mysql数据库补充知识2 查询数据库记录信息之单表查询

',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g

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

    SELECT * FROM employee;

    SELECT name,salary FROM employee;

#避免重复DISTINCT
    SELECT DISTINCT post FROM employee;    

#通过四则运算查询
    SELECT name, salary*12 FROM employee;
    SELECT name, salary*12 AS Annual_salary FROM employee;
    SELECT 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: ',name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一个参数为分隔符
   SELECT CONCAT_WS(':',name,salary*12)  AS Annual_salary 
   FROM employee;

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　　2、WHERE约束

　　where字句中可以使用：

　　1. 比较运算符：> < >= <= <> !=
　　2. between 80 and 100 值在10到20之间
　　3. in(80,90,100) 值是10或20或30
　　4. like 'egon%'
　　　　 pattern可以是%或_，
%表示任意多字符
_表示一个字符
　　5. 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 name FROM employee
        WHERE post='sale';
        
#2:多条件查询
    SELECT name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

#3:关键字BETWEEN AND
    SELECT name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

    SELECT 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 name,post_comment FROM employee 
        WHERE post_comment IS NULL;

    SELECT name,post_comment FROM employee 
        WHERE post_comment IS NOT NULL;
        
    SELECT 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 name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT 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 name LIKE 'eg%';

    通配符’_’
    SELECT * FROM employee 
            WHERE name LIKE 'al__';

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　　3、分组查询:GROUP BY

　　1、什么是分组？为什么要分组？　　　

#1、首先明确一点：分组发生在where之后，即分组是基于where之后得到的记录而进行的

#2、分组指的是：将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等

#3、为何要分组呢？
    取每个部门的最高工资
    取每个部门的员工数
    取男人数和女人数

小窍门：‘每’这个字后面的字段，就是我们分组的依据


#4、大前提：
    可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要借助于聚合函数

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　　2、ONLY_FULL_GROUP_BY

#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

#！！！注意
ONLY_FULL_GROUP_BY的语义就是确定select target list中的所有列的值都是明确语义，简单的说来，在ONLY_FULL_GROUP_BY模式下，target list中的值要么是来自于聚集函数的结果，要么是来自于group by list中的表达式的值。


#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';

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

mysql> select * from emp group by post; 
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
| id | name | sex    | age | hire_date  | post                       | post_comment | salary     | office | depart_id |
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
| 14 | 张野 | male   |  28 | 2016-03-11 | operation                  | NULL         |   10000.13 |    403 |         3 |
|  9 | 歪歪 | female |  48 | 2015-03-11 | sale                       | NULL         |    3000.13 |    402 |         2 |
|  2 | alex | male   |  78 | 2015-03-02 | teacher                    | NULL         | 1000000.31 |    401 |         1 |
|  1 | egon | male   |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL         |    7300.33 |    401 |         1 |
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
4 rows in set (0.00 sec)


#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的

mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)

mysql> quit #设置成功后，一定要退出，然后重新登录方可生效
Bye

mysql> use db1;
Database changed
mysql> select * from emp group by post; #报错
ERROR 1055 (42000): 'db1.emp.id' isn't in GROUP BY
mysql> select post,count(id) from emp group by post; #只能查看分组依据和使用聚合函数
+----------------------------+-----------+
| post                       | count(id) |
+----------------------------+-----------+
| operation                  |         5 |
| sale                       |         5 |
| teacher                    |         7 |
| 老男孩驻沙河办事处外交大使 |         1 |
+----------------------------+-----------+
4 rows in set (0.00 sec)

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　　3、 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(name) FROM employee GROUP BY post;#按照岗位分组，并查看组内成员名
    SELECT post,GROUP_CONCAT(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

强调：

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

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四聚合函数

#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 HAVING过滤

HAVING与WHERE不一样的地方在于!!!!!!

#！！！执行优先级从高到低：where > group by > having 
#1. 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。

#2. 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

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

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

mysql> select * from emp having salary > 100000;
ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause

mysql> select post,group_concat(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(name) from emp group by post having avg(salary) > 10000;
+-----------+-------------------------------------------------------+
| post | group_concat(name) |
+-----------+-------------------------------------------------------+
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
| teacher | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |
+-----------+-------------------------------------------------------+
2 rows in set (0.00 sec)

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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;
    SELECT * FROM employee ORDER BY salary DESC;

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，即先查询出第一条，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，即先查询出第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 name REGEXP '^ale';

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

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


小结：对字符串匹配的方式
WHERE name = 'egon';
WHERE name LIKE 'yua%';
WHERE name REGEXP 'on$';

mysql数据库补充知识2 查询数据库记录信息之单表查询

一单表查询的语法

二关键字的执行优先级(重点)

三SELECT语句关键字的定义顺序

四 SELECT语句关键字的执行顺序

五测试sql语句的执行顺序

六执行顺序分析

3、分组查询:GROUP BY

六 HAVING过滤

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

mysql数据库补充知识2 查询数据库记录信息之单表查询

一 单表查询的语法

二 关键字的执行优先级(重点)

三SELECT语句关键字的定义顺序

四 SELECT语句关键字的执行顺序

五 测试sql语句的执行顺序

六 执行顺序分析

3、 分组查询:GROUP BY

六 HAVING过滤

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

二关键字的执行优先级(重点)

五测试sql语句的执行顺序

六执行顺序分析

　　3、分组查询:GROUP BY

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