MySQL单表查询

一、单表查询的语法及关键字执行的优先级

1.1单表查询语法

SELECT DISTINCT 字段1,字段2... FROM 表名
                              WHERE 条件
                              GROUP BY field
                              HAVING 筛选
                              ORDER BY field
                              LIMIT 限制条数

1.2关键字执行的优先级

from：找到表
where：拿着where指定的约束条件，去文件/表中取出一条条记录
group by：将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组
select：执行select
distinct：去重
having：将分组的结果进行having过滤
order by：将结果按条件排序：order by
limit：限制结果的显示条数

二、简单查询

2.1建表和数据准备

company.employee
    员工id      id                  int             
    姓名        emp_name            varchar
    性别        sex                 enum
    年龄        age                 int
    入职日期     hire_date           date
    岗位        post                varchar
    职位描述     post_comment        varchar
    薪水        salary              double
    办公室       office              int
    部门编号     depart_id           int



# 创建表
create table employee(
id int not null unique auto_increment,
emp_name varchar(20) not null,
sex enum('male','female') not null default 'male',  #  大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int,  #  一个部门一个屋子
depart_id int
);


# 查看表结构
mysql> desc employee;
+--------------+-----------------------+------+-----+---------+----------------+
| Field        | Type                  | Null | Key | Default | Extra          |
+--------------+-----------------------+------+-----+---------+----------------+
| id           | int(11)               | NO   | PRI | NULL    | auto_increment |
| emp_name     | varchar(20)           | NO   |     | NULL    |                |
| sex          | enum('male','female') | NO   |     | male    |                |
| age          | int(3) unsigned       | NO   |     | 28      |                |
| hire_date    | date                  | NO   |     | NULL    |                |
| post         | varchar(50)           | YES  |     | NULL    |                |
| post_comment | varchar(100)          | YES  |     | NULL    |                |
| salary       | double(15,2)          | YES  |     | NULL    |                |
| office       | int(11)               | YES  |     | NULL    |                |
| depart_id    | int(11)               | YES  |     | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+

# 插入记录
# 三个部门：教学，销售，运营
insert into employee(emp_name,sex,age,hire_date,post,salary,office,depart_id) values
('nick','male',18,'20170301','老男孩驻上海虹桥最帅',7300.33,401,1),  #  以下是教学部
('jason','male',78,'20150302','teacher',1000000.31,401,1),
('sean','male',81,'20130305','teacher',8300,401,1),
('tank','male',73,'20140701','teacher',3500,401,1),
('oscar','male',28,'20121101','teacher',2100,401,1),
('mac','female',18,'20110211','teacher',9000,401,1),
('rocky','male',18,'19000301','teacher',30000,401,1),
('成龙','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),   #  以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('张野','male',28,'20160311','operation',10000.13,403,3),  #  以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;

#  ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk

# 简单查询
    SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;

    SELECT * FROM employee;

    SELECT emp_name,salary FROM employee;

# 避免重复DISTINCT
    SELECT DISTINCT post FROM employee;    

# 通过四则运算查询
    SELECT emp_name, salary*12 FROM employee;
    SELECT emp_name, salary*12 AS Annual_salary FROM employee;
    SELECT emp_name, salary*12 Annual_salary FROM employee;

# 定义显示格式
   CONCAT() 函数用于连接字符串
   SELECT CONCAT('姓名: ',emp_name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一个参数为分隔符
   SELECT CONCAT_WS(':',emp_name,salary*12)  AS Annual_salary 
   FROM employee;

   结合CASE语句：
   SELECT
       (
           CASE
           WHEN emp_name = 'mac' THEN
               emp_name
           WHEN emp_name = 'jason' THEN
               CONCAT(emp_name,'_BIGSB')
           ELSE
               concat(emp_name, 'SB')
           END
       ) as new_name
   FROM
       employee;

三、约束条件

where子句中可以使用：

比较运算符：> < >= <= <> !=
between 80 and 100 值在80到100之间
in(80,90,100) 值是80或90或100
like 'n%'
- 通配符可以是%或_，
  - %表示任意多字符
  - _表示一个字符
逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not

1. 单条件查询
    SELECT emp_name FROM employee
        WHERE post='sale';
        
2. 多条件查询
    SELECT emp_name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

3. 关键字BETWEEN AND
    SELECT emp_name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

    SELECT emp_name,salary FROM employee 
        WHERE salary NOT BETWEEN 10000 AND 20000;
    
4. 关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS)
    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment IS NULL;

    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment IS NOT NULL;
        
    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment=''; 注意''是空字符串，不是null
    ps：
        执行
        update employee set post_comment='' where id=2;
        再用上条查看，就会有结果了

5. 关键字IN集合查询
    SELECT emp_name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT emp_name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT emp_name,salary FROM employee 
        WHERE salary NOT IN (3000,3500,4000,9000) ;

6. 关键字LIKE模糊查询
    通配符’%’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'ni%';

    通配符’_’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'ja__';

四、分组(group by)

单独使用GROUP BY关键字分组
    SELECT post FROM employee GROUP BY post;
    注意：我们按照post字段分组，那么select查询的字段只能是post，想要获取组内的其他相关信息，需要借助函数

GROUP BY关键字和GROUP_CONCAT()函数一起使用
    SELECT post,GROUP_CONCAT(emp_name) FROM employee GROUP BY post;  # 按照岗位分组，并查看组内成员名
    SELECT post,GROUP_CONCAT(emp_name) as emp_members FROM employee GROUP BY post;

GROUP BY与聚合函数一起使用
    select post,count(id) as count from employee group by post;  #  按照岗位分组，并查看每个组有多少人

注意：如果我们用unique的字段作为分组的依据，则每一条记录自成一组，这种分组没有意义；多条记录之间的某个字段值相同，该字段通常用来作为分组的依据。

五、聚合函数

强调：聚合函数聚合的是组的内容，若是没有分组，则默认一组。

示例：

SELECT COUNT(*) FROM employee;
    SELECT COUNT(*) FROM employee WHERE depart_id=1;
    SELECT MAX(salary) FROM employee;
    SELECT MIN(salary) FROM employee;
    SELECT AVG(salary) FROM employee;
    SELECT SUM(salary) FROM employee;
    SELECT SUM(salary) FROM employee WHERE depart_id=3;

六、过滤(having)

6.1where和having的区别

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

6.1.1验证

mysql> select @@sql_mode;
+--------------------+
| @@sql_mode         |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
row in set (0.00 sec)

mysql> select * from emp where salary > 100000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | emp_name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
|  2 | jason | male |  78 | 2015-03-02 | teacher | NULL         | 1000000.31 |    401 |         1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
row in set (0.00 sec)

mysql> select post,group_concat(emp_name) from emp group by post having salary > 10000;#错误，分组后无法直接取到salary字段
ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
mysql> select post,group_concat(emp_name) from emp group by post having avg(salary) > 10000;
+-----------+-------------------------------------------------------+
| post | group_concat(emp_name) |
+-----------+-------------------------------------------------------+
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
| teacher | 成龙,rocky,mac,oscar,tank,sean,jason |
+-----------+-------------------------------------------------------+
rows in set (0.00 sec)

七、查询排序(order by)

按单列排序
    SELECT * FROM employee ORDER BY salary;
    SELECT * FROM employee ORDER BY salary ASC;  # 降序
    SELECT * FROM employee ORDER BY salary DESC; # 升序

按多列排序:先按照age排序，如果年纪相同，则按照薪资排序
    SELECT * from employee
        ORDER BY age,
        salary DESC;

八、限制查询的记录数(limit)

示例：

SELECT * FROM employee ORDER BY salary DESC 
    LIMIT 3;                    #默认初始位置为0 

SELECT * FROM employee ORDER BY salary DESC
    LIMIT 0,5; #从第0开始，即先出第一条，然后包含这一条在内往后查5条

SELECT * FROM employee ORDER BY salary DESC
    LIMIT 5,5; #从第5开始，即先出第6条，然后包含这一条在内往后查5条

九、使用正则表达式查询

SELECT * FROM employee WHERE emp_name REGEXP '^jas';

SELECT * FROM employee WHERE emp_name REGEXP 'on$';

SELECT * FROM employee WHERE emp_name REGEXP 'm{2}';


小结：对字符串匹配的方式
WHERE emp_name = 'nick';
WHERE emp_name LIKE 'sea%';
WHERE emp_name REGEXP 'on$';

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原文地址：https://www.cnblogs.com/Lin2396/p/11631499.html

MySQL单表查询

MySQL单表查询

一、单表查询的语法及关键字执行 的优先级

1.1单表查询语法

1.2关键字执行的优先级

二、简单查询

2.1建表和数据准备

三、约束条件

四、分组(group by)

五、聚合函数

六、过滤(having)

6.1where和having的区别

6.1.1验证

七、查询排序(order by)

八、限制查询的记录数(limit)

九、使用正则表达式查询

一、单表查询的语法及关键字执行的优先级