select*from employees; select first_name from employees; select first_name as hhh from employees; select first_name hhh from employees; selectdistinct department_id from employees; select ifnull(commission_pct, 0) as 奖金率, commission_pct from employees; select concat(first_name,'___' ,last_name,'___',ifnull(commission_pct, 0)) as output from employees; select100; select'xym'; select100*20as resulet; select version();
#------------------------------------------------------------------------------------------------------------------------------------ /* select 查询列表 from 表名 where 筛选条件; */
select*from employees where salary>12000; select last_name, department_id from employees where department_id <>90;
select last_name, salary, commission_pct from employees where salary >=10000and salary <=20000;
select* from employees wherenot(department_id <90or department_id>110) or salary >15000;
/*通配符:%任意多个字符 _任意单个字符 */ select*from employees where last_name like'%a%'; select*from employees where last_name like'_e_l';
select* from employees where last_name like'_$_%'escape'$';*//*查询第二个字符为_的员工,转义*/
select* from employees where employee_id between100and120; /*包含临界值(闭区间),不能颠倒大小*/
select last_name, job_id from employees #where job_id='IT_PROT'or job_id='AD_VP'or job_id ='AD_PRES'; where job_id in('IT_PROT','AD_VP','AD_PRES')
/* is null:仅可以判断 null <=>:既可以判断 null,又可以判断数值 */
select last_name, commission_pct from employees #where commission_pct isnull; where commission_pct isnotnull;
select last_name, commission_pct from employees where commission_pct <=>null; /*安全等于*/
select last_name, commission_pct, salary from employees where salary <=>12000; /*安全等于*/