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  • 单表查询

    一 介绍

    本节内容:

    查询语法

    关键字的执行优先级

    简单查询

    单条件查询:WHERE

    分组查询:GROUP BY

    HAVING

    查询排序:ORDER BY

    限制查询的记录数:LIMIT

    使用聚合函数查询

    使用正则表达式查询

    创建表
    create table employee(
    id int not null unique auto_increment,
    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
    );
    #插入记录
    #三个部门:教学,销售,运营
    insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
    ('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部
    ('alex','male',78,'20150302','teacher',1000000.31,401,1),
    ('wupeiqi','male',81,'20130305','teacher',8300,401,1),
    ('yuanhao','male',73,'20140701','teacher',3500,401,1),
    ('liwenzhou','male',28,'20121101','teacher',2100,401,1),
    ('jingliyang','female',18,'20110211','teacher',9000,401,1),
    ('jinxin','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)
    ;

    二 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


    三 关键字的执行优先级(重点)

    from
    where
    group by
    having
    select
    distinct
    order by
    limit

    1.找到表:from

    2.按条件过滤表记录:where(过滤的结果是一条条记录)

    3.将查出的结果按条件分组:group by

    4.将分组的结果再次过滤:having(与where相同之处是都可以进行过滤,不同之处是having是基于分组之后的结果进行过滤,过滤的结果也是一个个的组)

    5.查出结果:select

    6.去重

    7.将6的结果按条件排序:order by

    8.将7的结果限制显示条数

    详细见:http://www.cnblogs.com/linhaifeng/articles/7372774.html

    简单查询

    #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;
    View Code

    五 单条件查询:WHERE

    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__';
    View Code

    六 分组查询:GROUP BY

    大前提:可以按照任意字段分组,但分完组后,只能查看分组的那个字段,要想取的组内的其他字段信息,需要借助函数

    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
    View Code
    #参考链接:http://www.ywnds.com/?p=8184
    #分组查询的常见问题:
    mysql> select id,count from tt group by id;
    ERROR 1055 (42000): Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'test.tt.count' which is 
    not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=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
    
    #去掉ONLY_FULL_GROUP_BY模式,如下操作:
    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';
    
    
    #!!!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。
    注意

    七 HAVING

    HAVING与WHERE用法一样
    select * from employee where salary > 10000;
    select * from employee having salary > 10000;
    
    不同之处在于:having是分组后过滤,而wehre是分组前过滤
    语法:having可以放到group by之后,而where只能放到group by之前
    select post,group_concat(name) from employee group by post having salary > 10000;#错误,分组后无法直接取到salary字段
    select post,group_concat(name) from employee group by post having avg(salary) > 10000;
    
    执行优先级:having比where的执行优先级更低
    select count(id) from employee where salary > 10000; #正确:where先执行,后执行select
    select count(id) from employee having salary > 10000;#错误:select先执行,后执行having,所有无法统计count
    
    #以上两条sql的顺序是
    #1:
    找到表employee--->用where过滤---->没有分组则默认一组--->select执行查看组内id数目
    #2
    找到表employee--->用where过滤---->没有分组则默认一组--->select执行查看组内id数目---->having 基于上一步得到的结果进行过滤,无法从组内组取到salary这个值

    八 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;

    九 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

    十 使用聚合函数查询

    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;

    十一 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$';



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  • 原文地址:https://www.cnblogs.com/pythonclass/p/7490420.html
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