mysql简单的单表查询详解

mysql简单的单表查询详解

MySQL的查询操作：
单表查询：简单查询
多表查询：连续查询
联合查询：
选择和投影：
投影：挑选要显示的字段
选择：挑选符合条件的行
投影：SELECT 字段1, 字段2, ... FROM tb_name;
SELECT * FROM tb_name;
选择：SELECT 字段1, ... FROM tb_name WHERE 子句;
布尔条件表达式

mysql> CREATE TABLE students (SID INT UNSIGNED AUTO_INCREMENT NOT NULL UNIQUE KEY ,Name CHAR (30) NOT NULL , Age TINYINT UNSIGNED NOT NULL , Gender ENUM('F','M') NOT NULL,Tutor CHAR(30));
Query OK, 0 rows affected (0.01 sec)
mysql> desc students;
+--------+---------------------+------+-----+---------+----------------+
| Field  | Type                | Null | Key | Default | Extra          |
+--------+---------------------+------+-----+---------+----------------+
| SID    | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| Name   | char(30)            | NO   |     | NULL    |                |
| Age    | tinyint(3) unsigned | NO   |     | NULL    |                |
| Gender | enum('F','M')       | NO   |     | NULL    |                |
| Tutor  | char(30)            | YES  |     | NULL    |                |
+--------+---------------------+------+-----+---------+----------------+

【插入数据】

mysql> insert into students values (1,'Guo Jing',27,'M','Song Jiang'),(2,'Yang Guo',28,'M','Hu Sanniang'),(3,'Guo polu',21,'M','Jii Baoyu');

mysql> INSERT INTO students VALUES (4,'Xue Baochai',19,'F','Rong Momo'),(5,'Xia Yuhe',37,'F','Shi Qian'),(6,'Wu Yong',51,'M','Lin Daiyu');

【显示数据】

mysql> select * from students;
+-----+-------------+-----+--------+-------------+
| SID | Name        | Age | Gender | Tutor       |
+-----+-------------+-----+--------+-------------+
|   1 | Guo Jing    |  27 | M      | Song Jiang  |
|   2 | Yang Guo    |  28 | M      | Hu Sanniang |
|   3 | Guo polu    |  21 | M      | Jii Baoyu   |
|   4 | Xue Baochai |  19 | F      | Rong Momo   |
|   5 | Xia Yuhe    |  37 | F      | Shi Qian    |
|   6 | Wu Yong     |  51 | M      | Lin Daiyu   |
+-----+-------------+-----+--------+-------------+

【只显示25到40岁的】

mysql> select * from students where Age between 25 and 40;
+-----+----------+-----+--------+-------------+
| SID | Name     | Age | Gender | Tutor       |
+-----+----------+-----+--------+-------------+
|   1 | Guo Jing |  27 | M      | Song Jiang  |
|   2 | Yang Guo |  28 | M      | Hu Sanniang |
|   5 | Xia Yuhe |  37 | F      | Shi Qian    |
+-----+----------+-----+--------+-------------+

【名字，年龄 只显示25到40岁的】 between and

mysql> select Name,Age from students where Age between 25 and 40;
+----------+-----+
| Name     | Age |
+----------+-----+
| Guo Jing |  27 |
| Yang Guo |  28 |
| Xia Yuhe |  37 |
+----------+-----+

【更新表的数据】

mysql> update students set Gender='M'where Name='Wu Yong';

【显示 以 X开头的名字】like ‘X%’

mysql> select * from students where Name like 'X%';
+-----+-------------+-----+--------+-----------+
| SID | Name        | Age | Gender | Tutor     |
+-----+-------------+-----+--------+-----------+
|   4 | Xue Baochai |  19 | F      | Rong Momo |
|   5 | Xia Yuhe    |  37 | F      | Shi Qian  |
+-----+-------------+-----+--------+-----------+

【显示 以 X开头的名字】增则表达式 RLIKE

mysql> select * from students where Name Rlike '^X.*';
+-----+-------------+-----+--------+-----------+
| SID | Name        | Age | Gender | Tutor     |
+-----+-------------+-----+--------+-----------+
|   4 | Xue Baochai |  19 | F      | Rong Momo |
|   5 | Xia Yuhe    |  37 | F      | Shi Qian  |
+-----+-------------+-----+--------+-----------+

【添加null值】

mysql> insert into students values(7,'tom',11,'M','jerry'),(8,'tomy',13,'M',null)；
mysql> select * from students；
+-----+-------------+-----+--------+-------------+
| SID | Name        | Age | Gender | Tutor       |
+-----+-------------+-----+--------+-------------+
|   1 | Guo Jing    |  27 | M      | Song Jiang  |
|   2 | Yang Guo    |  28 | M      | Hu Sanniang |
|   3 | Guo polu    |  21 | M      | Jii Baoyu   |
|   4 | Xue Baochai |  19 | F      | Rong Momo   |
|   5 | Xia Yuhe    |  37 | F      | Shi Qian    |
|   6 | Wu Yong     |  51 | M      | Lin Daiyu   |
|   7 | tom         |  11 | M      | jerry       |
|   8 | tomy        |  13 | M      | NULL        |
+-----+-------------+-----+--------+-------------+

【查找显示空值 】is null

mysql> select * from students where Tutor is null;
+-----+------+-----+--------+-------+
| SID | Name | Age | Gender | Tutor |
+-----+------+-----+--------+-------+
|   8 | tomy |  13 | M      | NULL  |
+-----+------+-----+--------+-------+

【查找显示非空值 】is not null

mysql> select * from students where Tutor is not null;
+-----+-------------+-----+--------+-------------+
| SID | Name        | Age | Gender | Tutor       |
+-----+-------------+-----+--------+-------------+
|   1 | Guo Jing    |  27 | M      | Song Jiang  |
|   2 | Yang Guo    |  28 | M      | Hu Sanniang |
|   3 | Guo polu    |  21 | M      | Jii Baoyu   |
|   4 | Xue Baochai |  19 | F      | Rong Momo   |
|   5 | Xia Yuhe    |  37 | F      | Shi Qian    |
|   6 | Wu Yong     |  51 | M      | Lin Daiyu   |
|   7 | tom         |  11 | M      | jerry       |
+-----+-------------+-----+--------+-------------+

【布尔条件表达式操作符】：
=
<=>
<>
<
<=
>
>=
IS NULL
IS NOT NULL
LIKE: 支持的通配符: %(任意长度的任意字符)， _（任意单个字符）
RLIKE, REGEXP: 支持使用正则表达式
IN: 判断指定字段的值是否在给定在列表中；
【组合条件测试】：
NOT, !
AND, &&
OR, ||
【多条件】BETWEEN ... AND ...: 位于指定的范围之间 where Age>25 and Gender='M'

mysql> select * from students where Age>25 and Gender='M';
mysql> select * from students where Age>25 && Gender='M';
mysql> select * from students where Age > 25 and Gender = 'M';
+-----+----------+-----+--------+-------------+
| SID | Name     | Age | Gender | Tutor       |
+-----+----------+-----+--------+-------------+
|   1 | Guo Jing |  27 | M      | Song Jiang  |
|   2 | Yang Guo |  28 | M      | Hu Sanniang |
|   6 | Wu Yong  |  51 | M      | Lin Daiyu   |
+-----+----------+-----+--------+-------------+

【显示 顺序】 order by Name ；asc升序；desc降序；

mysql> select * from students where Age>25 and Gender='M' order by Name desc;
+-----+----------+-----+--------+-------------+
| SID | Name     | Age | Gender | Tutor       |
+-----+----------+-----+--------+-------------+
|   2 | Yang Guo |  28 | M      | Hu Sanniang |
|   6 | Wu Yong  |  51 | M      | Lin Daiyu   |
|   1 | Guo Jing |  27 | M      | Song Jiang  |
+-----+----------+-----+--------+-------------+

【聚合函数】：
SUM(), AVG(), MAX(), MIN(), COUNT()个数

【计算年龄之和】

mysql> select sum(Age) from students;
+----------+
| sum(Age) |
+----------+
|      207 |
+----------+

【支持where】

mysql> select sum(Age) from students where Age>25 && Gender='M' ;
+----------+
| sum(Age) |
+----------+
|      106 |
+----------+

【 GROUP BY 分组：统计F、M 各个年龄之和】

mysql> select Gender,sum(Age) from students group by Gender;
+--------+----------+
| Gender | sum(Age) |
+--------+----------+
| F      |       56 |
| M      |      151 |
+--------+----------+

【插入一个字段】

mysql> alter table students add ClassID TINYINT UNSIGNED;
mysql> select * from students;
+-----+-------------+-----+--------+-------------+---------+
| SID | Name        | Age | Gender | Tutor       | ClassID |
+-----+-------------+-----+--------+-------------+---------+
|   1 | Guo Jing    |  27 | M      | Song Jiang  |    NULL |
|   2 | Yang Guo    |  28 | M      | Hu Sanniang |    NULL |
|   3 | Guo polu    |  21 | M      | Jii Baoyu   |    NULL |
|   4 | Xue Baochai |  19 | F      | Rong Momo   |    NULL |
|   5 | Xia Yuhe    |  37 | F      | Shi Qian    |    NULL |
|   6 | Wu Yong     |  51 | M      | Lin Daiyu   |    NULL |
|   7 | tom         |  11 | M      | jerry       |    NULL |
|   8 | tomy        |  13 | M      | NULL        |    NULL |
+-----+-------------+-----+--------+-------------+---------+

【update 数值 】

mysql> update students set ClassID=2 where SID=1;
mysql> select * from students;
+-----+-------------+-----+--------+-------------+---------+
| SID | Name        | Age | Gender | Tutor       | ClassID |
+-----+-------------+-----+--------+-------------+---------+
|   1 | Guo Jing    |  27 | M      | Song Jiang  |       2 |
|   2 | Yang Guo    |  28 | M      | Hu Sanniang |       3 |
|   3 | Guo polu    |  21 | M      | Jii Baoyu   |       3 |
|   4 | Xue Baochai |  19 | F      | Rong Momo   |       1 |
|   5 | Xia Yuhe    |  37 | F      | Shi Qian    |       2 |
|   6 | Wu Yong     |  51 | M      | Lin Daiyu   |       1 |
|   7 | tom         |  11 | M      | jerry       |       1 |
|   8 | tomy        |  13 | M      | NULL        |       4 |
+-----+-------------+-----+--------+-------------+---------+

【统计每个ClassID 对应的人数 】

mysql> select ClassID,count(*) from students group by ClassID;
+---------+----------+
| ClassID | count(*) |
+---------+----------+
|       1 |        3 |
|       2 |        2 |
|       3 |        2 |
|       4 |        1 |
+---------+----------+

【统计每个ClassID 对应的人数，并显示年龄之和 】

mysql> select ClassID,count(*) ,sum(Age) from students group by ClassID;
+---------+----------+----------+
| ClassID | count(*) | sum(Age) |
+---------+----------+----------+
|       1 |        3 |       81 |
|       2 |        2 |       64 |
|       3 |        2 |       49 |
|       4 |        1 |       13 |
+---------+----------+----------+

【having 统计显示，每个ClassID人数大于等于2的】

mysql> select ClassID from students group by ClassID having count(Name) >=2;
+---------+
| ClassID |
+---------+
|       1 |
|       2 |
|       3 |
+---------+

【统计显示，每个ClassID，年龄之和大于等于2的】

mysql> select ClassID from students group by ClassID having sum(Age) <=50;
+---------+
| ClassID |
+---------+
|       3 |
|       4 |
+---------+

【limit】
limit 一个参数就显示几行

mysql> select *from students limit 3;
+-----+----------+-----+--------+-------------+---------+
| SID | Name     | Age | Gender | Tutor       | ClassID |
+-----+----------+-----+--------+-------------+---------+
|   1 | Guo Jing |  27 | M      | Song Jiang  |       2 |
|   2 | Yang Guo |  28 | M      | Hu Sanniang |       3 |
|   3 | Guo polu |  21 | M      | Jii Baoyu   |       3 |
+-----+----------+-----+--------+-------------+---------+

limit 两个参数用，隔开，第一个为跳过，第二个显示行数

mysql> select *from students limit 2,4;
+-----+-------------+-----+--------+-----------+---------+
| SID | Name        | Age | Gender | Tutor     | ClassID |
+-----+-------------+-----+--------+-----------+---------+
|   3 | Guo polu    |  21 | M      | Jii Baoyu |       3 |
|   4 | Xue Baochai |  19 | F      | Rong Momo |       1 |
|   5 | Xia Yuhe    |  37 | F      | Shi Qian  |       2 |
|   6 | Wu Yong     |  51 | M      | Lin Daiyu |       1 |
+-----+-------------+-----+--------+-----------+---------+

练习：导入hellodb.sql，以下操作在students表上执行
1、以ClassID分组，显示每班的同学的人数；
2、以Gender分组，显示其年龄之和；
3、以ClassID分组，显示其平均年龄大于25的班级；
4、以Gender分组，显示各组中年龄大于25的学员的年龄之和；

1，mysql> select ClassID ,count(Name) from students group by ClassID;
+---------+-------------+
| ClassID | count(Name) |
+---------+-------------+
|       1 |           3 |
|       2 |           2 |
|       3 |           2 |
|       4 |           1 |
+---------+-------------+
2，mysql> select Gender,sum(Age) from students group by Gender;
+--------+----------+
| Gender | sum(Age) |
+--------+----------+
| F      |       56 |
| M      |      151 |
+--------+----------+
3，mysql> select ClassID,avg(Age) from students group by ClassID having avg(Age) >=25 ;
+---------+----------+
| ClassID | avg(Age) |
+---------+----------+
|       1 |  27.0000 |
|       2 |  32.0000 |
+---------+----------+
4，mysql> select Gender,sum(Age) from students where Age >=25 group by Gender ;
+--------+----------+
| Gender | sum(Age) |
+--------+----------+
| F      |       37 |
| M      |      106 |
+--------+----------+

【重复数据只显示一次】distinct

mysql> select ClassID from students;
+---------+
| ClassID |
+---------+
|       2 |
|       3 |
|       3 |
|       1 |
|       2 |
|       1 |
|       1 |
|       4 |
+---------+
mysql> select distinct ClassID from students;
+---------+
| ClassID |
+---------+
|       2 |
|       3 |
|       1 |
|       4 |
+---------+

【SELECT语句的执行流程：】
FROM clause --> WHERE clause --> GROUP BY --> HAVING clause --> ORDER BY ... --> SELECT --> LIMIT

【SELECT语句:】mysql> help select;
DISTINCT：指定的结果相同的只显示一次；mysql> help distinct;
SQL_CACHE：缓存于查询缓存中；
SQL_NO_CACHE：不缓存查询结果；