MySQL单表数据查询(DQL)

数据准备工作：

CREATE TABLE student(
    sid INT PRIMARY KEY AUTO_INCREMENT,
    sname VARCHAR(10),
    age TINYINT,
    city VARCHAR(10),
    score TINYINT
);

INSERT INTO student VALUES (NULL,"曹操",28,"洛阳",95);
INSERT INTO student VALUES (NULL,"刘备",27,"成都",80);
INSERT INTO student VALUES (NULL,"孙权",17,"建业",85);
INSERT INTO student VALUES (NULL,"孔明",21,"成都",98);
INSERT INTO student VALUES (NULL,"关羽",25,"成都",70);
INSERT INTO student VALUES (NULL,"张飞",24,"成都",50);
INSERT INTO student VALUES (NULL,"鲁肃",22,"建业",90);
INSERT INTO student VALUES (NULL,"周瑜",19,"建业",97);
INSERT INTO student VALUES (NULL,"郭嘉",23,"洛阳",98);
INSERT INTO student VALUES (NULL,"小乔",18,"建业",70);
INSERT INTO student VALUES (NULL,"貂蝉",26,NULL,65);

一、简单查询

语法：select select选项 字段列表 from 数据源;

select选项：系统该如何对待查询得到的结果

all：默认的，表示保存所有的记录

distinct：去重，去除重复的记录（所有的字段值都相同），只保留一条

字段列表：对查询结果显示哪些列名

*：全部列名

as：给列名起别名，解决从多张表获取数据存在列名冲突的问题

from数据源：只要是一个符合二维表结构的数据即可

单表数据：from 表名

多表数据：from 表名1，表名2...

动态数据：from (select select选项 字段列表 from 数据源) as 别名

1.1 查询所有学生

查询指令：select * from student;

mysql> select * from student;
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   1 | 曹操   |   28 | 洛阳   |    95 |
|   2 | 刘备   |   27 | 成都   |    80 |
|   3 | 孙权   |   17 | 建业   |    85 |
|   4 | 孔明   |   21 | 成都   |    98 |
|   5 | 关羽   |   25 | 成都   |    70 |
|   6 | 张飞   |   24 | 成都   |    50 |
|   7 | 鲁肃   |   22 | 建业   |    90 |
|   8 | 周瑜   |   19 | 建业   |    97 |
|   9 | 郭嘉   |   23 | 洛阳   |    98 |
|  10 | 小乔   |   18 | 建业   |    70 |
|  11 | 貂蝉   |   26 | NULL   |    65 |
+-----+--------+------+--------+-------+
11 rows in set (0.03 sec)

1.2 查询学生姓名和年龄

查询指令：select sname,age from student;

mysql> select sname,age from student;
+--------+------+
| sname  | age  |
+--------+------+
| 曹操   |   28 |
| 刘备   |   27 |
| 孙权   |   17 |
| 孔明   |   21 |
| 关羽   |   25 |
| 张飞   |   24 |
| 鲁肃   |   22 |
| 周瑜   |   19 |
| 郭嘉   |   23 |
| 小乔   |   18 |
| 貂蝉   |   26 |
+--------+------+
11 rows in set (0.00 sec)

1.3 运算查询：将所有学生的年龄+10进行显示

查询指令：select sname,age+10 from student;

mysql> select sname,age+10 from student;
+--------+--------+
| sname  | age+10 |
+--------+--------+
| 曹操   |     38 |
| 刘备   |     37 |
| 孙权   |     27 |
| 孔明   |     31 |
| 关羽   |     35 |
| 张飞   |     34 |
| 鲁肃   |     32 |
| 周瑜   |     29 |
| 郭嘉   |     33 |
| 小乔   |     28 |
| 貂蝉   |     36 |
+--------+--------+
11 rows in set (0.00 sec)

 1.4 别名查询：将列名sname,age分别用中文显示

查询指令：select sname as 姓名,age as 年龄 from student;

mysql> select sname as 姓名,age as 年龄 from student;
+--------+--------+
| 姓名   | 年龄   |
+--------+--------+
| 曹操   |     28 |
| 刘备   |     27 |
| 孙权   |     17 |
| 孔明   |     21 |
| 关羽   |     25 |
| 张飞   |     24 |
| 鲁肃   |     22 |
| 周瑜   |     19 |
| 郭嘉   |     23 |
| 小乔   |     18 |
| 貂蝉   |     26 |
+--------+--------+
11 rows in set (0.00 sec)

二、高级查询

语法：select select选项 字段列表 from 数据源 where 条件 group by 分组 having 条件 order by 排序 limit 分页 ;

2.1 where 子句

作用：针对表去对应的磁盘处获取所有的记录，where的作用就是在拿到一条结果就开始进行判断，判断是否符合

条件，如果符合就获取放到内存中去，如果不符合条件就不获取。

2.1.1 查询年龄大于24的学生

查询指令：select * from student where age > 24;

mysql> select * from student where age > 24;
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   1 | 曹操   |   28 | 洛阳   |    95 |
|   2 | 刘备   |   27 | 成都   |    80 |
|   5 | 关羽   |   25 | 成都   |    70 |
|  11 | 貂蝉   |   26 | NULL   |    65 |
+-----+--------+------+--------+-------+
4 rows in set (0.00 sec)

2.1.2 查询年龄是18到22的学生

查询指令：select * from student where age between 18 and 22;

mysql> select * from student where age between 18 and 22;
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   4 | 孔明   |   21 | 成都   |    98 |
|   7 | 鲁肃   |   22 | 建业   |    90 |
|   8 | 周瑜   |   19 | 建业   |    97 |
|  10 | 小乔   |   18 | 建业   |    70 |
+-----+--------+------+--------+-------+
4 rows in set (0.00 sec)

2.1.3 查询地址是成都或洛阳的学生

查询指令：select * from student where city in ("成都","洛阳");

mysql> select * from student where city in ("成都","洛阳");
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   1 | 曹操   |   28 | 洛阳   |    95 |
|   2 | 刘备   |   27 | 成都   |    80 |
|   4 | 孔明   |   21 | 成都   |    98 |
|   5 | 关羽   |   25 | 成都   |    70 |
|   6 | 张飞   |   24 | 成都   |    50 |
|   9 | 郭嘉   |   23 | 洛阳   |    98 |
+-----+--------+------+--------+-------+
6 rows in set (0.00 sec)

2.1.4 查询姓孙的学生

查询指令：select * from student where sname like "孙%";

mysql> select * from student where sname like "孙%";
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   3 | 孙权   |   17 | 建业   |    85 |
+-----+--------+------+--------+-------+
1 row in set (0.00 sec)

2.1.5 查询地址不为空的学生

查询指令：select * from student where city is not null;

mysql> select * from student where city is not null;
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   1 | 曹操   |   28 | 洛阳   |    95 |
|   2 | 刘备   |   27 | 成都   |    80 |
|   3 | 孙权   |   17 | 建业   |    85 |
|   4 | 孔明   |   21 | 成都   |    98 |
|   5 | 关羽   |   25 | 成都   |    70 |
|   6 | 张飞   |   24 | 成都   |    50 |
|   7 | 鲁肃   |   22 | 建业   |    90 |
|   8 | 周瑜   |   19 | 建业   |    97 |
|   9 | 郭嘉   |   23 | 洛阳   |    98 |
|  10 | 小乔   |   18 | 建业   |    70 |
+-----+--------+------+--------+-------+
10 rows in set (0.00 sec)

2.1.6 查询地址为成都或洛阳，且年龄大于25的学生

查询指令：select * from student where city in ("成都","洛阳") and age > 25;

mysql> select * from student where city in ("成都","洛阳") and age > 25;
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   1 | 曹操   |   28 | 洛阳   |    95 |
|   2 | 刘备   |   27 | 成都   |    80 |
+-----+--------+------+--------+-------+
2 rows in set (0.00 sec)

2.1.7 查询地址不是成都或洛阳的学生

查询指令：select * from student where city not in ("成都","洛阳");

mysql> select * from student where city not in ("成都","洛阳");
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   3 | 孙权   |   17 | 建业   |    85 |
|   7 | 鲁肃   |   22 | 建业   |    90 |
|   8 | 周瑜   |   19 | 建业   |    97 |
|  10 | 小乔   |   18 | 建业   |    70 |
+-----+--------+------+--------+-------+
4 rows in set (0.00 sec)

2.2 group by 子句

作用：根据指定的字段，将数据进行分组，分组的目标是为了统计。

聚合函数：对分组数据进行统计操作。

count()：统计每组中的数量，如果统计目标是字段，那么不统计为空NULL字段，如果为*，则统计记录数

avg()：求平均值

sum()：求和

max()：求最大值

min()：求最小值

2.2.1 统计各个地方的人数

查询指令：select city,count(*) from student group by city;

mysql> select city,count(*) from student group by city;
+--------+----------+
| city   | count(*) |
+--------+----------+
| NULL   |        1 |
| 建业   |        4 |
| 成都   |        4 |
| 洛阳   |        2 |
+--------+----------+
4 rows in set (0.02 sec)

2.2.2 统计各个地方的平均年龄

查询指令：select city,avg(age) from student group by city;

mysql> select city,avg(age) from student group by city;
+--------+----------+
| city   | avg(age) |
+--------+----------+
| NULL   |  26.0000 |
| 建业   |  19.0000 |
| 成都   |  24.2500 |
| 洛阳   |  25.5000 |
+--------+----------+
4 rows in set (0.01 sec)

2.2.3 统计各个地方的成绩总和

查询指令： select city,sum(score) from student group by city;

mysql> select city,sum(score) from student group by city;
+--------+------------+
| city   | sum(score) |
+--------+------------+
| NULL   |         65 |
| 建业   |        342 |
| 成都   |        298 |
| 洛阳   |        193 |
+--------+------------+
4 rows in set (0.00 sec)

2.2.4 统计各个地方的最大成绩与最小成绩

查询指令：select city,max(score),min(score) from student group by city;

mysql> select city,max(score),min(score) from student group by city;
+--------+------------+------------+
| city   | max(score) | min(score) |
+--------+------------+------------+
| NULL   |         65 |         65 |
| 建业   |         97 |         70 |
| 成都   |         98 |         50 |
| 洛阳   |         98 |         95 |
+--------+------------+------------+
4 rows in set (0.01 sec)

2.3 having 子句

作用：对分组后的数据进行筛选

having与where的区别：

1.having是在分组后对数据进行过滤，where是在分组前对数据进行过滤

2.having后面可以使用统计函数过滤数据，where后面不可以使用统计函数

2.3.1 统计各个地方的学生人数，获取人数大于2的数据

查询指令：select city,count(*) as num from student group by city having num > 2;

mysql> select city,count(*) as num from student group by city having num > 2;
+--------+-----+
| city   | num |
+--------+-----+
| 建业   |   4 |
| 成都   |   4 |
+--------+-----+
2 rows in set (0.00 sec)

注意：having是在group by之后，group by是在where之后，where的时候表示将数据从磁盘拿到内存，

where之后的所有操作都是内存操作。

2.4 order by 子句

作用：对查询结果进行排序显示，默认按升序，可以对多个字段进行排序

asc：升序排序

desc：降序排序

2.4.1 把所有学生先按成绩进行降序排序，再按年龄进行升序排序

查询指令：select * from student order by score desc,age asc;

mysql> select * from student order by score desc,age asc;
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   4 | 孔明   |   21 | 成都   |    98 |
|   9 | 郭嘉   |   23 | 洛阳   |    98 |
|   8 | 周瑜   |   19 | 建业   |    97 |
|   1 | 曹操   |   28 | 洛阳   |    95 |
|   7 | 鲁肃   |   22 | 建业   |    90 |
|   3 | 孙权   |   17 | 建业   |    85 |
|   2 | 刘备   |   27 | 成都   |    80 |
|  10 | 小乔   |   18 | 建业   |    70 |
|   5 | 关羽   |   25 | 成都   |    70 |
|  11 | 貂蝉   |   26 | NULL   |    65 |
|   6 | 张飞   |   24 | 成都   |    50 |
+-----+--------+------+--------+-------+
11 rows in set (0.00 sec)

2.5 limit 子句

作用：利用limit来限制获取指定区间的数据。

基本语法：limit offset,length;

offset：偏移量，从哪条记录开始，第一条记录为0

length：长度，获取多少条记录

2.5.1 对成绩进行排名，获取成绩是第6名到第10名的学生

查询指令：select * from student order by score desc limit 5,5;

mysql> select * from student order by score desc limit 5,5;
+-----+--------+------+--------+-------+
| sid | sname  | age  | city   | score |
+-----+--------+------+--------+-------+
|   3 | 孙权   |   17 | 建业   |    85 |
|   2 | 刘备   |   27 | 成都   |    80 |
|   5 | 关羽   |   25 | 成都   |    70 |
|  10 | 小乔   |   18 | 建业   |    70 |
|  11 | 貂蝉   |   26 | NULL   |    65 |
+-----+--------+------+--------+-------+
5 rows in set (0.00 sec)