【Oracle】第一单元 select

select

• 列选择查询
• 查询的字段可以进行”加减乘除“
• null不可用、不赋值、不适用，加减乘除后还是null
• 连接
• distinct去除重复行

列选择查询

select last_name, salary from employees;

select * from employees;

查询的字段可以进行”加减乘除“

select last_name, salary,salary+300, salary*2 from employees;

null不可用、不赋值、不适用，加减乘除后还是null

select e.last_name, e.commission_pct, e.commission_pct+0.1, e.commission_pct*2 from employees e;

注：空的加减乘除后还是空的 null

连接

select last_name||job_id as "Employess" from employees;

select last_name||'is a'||job_id as "Employess" from employees;

distinct去除重复行

select distinct department_id from employees;