标签:esc rom order 多行注释 筛选 rtm 没有 code 查看
1.查看指定数据库中的所有表
show tables from mysql;
2.查看自己在哪个数据库中
select database();
3.查看表的结构
desc user;
4.查看当前数据库版本
select version();
5.终端里查看
mysql --version 或 mysql -V
6.注释
(1)单行注释:#注释语句
(2)单行注释:-- 注释语句
(3)多行注释:/*注释语句*/
# 进阶1:基础查询
/*
1.语法
select 查询列表 from 表明;
2.特点
(1)查询列表可以是:表中的字段、常量值、表达式、函数
(2)查询的结果是一个虚拟的表格
*/
USE myemployees ;
# 1.单字段查询
SELECT `last_name` FROM employees;
# 2.多字段查询
SELECT `last_name`, `first_name`, `email` FROM employees;
# 3.所有字段查询
SELECT * FROM employees;
# 4.起别名,两种方式:AS或空格;如果别名有特殊符号用引号
SELECT 100%98 AS 结果;
SELECT last_name AS '姓', first_name AS '名' FROM employees;
SELECT last_name 姓, first_name 名 FROM employees;
# 5.去重
# 案例:查询员工表中涉及到所有部门编号
SELECT DISTINCT `department_id` FROM employees;
# + 作用:只作为数值相加
# 案例:员工姓、名连接成一个字段
SELECT CONCAT(last_name, first_name) AS '姓名' FROM employees;
# 进阶2:条件查询
/*
1.语法
select 查询列表 from 表名 where 筛选条件;
2.分类
(1)按条件表达式筛选[> = < != <> >= <=]
(2)按逻辑表达式筛选[&& || ! and or not]
(3)按模糊筛选[like、between and、in、is null]
*/
# 一、条件查询
# 案例1:查询工资大于12000员工信息
SELECT * FROM employees WHERE salary > 12000;
# 案例2:查询部门编号!=90的员工名和部门编号
SELECT first_name, department_id FROM employees WHERE department_id<>90;
# 二、逻辑表达式
# 案例1:查询工资在10000到20000之间的员工名、工资以及奖金
SELECT first_name, salary, commission_pct FROM employees WHERE salary >=10000 AND salary <=20000;
# 案例2:查询部门编号不是90到110之间,或者工资高于15000的员工信息
SELECT * FROM employees WHERE NOT(department_id >= 90 AND department_id <= 110) OR salary >15000;
# 三、模糊查询
/*
通配符:
① % 任意多个字符,包含0个字符
② _ 任意单个字符
*/
# 案例1:查询员工名中包含字母a的员工信息(like关键字)
SELECT * FROM employees WHERE last_name LIKE '%a%'
# 案例2:查询员工名中第三个字母n,第五个字符为l的员工名、工资
SELECT last_name, salary FROM employees WHERE last_name LIKE '__n_l%';
# 案例3:查询员工名中第二个字符为 _ 的员工名,需要使用\对_进行转义
SELECT last_name, salary FROM employees WHERE last_name LIKE '_\_%';
# 案例4:查询员工编号在100到120之间的员工信息(between and 是包含临界值的)
SELECT * FROM employees WHERE employee_id BETWEEN 100 AND 120;
# 案例5:查询员工的工种ID是IT_PROG/AD_VP/PU_MAN中之一的员工名工种ID(in 关键字)
SELECT last_name, job_id FROM employees WHERE job_id IN ('IT_PROG', 'AD_VP', 'PU_MAN');
# 案例6:查询没有奖金的员工名和奖金率(is null、is not null)
SELECT last_name, commission_pct FROM employees WHERE commission_pct IS NULL;
SELECT last_name, commission_pct FROM employees WHERE commission_pct IS NOT NULL;
# 案例7:安全等于
SELECT last_name, commission_pct FROM employees WHERE commission_pct <=> NULL;
SELECT last_name, commission_pct FROM employees WHERE commission_pct <=> 0.20;
/*
IS NULL:仅仅可以判断NULL值(√)
<=>:既可判断NULL值,也可判断数值
*/
# 查询员工号为176的员工姓名和部门号、年薪(如果利率为NULL则为0计算)
SELECT
last_name, department_id, salary*12*(1+IFNULL(commission_pct, 0)) AS annual_salary
FROM
employees
WHERE
employee_id=176;
# 进阶3:排序查询
/*
语法:
select 查询列表 ③
from 表 ①
where 筛选条件 ②
order by 排序列表[asc | desc] ④
特点:
① 如果不写,默认升序
② order by 支持单个字段、多个字段、表达式、函数、别名
③ order by 一般放查询语句最后面,limit子句除外
*/
# 案例1:查询员工信息,要求工资从高到低排序(asc升序,desc降序)
SELECT * FROM employees ORDER BY salary DESC;
SELECT * FROM employees ORDER BY salary ASC;
# 案例2:查询部门编号>=90的员工信息,按入职时间的先后进行排序
SELECT * FROM employees WHERE department_id >= 90 ORDER BY hiredate ASC;
# 案例3;按年薪高低显示员工信息和薪资
SELECT *, salary*12*(1+IFNULL(commission_pct, 0)) AS annual_salary
FROM employees
ORDER BY annual_salary DESC;
# 案例4:按员工名长度显示员工姓名和工资
SELECT last_name, salary, LENGTH(last_name) AS name_length
FROM employees
ORDER BY name_length DESC;
# 案例5:查询员工信息,要求先按工资排序,再按员工编号排序
# 先按照工资升序,如果工资相同再按照员工id降序
SELECT * FROM employees ORDER BY salary ASC, employee_id DESC;
标签:esc rom order 多行注释 筛选 rtm 没有 code 查看
原文地址:https://www.cnblogs.com/hq82/p/12240741.html