1.基础查询

语法：
SELECT 要查询的东西
【FROM 表名】;

#查询employees表中所有字段
select * from employees;#查询employees表中的first_name字段
select first_name from employees;#查询employees表中的多个字段
select first_name,salary FROM employees;#查询常量值
select 100+123;
select "zhaoxr"#查询表达式
select 100*98;#查询函数
select version();#起别名
select 98*98 as result;
select last_name as "姓",first_name as "名" from employees;
select last_name "姓",first_name "名" from employees;#去重
select distinct department_id from employees;# +的作用
select 100+99;
select "123"+99;
select "zhaoxr"+"linux";
select null+16;# concat函数
select concat(last_name," ",first_name) "姓名" from employees;

类似于Java中 :System.out.println(要打印的东西);
特点：
① 通过select查询完的结果 ，是一个虚拟的表格，不是真实存在
② 要查询的东西 可以是常量值、可以是表达式、可以是字段、可以是函数

2.条件查询

条件查询：根据条件过滤原始表的数据，查询到想要的数据
语法：
select 要查询的字段|表达式|常量值|函数
from 表
where 条件 ;分类：
一、条件表达式示例：salary>10000条件运算符：> < >= <= = != <>二、逻辑表达式
示例：salary>10000 && salary<20000逻辑运算符：and（&&）:两个条件如果同时成立，结果为true，否则为falseor(||)：两个条件只要有一个成立，结果为true，否则为falsenot(!)：如果条件成立，则not后为false，否则为true三、模糊查询
like
between and
in
is null | is not null通配符：
%：任意多个字符，包含0个字符
_：单个字符
\：转义字符，"\_"代表字符_示例：last_name like 'a%'

# 查询工资大于等于12000的员工ID,姓名和薪水
select employee_id,CONCAT(last_name," ",first_name) as "姓名",salary 
from employees
where salary>=12000;# 查询部分编号不等于90的员工姓名和部门编号
select CONCAT(last_name," ",first_name) as "姓名",department_id
from employees
where department_id!=90;#查询工资在10000到20000之间的员工ID，姓名和工资
select employee_id,concat(last_name," ",first_name) as "姓名",salary
from employees
where salary>=10000 and salary<=20000;#查询员工姓中含有a的员工信息
select *
from employees
where last_name like "%a%";#查询员工姓第三个字母是l,第五个是e的员工信息
select *
from employees
where last_name like "__l_e%"; #查询员工姓的第二个字符是_的员工信息
select *
from employees
where last_name like "_\_%";#查询员工ID在100到200之间的员工信息,包含100和200
select *
from employees
where employee_id between 100 and 200;#查询工种编号是IT_PROG,PU_CLERK,ST_CLERK的员工信息
select *
from employees
where job_id in("IT_PROG","PU_CLERK","ST_CLERK");#查询奖金率为null的员工信息
select *
from employees
where commission_pct is null;

3.排序查询

语法：
select要查询的东西
from表
where 条件order by 排序的字段|表达式|函数|别名 【asc|desc】

#查询员工信息，工资由低到高排列
select *
from employees
order by salary asc;#查询员工信息，部门编号大于等于90，并且按照入职先后排列
select *
from employees
where department_id>=90
order by hiredate asc;#查询员工信息和年薪，并按照年薪从低到高排列
select *,salary*12*(1+ifnull(commission_pct,0)) as "年薪"
from employees
order by salary*12*(1+ifnull(commission_pct,0)) asc;#查询员工的姓名和工资，按照姓名的长度从小到大排列
select CONCAT(last_name," ",first_name) as "姓名",length(concat(last_name," ",first_name)) as "姓名长度",salary
from employees
order by length(concat(last_name," ",first_name)) asc;#查询员工信息，先按工资从低到高排名，再按员工编号从低到高排序
select *
from employees
order by salary asc,employee_id asc;

4.常见函数

4.1 单行函数

4.1.1 字符函数

	concat拼接substr截取子串upper转换成大写lower转换成小写trim去前后指定的空格和字符ltrim去左边空格rtrim去右边空格replace替换lpad左填充rpad右填充instr返回子串第一次出现的索引length 获取字节个数

#length函数|查询员工的姓，以及姓的长度
select last_name,length(last_name)
from employees;#concat函数|查询员工的姓名
select concat(last_name," ",first_name) as "姓名"
from employees;#upper函数和lower函数|查询员工的姓名，姓大写，名小写
select concat(upper(last_name)," ",lower(first_name)) as "姓名"
from employees;#substr截取函数|注意：截取函数从1开始
#返回"i love china"中的"china"
select substr("i love china",8);#返回"我爱你中国"中"中国"
select substr("我爱你中国",4);#返回"i love china"中的"love"
select substr("i love china",3,4);#instr返回子串第一次出现的索引|返回"i love china"中的'c'在第几个位置
select instr("i love china","c");#trim去空格|删除"  love   "左右的空格
select trim("   love   ");#trim去除指定的字符|删除"aaaaaloveaaaa"左右的'a'
select trim('a' from "aaaaaaaloveaaaaa");#lpad填充|使用'0'填充字符"我爱你",总字符数10个，填充结果为"0000000我爱你"
select lpad("我爱你",10,'0');#replace替换|将"我爱china",替换为"我爱中国"
select replace("我爱china","china","中国");

4.1.2 数学函数

	round 四舍五入rand 随机数floor向下取整ceil向上取整mod取余truncate截断

#数学函数
#round四舍五入|2.55四舍五入保留整数部分
select round(2.55);#ceil向上取整，返回>=该数的最小整数|取整2.0001,2.0000
select ceil(2.0001),ceil(2.0000);#floor向下取整，返回<=该数的最大整数|取整-2.0001,2.0000
select floor(-2.0001),floor(2.0000);#truncate截断，保留小数几位|1.99999保留两位小数
select truncate(1.99999,2);#mod取余|10/3的余数
select mod(10,3);

4.1.3 日期函数

	now当前系统日期+时间curdate当前系统日期curtime当前系统时间str_to_date 将字符转换成日期date_format将日期转换成字符

#时间函数
#查询当前时间
select now();#获取指定的年月日，获取当前年份,月份,日
select year(now()) as "年",month(now()) as "月",day(now()) as "日";#获取员工姓名和入职年份
select concat(last_name," ",first_name) as "姓名",year(hiredate) as "入职年份"
from employees;#str_to_date将日期格式的字符转换为规定的日期格式，规定的日期格式"2021-04-29"
select str_to_date("2021年4月29日","%Y年%m月%d日"),str_to_date("04/29/2021","%m/%d/%Y");#date_format将日期转换为自己想要的日期字符|查询员工的姓名和入职时间，入职时间显示为"2021年4月29日"的格式
select concat(last_name," ",first_name) as "姓名",date_format(hiredate,"%Y年%m月%d日") as "日期"
from employees;

4.1.4 流程控制函数

	if 处理双分支if(判断条件,真就执行此处，假就执行此处)case 要判断的字段或者表达式when 常量1 then 要显示的值1或者语句1when 常量2 then 要显示的值2或者语句2。。。。。else 要显示的值n或者语句nendcase when 条件1 then 要显示的值1或者语句1when 条件2 then 要显示的值2或者语句2。。。。。else 要显示的值n或者语句nend

#流程控制函数
#if函数|查询员工的姓，奖金率，如果有奖金，返回有奖金，没有奖金返回没奖金
select last_name,commission_pct,if(commission_pct is null,"没奖金","有奖金") as "奖金"
from employees;#case|查询员工的姓名，工资，部门ID#如果部门=90，salary*1.1;如果部门=100，salary*1.2;如果部门=110，salary*1.3;其它salary不变;并显示
select concat(last_name," ",first_name),department_id,salary as "原始工资",case department_idwhen 90 then salary*1.1when 100 then salary*1.2when 110 then salary*1.3else salaryend as "当前工资"
from employees;#case|查询员工的姓名，工资#如果工资小于10000，显示低工资；#如果工资大于等于10000，小于20000，显示中等工资；#如果工资大于等于20000，显示高工资
select concat(last_name," ",first_name),salary,case when salary<10000 then "低工资"when salary>=10000 and salary<20000 then "中等工资"when salary>20000 then "高工资"end as "工资区间"
from employees;

4.1.5 其他函数

	version版本database当前库user当前连接用户

#其它函数
#version查看版本号
select version();#database查看当前数据库
select database();#user查看当前用户
select user();

4.2 分组函数/统计函数/聚合函数

	sum 求和max 最大值min 最小值avg 平均值count 计数

#分组函数
#简单使用
select sum(salary) as "总工资",max(salary) as "最高工资",min(salary) as "最低工资",avg(salary) as "平均工资",count(salary) as "工资个数"
from employees;#查询有多少中工资
select count(distinct salary)
from employees;#查询employees表有多少行
select count(*) as "行数",count(1) as "行数"
from employees;

	特点：1、以上五个分组函数都忽略null值，除了count(*)2、sum和avg一般用于处理数值型max、min、count可以处理任何数据类型3、都可以搭配distinct使用，用于统计去重后的结果4、count的参数可以支持：字段、*、常量值，一般放1建议使用 count(*)

5.分组查询

语法：
select 查询的字段，分组函数
from 表
group by 分组的字段特点：
1、可以按单个字段分组
2、和分组函数一同查询的字段最好是分组后的字段
3、分组筛选针对的表	位置			关键字
分组前筛选：	原始表		group by的前面		where
分组后筛选：	分组后的结果集	group by的后面		having4、可以按多个字段分组，字段之间用逗号隔开
5、可以支持排序
6、having后可以支持别名

#查询每个部门的平均工资
select avg(salary),department_id
from employees
group by department_id;#查询每个工种的最高工资
select max(salary),job_id
from employees
group by job_id;#查询工资大于10000的每个工种的最高工资
select max(salary),job_id
from employees
where salary>10000
group by job_id;#查询哪个部门员工的个数大于2
select count(*),department_id
from employees
group by department_id
HAVING count(*)>2;#按员工的姓名长度分组，查询每一组员工的个数，筛选员工个数大于5的有哪些
select count(*),length(last_name) as "len_name"
from employees
group by length(last_name)
having count(*)>5;#查询每个部门每个工种的平均工资
select avg(salary),department_id,job_id
from employees
group by department_id,job_id;#查询每个部门每个工种的平均工资,并按照降序排列
select avg(salary),department_id,job_id
from employees
group by department_id,job_id
order by avg(salary) desc;