本人SQL新手，五一期间自学了《SQL必知必会》一书，在此简要分享学习心得，若有差错，请各位大佬们多多指教呀！

本人的SQL学习计划是先根据《SQL必知必会》一书入门，了解SQL的整体框架，语法等，在闲暇时间刷题巩固。

首先奉上本人整理的SQL涉及的功能及语法的思维导图，思维导图中整理了各个SQL功能以及实现功能的代码和函数等，还在不断完善更新中~（本人新手，可能有部分功能理解有误，请大家不吝赐教，(●'◡'●)）

另外，本着学以致用的心态，本人在SQL学习过程中，坚持码代码实现相关功能，文末也将奉上本人编写的SQL代码以及相关功能说明~（工具是SQLyog，思路依据的是《SQL必知必会》一书），老规矩，同学们觉得有问题或有优化空间的话，请多多留言指教哇！

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/* 检索固定位置和固定行数

SELECT *

FROM employees;

Select distinct employee_id

from employees;

select employee_id

from employees

limit 4,4

order by employee_id; */

/* 多列排序 第一个排序列的值不唯一，需要多列区分

select employee_id, first_name, last_name

from employees

order by first_name,last_name;

*/

/* 升序排序和降序排序

select first_name, last_name, salary

from employees

order by salary desc;

*/

/* 找到工资3000的员工；where 过滤指定的是行。

select first_name,last_name,salary

from employees

where salary >= 3000;

*/

/* 不匹配查询--字符串要用单引号引用

select

first_name,last_name,salary

from

employees

where job_id = 'AD_VP' ;

*/

/* 范围查询

select first_name,last_name,salary

from employees

where salary between 3000 and 5000

order by salary desc;

*/

/* 查询空值

select first_name,last_name,email

from employees

where email is null;

*/

/*逻辑操作符*/

/* AND 操作符

select first_name,last_name,salary,job_id

from employees

where job_id = 'IT_PROG' and salary >5000

order by salary;

*/

/* OR 操作符

select salary,first_name,last_name,job_id

from employees

where job_id ='IT_PROG'

or salary >=10000

order by salary;

*/

/* 求值顺序--AND的优先级更高，会优先和其它语句组合，类似数学运算的乘除；

OR类似加减，

可用圆括号（优先级更高）强制改变运算顺序

select job_id, first_name,last_name,salary

from employees

where (job_id = 'IT_PROG'

or job_id = 'AD_VP')

and salary > 6000;

*/

/* IN 的用法————类似“集合”概念；完成类似 ”OR“语句的操作，学会使用“IN”替代“OR”

select first_name,last_name,salary,job_id

from employees

where job_id in ('AD_VP','IT_PROG')

order by salary;

*/

/*

select first_name,last_name,salary,job_id

from employees

where not job_id in ('AD_VP','IT_PROG')

and salary > 10000

order by salary;

*/

/* 通配符% 匹配多个字符 搜百分号放字符前面表示以百分号后的字符结尾的量，放字符后面表示搜索以字符开头的量

select *

from jobs

where job_title like '%manager%'

order by min_salary;

*/

/*通配符 下划线 一个下划线只匹配单个字符而不是多个字符

select *

from jobs

where job_title like '__________ Manager';

*/

/*组合使用

select *

from employees

where first_name NOT like 'N%'

or first_name Not like 'L%'

order by salary desc;

*/

/* 使用concat拼接两列,并指定别名

select concat(first_name,'(',last_name,')')

from employees

as name

order by salary;

*/

/* 返回当前日期

select now()

*/

/* 文本处理，改变字符串首字母大小写

select first_name,lower(first_name)as first_name_locase

from employees

order by first_name_locase;

*/

/*搜索发音相同的字符

select first_name

from employees

where soundex(first_name)=soundex('vali');

*/

/* 提取时间 年

select first_name,last_name,salary

from employees

where year(hiredate)=2004

order by salary;

/* 提取时间 月

select first_name,last_name,salary

from employees

where month(hiredate)=3

order by salary;

*/

/* 提取时间 日

select first_name,last_name,salary

from employees

where day(hiredate)=3

order by salary;

*/

/* 求平均值，列名不是字符串

select avg(salary)

as avg_salary

from employees;

*/

/* 计数函数的使用，是否将NULL计算在内

select count(*)

from employees;

select count(manager_id)

from employees;

*/

/*最大值 最小值函数使用

select max(first_name)

from employees;

select min(first_name)

from employees;

*/

/*分组数据，Group by和Having*/

/* group by 使用

select job_id,count(*)as num_job

from employees

group by job_id

*/

/* having函数使用，对“列”过滤,再排序

select job_id,count(*)as num_job

from employees

group by job_id

having num_job>=10

order by num_job;

*/

/* 子查询

select first_name,last_name,salary

from employees

where department_id in (select department_id

from departments

where location_id = 1700);

*/

/*作为计算字段使用子查询

select department_name,

manager_id,

(select count(*)

from employees

where employees.department_id=departments.department_id) as depar

from departments

order by manager_id;

*/

/* 内联结 选定的列位于不同的表，如何选取？如何使用表名的缩写-表别名？

select d.`department_name`,e.first_name,e.last_name

from `departments` as d,employees as e

where d.`department_id` = e.`department_id`;

*/

/* 自联结 同一张表中寻找拥有同种属性的对象--问题：相同列会出现多次,下面代码结果同一个对象重复出现

select e1.`employee_id`,e1.first_name,e1.last_name,e1.department_id

from employees as e1,employees as e2

where e1.`department_id`=e2.`department_id`

and e2.`department_id`=100;

*/

/*自然联结--避免自联结结果项重复出现，重点在于采用另一张明确的表来联结

select e.`department_id`,e.`first_name`,e.`last_name`,e.`salary`

from employees as e,departments as d

where e.`department_id`=d.`department_id`

and d.`department_id`=100;

*/

/* 外联结-关键：可以显示没有关联行的行；注意：left outer join指出语句左边表（默认按左边表排序），right outer join指出语句右边表（默认按右边表排序）

select e.`employee_id`,e.`first_name`,e.`last_name`,d.`department_name`,e.`department_id`

from employees as e left outer join departments as d

on e.`department_id`=d.`department_id`;

*/

/*使用带聚集函数的联结。

select d.`department_name`,e.`employee_id`,

count(e.`department_id`) as num_emp

from employees as e inner join departments as d

on e.`department_id`=d.`department_id`

group by e.department_id;

*/

/* 组合查询，多个查询条件，类似OR的功能，在面对多个表调用时，采用这个语句，结构会清晰一些。

select e.`employee_id`,e.`first_name`,e.`last_name`,e.`email`,e.`department_id`

from employees as e

where e.`department_id` in (100,110,120)

union

select e.`employee_id`,e.`first_name`,e.`last_name`,e.`email`,e.`department_id`

from employees as e

where e.`first_name`='Steven'

order by e.`department_id`;

*/

/* 用OR语句替代上述组合查询语句，实现相同功能

select e.`employee_id`,e.`first_name`,e.`last_name`,e.`email`,e.`department_id`

from employees as e

where e.`department_id` in (100,110,120)

or e.`first_name`='Steven'

order by e.`department_id`;

*/

/* 插入

insert into employeess(*)

values(01,

'Lynn',

'Zheng',

'lingling66zh',

'1888',

'AD_VP',

17000,

null,

null,

100,

2020-08-25);

*/

/*插入检索的数据

insert into employees(*)

select(*)

from another_tbl;

select *

into custcopy

from customer

*/

/*更新数据

update employees

set employee_id =250

where employee_id =1;

*/

/*删除数据-注意：这里删除的是整行，部分删除要使用update

delete from employees

where employee_id = 250;

*/

/*创建表，必须给出三个项： 列名，数据类型，是否允许值为null；如何给定默认值

create table test.product250

(prod_id char(10) not null,

department_id int(4) default 1,

prod_name char(254) default 1);

*/

/*更改设定表,添加用add，删除用drop column 列名

alter table test.`product250`

add prod_price int(10) default 1;

*/