语法:
select 查询列表 from 表名;
类似于:System.out.println(打印东西);
特点:
USE myemployees;
select last_name
from employees;
select last_name,salary,email
from employees;
select
`employee_id`,
`first_name`,
`last_name`,
`phone_number`,
`last_name`,
`job_id`,
`phone_number`,
`job_id`,
`salary`,
`commission_pct`,
`manager_id`,
`department_id`,
`hiredate`
from employees;
select * from employees;
select 100;
select 'mike';
select 100*98;
select 100%98;
select VERSION();
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;
SELECT salary AS "out put" FROM employees;
SELECT DISTINCT department_id FROM employees;
java中的+号:
①运算符,两个操作数都为数值型
②连接符,只要有一个操作数为字符串
mysql中的+号:
仅仅只有一个功能:运算符
select 100+90; 两个操作数都为数值型,则做加法运算 结果:190
select '123'+90;只要其中一方为字符型,试图将字符型数值转换成数值型
如果转换成功,则继续做加法运算 结果:213
select 'john'+90; 如果转换失败,则将字符型数值转换成0 结果:90
select null+10; 只要其中一方为null,则结果肯定为null 结果:null
SELECT CONCAT('a','b','c') AS 结果;
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees;