一、SELECT--SQL基础

1.字符串连接，'||'

select last_name||job_id as 'employees' from employees;
select last_name||'is a'||job_id as 'employee details' from employees;

2.去除重复行distinct

用于字段前，原理是在临时表空间中排序，结果出来从大到小

select distinct dept_id from employees;