from student group by hoc_group;

+-----------+
| hoc_group |
+-----------+
| fifth |
| first |
| fourth |
| second |
| third |
+-----------+

如果不使用分组，则会产生重复的信息

mysql> select hoc_group from student;
+-----------+
| hoc_group |
+-----------+
| first |
| second |
| first |
| third |
| fifth |
| second |
| third |
| fourth |
| fifth |
| fourth |
+-----------+

6.2、group_concat

用什么字段名进行分组，在select查询时就只能查那个用于分组的字段，查询别的字段会抛出异常，会提示sql_mode异常。

我们将Js成绩大于80分的同学筛选出来并且按照gender字段进行分组，此外我们还想查看其所有满足条件同学的名字。

以下这样操作会抛出异常。

mysql> select gender,name from student where js > 80 group by gender;

ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'school.student.name' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

必须借助group_concat()函数来进行操作才能使我们的需求圆满完成。

select gender, group_concat(name) from student where js > 80 group by gender;

+--------+--------------------+
| gender | group_concat(name) |
+--------+--------------------+
| male | Jack,Jason,Tom |
| famale | Bella  |
+--------+--------------------+

6.3、分组模式

ONLY_FULL_GROUP_BY要求select中的字段是在与group by中使用的字段

如果group by是主键或unique not null时可以在select中列出其他字段

#查看MySQL 5.7默认的sql_mode如下：
mysql> select @@global.sql_mode;
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION

#设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式)：
mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

6.4、聚合函数

聚合函数可以在where执行后的所有语句中使用，比如having，select等。

聚合函数一般是同分组进行配套使用，以下是常用的聚合函数。

以下示例将展示求每组的成绩总和

select hoc_group, sum(js+html+css) from student group by hoc_group;

+-----------+-------------------+
| hoc_group | sum(js+html+css) |
+-----------+-------------------+
| fifth |  366 |
| first |  488 |
| fourth |  449 |
| second |  391 |
| third |  298 |
+-----------+-------------------+

以下示例将展示整个班级的平均成绩及总成绩（round()用于四舍五入操作）

select round(avg(html+js+css)) as 平均分 ,sum(html+js+css) as 总分 from student;

+-----------+--------+
| 平均分 | 总分 |
+-----------+--------+
| 199 | 1992 |
+-----------+--------+

以下示例将展示打印出总科成绩最高分数

select max(js+css+html) from student;

+------------------+
| max(js+css+html) |
+------------------+
|  273 |
+------------------+

以下示例将展示查看本班有多少男生，多少女生

select gender, count(id) from student group by gender;

+--------+-----------+
| gender | count(id) |
+--------+-----------+
| male |  5 |
| famale |  5 |
+--------+-----------+

7、having

having也可用于过滤操作

7.1、区别差异

执行优先级从高到低：where> group by > having

where发生在分组group by之前，因而where中可以有任意字段，但是绝对不能使用聚合函数。

having发生在分组group by之后，因而having中可以使用分组的字段，无法直接取到其他字段，可以使用聚合函数

7.2、示例演示

以下示例将展示使用having过滤取出每组总分数大于400的小组

select hoc_group, sum(html+css+js) from student group by hoc_group having sum(html+css+js) > 400;

+-----------+------------------+
| hoc_group | sum(html+css+js) |
+-----------+------------------+
| first  |    488 |
| fourth |    449 |
+-----------+------------------+

以下示例将展示使用having过滤取出有处分的同学。（可以使用分组的字段，但不能使用其他字段）

select sanction, group_concat(name) from student group by sanction having sanction != "无";

+-----------+--------------------+
| sanction | group_concat(name) |
+-----------+--------------------+
| 大处分 | Dairis,Kyle,Ken |
| 小处分 | Bella    |
+-----------+--------------------+

8、ordery by

ordery by用于对查询结果进行排序

默认的排序是按照主键进行排序的

8.1  asc

asc用于升序排列，以下示例将展示按照每位同学的年龄进行升序排列，如果年龄相同则依照总成绩进行升序排列。

select id, name, age, html+css+js as 总成绩 from student order by age, html+css+js asc;

+----+--------+-----+-----------+
| id | name | age | 总成绩 |
+----+--------+-----+-----------+
| 7 | Ken | 16 |  112 |
| 6 | Alice | 16 |  130 |
| 3 | Bella | 17 |  231 |
| 2 | Jack | 17 |  261 |
| 4 | Dairis | 18 |  186 |
| 1 | Yunya | 18 |  257 |
| 5 | Kyle | 19 |  115 |
| 10 | Fiona | 19 |  176 |
| 9 | Tom | 20 |  251 |
| 8 | Jason | 21 |  273 |
+----+--------+-----+-----------+

8.2、desc

desc用于降序排列，以下示例将展示按照每位同学的年龄进行降序排列。

select id, name, age, html+css+js as 总成绩 from student order by age desc;

+----+--------+-----+-----------+
| id | name | age | 总成绩 |
+----+--------+-----+-----------+
| 8 | Jason | 21 |  273 |
| 9 | Tom | 20 |  251 |
| 5 | Kyle | 19 |  115 |
| 10 | Fiona | 19 |  176 |
| 1 | Yunya | 18 |  257 |
| 4 | Dairis | 18 |  186 |
| 2 | Jack | 17 |  261 |
| 3 | Bella | 17 |  231 |
| 6 | Alice | 16 |  130 |
| 7 | Ken | 16 |  112 |
+----+--------+-----+-----------+

9、limit

limit用于控制显示的条数

9.1、示例演示

按照总成绩进行降序排序，只打印1-5名。

select id, name, age, html+css+js as 总成绩 from student order by html+css+js desc limit 5;

+----+-------+-----+-----------+
| id | name | age | 总成绩 |
+----+-------+-----+-----------+
| 8 | Jason | 21 |  273 |
| 2 | Jack | 17 |  261 |
| 1 | Yunya | 18 |  257 |
| 9 | Tom | 20 |  251 |
| 3 | Bella | 17 |  231 |
+----+-------+-----+-----------+

按照总成绩进行降序排序，只打印6-8名。

select id, name, age, html+css+js as 总成绩 from student order by html+css+js desc limit 5,3; # 从第五名开始，打印三条。 6，7，8

+----+--------+-----+-----------+
| id | name | age | 总成绩 |
+----+--------+-----+-----------+
| 4 | Dairis | 18 |  186 |
| 10 | Fiona | 19 |  176 |
| 6 | Alice | 16 |  130 |
+----+--------+-----+-----------+

来源：https://www.cnblogs.com/Yunya-Cnblogs/p/13597456.html