5.1SELECT语句

Oracle

SELECT语句

单表查询

SELECT Column1, Column2 
from table1

多表连接查询

SELECT table1.Column1,table2.Column2 
from table1,table2

对选择可运算的列进行运算操作

select salary, salary * 2  
from employees

NULL

NULL表示 不可用、未赋值、不知道、不适用，它既不是0 也不是空格。

给查询的列起别名

select last_name 
AS name, employee_id id  
from employees

select last_name AS "Name", salary * 2 "Annual Salary"  
from employees

字符串连接操作符

SELECT last_name, job_id AS "Employees"
FROM employees;

SELECT last_name||job_id AS "Employees"
FROM employees;

SELECT last_name ||' is a '||job_id AS "Employee Details"
FROM employees;

DISTINCT去除重复行

默认情况，返回所有行，包括重复行

SELECT department_id 
FROM employees;

使用DISTINCT消除重复结果行

SELECT DISTINCT department_id 
FROM employees;