MySQL怎么进行单表查询？单表查询的语句

首先创建数据表

# 创建表
    mysql> create table company.employee5(
    id int primary key AUTO_INCREMENT not null,
    name varchar(30) not null,
    sex enum('male','female') default 'male' not null,
    hire_date date not null,
    post varchar(50) not null,
    job_description varchar(100),
    salary double(15,2) not null,
    office int,
    dep_id int
    );
    
# 插入数据
    mysql> insert into company.employee5(name,sex,hire_date,post,job_description,salary,office,dep_id) values 
    ('jack','male','20180202','instructor','teach',5000,501,100),
    ('tom','male','20180203','instructor','teach',5500,501,100),
    ('robin','male','20180202','instructor','teach',8000,501,100),
    ('alice','female','20180202','instructor','teach',7200,501,100),
    ('tianyun','male','20180202','hr','hrcc',600,502,101),
    ('harry','male','20180202','hr',NULL,6000,502,101),
    ('emma','female','20180206','sale','salecc',20000,503,102),
    ('christine','female','20180205','sale','salecc',2200,503,102),
    ('zhuzhu','male','20180205','sale',NULL,2200,503,102),
    ('gougou','male','20180205','sale','',2200,503,102);
    
# 查看表结构
    mysql> desc employee5;
+-----------------+-----------------------+------+-----+---------+----------------+
  Field             Type                    Null   Key   Default   Extra           
+-----------------+-----------------------+------+-----+---------+----------------+
  id                int(11)                 NO     PRI   NULL      auto_increment  
  name              varchar(30)             NO           NULL                      
  sex               enum('male','female')   NO           male                      
  hire_date         date                    NO           NULL                      
  post              varchar(50)             NO           NULL                      
  job_description   varchar(100)            YES          NULL                      
  salary            double(15,2)            NO           NULL                      
  office            int(11)                 YES          NULL                      
  dep_id            int(11)                 YES          NULL                      
+-----------------+-----------------------+------+-----+---------+----------------+

查询语法

SELECT 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；

查看表中所有数据

mysql> select * from employee5;
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
  id   name        sex      hire_date    post         job_description   salary     office   dep_id  
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
   1   jack        male     2018-02-02   instructor   teach              5000.00      501      100  
   2   tom         male     2018-02-03   instructor   teach              5500.00      501      100  
   3   robin       male     2018-02-02   instructor   teach              8000.00      501      100  
   4   alice       female   2018-02-02   instructor   teach              7200.00      501      100  
   5   tianyun     male     2018-02-02   hr           hrcc                600.00      502      101  
   6   harry       male     2018-02-02   hr           NULL               6000.00      502      101  
   7   emma        female   2018-02-06   sale         salecc            20000.00      503      102  
   8   christine   female   2018-02-05   sale         salecc             2200.00      503      102  
   9   zhuzhu      male     2018-02-05   sale         NULL               2200.00      503      102  
  10   gougou      male     2018-02-05   sale                            2200.00      503      102  
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
10 rows in set (0.00 sec)

简单查询

简单查询

mysql> SELECT * FROM employee5;

mysql> SELECT name, salary, dep_id FROM employee5;

去重DISTINCT

mysql> SELECT post FROM employee5;

mysql> SELECT DISTINCT post  FROM employee5;

注：不能部分使用DISTINCT，通常仅用于某一字段。

通过四则运算查询

mysql> SELECT name, salary, salary*14 FROM employee5;

mysql> SELECT name, salary, salary*14 AS Annual_salary FROM employee5;

mysql> SELECT name, salary, salary*14 Annual_salary FROM employee5;

定义显示格式

CONCAT() 函数用于连接字符串

mysql> SELECT CONCAT(name, ' annual salary: ', salary*14)  AS Annual_salary FROM employee5;

条件查询

a、语法
    select * from 表名 where 条件
    
b、比较运算符
    大于    小于    大于等于    小于等于    不等于    >        <        >=            <=            !=或<>

c、逻辑运算符
    并且    或者    非    and        or        not

d、模糊查询
    like    %           表示任意多个任意字符
    _           表示一个任意字符

e、范围查询    in                  表示在一个非连续的范围内
    between...and...    表示在一个连续的范围内

f、空判断
    判断空：is null
    判断非空：is not null 

g、优先级
    小括号,not 比较运算符， 逻辑运算符
    and比or优先级高，如果同时出现并希望先选or，需要结合()来使用

单条件查询

mysql> SELECT name,post FROM employee5 WHERE post='hr';

多条件查询

mysql> SELECT name,salary FROM employee5 WHERE post='hr' AND salary>5000;

关键字 BETWEEN AND查询

mysql> SELECT name,salary FROM employee5 WHERE salary BETWEEN 5000 AND 15000;

mysql> SELECT name,salary FROM employee5 WHERE salary NOT BETWEEN 5000 AND 15000;

关键字 IS NULL 查询

mysql> SELECT name,job_description FROM employee5 WHERE job_description IS NULL;

mysql> SELECT name,job_description FROM employee5 WHERE job_description IS NOT NULL;

mysql> SELECT name,job_description FROM employee5 WHERE job_description='';

关键字IN集合查询

mysql> SELECT name, salary FROM employee5 WHERE salary=4000 OR salary=5000 OR salary=6000 OR salary=9000 ;

mysql> SELECT name, salary FROM employee5 WHERE salary IN (4000,5000,6000,9000) ;

mysql> SELECT name, salary FROM employee5 WHERE salary NOT IN (4000,5000,6000,9000) ;

关键字LIKE模糊查询

通配符’%’
mysql> SELECT * FROM employee5 WHERE name LIKE 'al%';

通配符’_’
mysql> SELECT * FROM employee5 WHERE name LIKE 'al___';

查询排序

按单列排序

mysql> SELECT * FROM employee5 ORDER BY salary;

mysql> SELECT name, salary FROM employee5 ORDER BY salary ASC;

mysql> SELECT name, salary FROM employee5 ORDER BY salary DESC;

按多列排序

mysql> SELECT * FROM employee5 ORDER BY hire_date DESC,salary ASC;

# 先按入职时间，再按薪水排序
mysql> SELECT * FROM employee5 ORDER BY hire_date DESC, salary DESC;

# 先按职位，再按薪水排序
mysql> SELECT * FROM employee5 ORDER BY post, salary DESC;

分页查询 limit

mysql> SELECT * FROM employee5 ORDER BY salary DESC LIMIT 5;      //默认初始位置为0

mysql> SELECT * FROM employee5 ORDER BY salary DESC LIMIT 0,5;

mysql> SELECT * FROM employee5 ORDER BY salary DESC LIMIT 3,5;      //从第4条开始，共显示5条

聚合函数查询

    a、count(*)       表示计算总行数，括号中可以写*和列名
    b、max(列)        表示求此列的最大值
    c、min(列)        表示求此列的最小值
    d、sun(列)        表示求此列的和
    e、avg(列)        表示求此列的平均值
    
mysql> SELECT COUNT(*) FROM employee5;
mysql> SELECT COUNT(*) FROM employee5 WHERE dep_id=101;
mysql> SELECT MAX(salary) FROM employee5;
mysql> SELECT MIN(salary) FROM employee5;
mysql> SELECT AVG(salary) FROM employee5;
mysql> SELECT SUM(salary) FROM employee5;
mysql> SELECT SUM(salary) FROM employee5 WHERE dep_id=101;

分组查询

单独使用GROUP BY关键字分组
mysql> SELECT post FROM employee5 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

# 按照id分组，并查看组内成员
mysql> SELECT dep_id,GROUP_CONCAT(name) FROM employee5 GROUP BY dep_id;

mysql> SELECT dep_id,GROUP_CONCAT(name) as emp_members FROM employee5 GROUP BY dep_id;

GROUP BY和集合函数一起使用

# 按照dep_id 分组， 并计算组内成员工资总和
mysql> SELECT dep_id,SUM(salary) FROM employee5 GROUP BY dep_id;

# 按照dep_id分组，并计算组内成员工资平均值
mysql> SELECT dep_id,AVG(salary) FROM employee5 GROUP BY dep_id;

正则表达式查询

mysql> SELECT * FROM employee5 WHERE name REGEXP '^j';

mysql> SELECT * FROM employee5 WHERE salary REGEXP '[5]+.*';

mysql> SELECT * FROM employee5 WHERE salary REGEXP '[5]{2}.*';

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