Adapted from Chapter 2 on official documentation

1： 基础概念

– 关系型数据库管理系统（Relational Database Management System）RDBMS; Relation 是Table 对应的数学语言

– tables are grouped into databases, and a collection of databases managed by a single PostgreSQL server instance constitutes a database cluster(集群)

2: 建立新表格

– 在SQL命令中可以自由的使用空白（空格/tab/换行符）

– （–）双划线引入comment

– varchar(80) specify a data type that can store arbitrary character strings up to 80 characters in length

– real              type for store single precision floating-point numbers

3： 标准SQL Types

– int

– smallint

– real

– double precision

– char(N)

– varchar(N)

– date

– time

– timestamp

– interval

4: 删除表格

DROP TABLE tablename;

5: 向表格中加入行  INSERT

非数字的值，通常都在‘’ 内。COPY从文本文件中装载大量数据。

COPY weather FROM ‘/home/user/weather.txt’; (在表和文档之间拷贝数据)

6： 查询一个表 SELECT

AS —> relabel output column

WHERE —> specify which rows are wanted

ORDER BY； DISTINCT and etc.

7: 连接两张表

外链接（outer join）: left/right/full 取决于which part of row output you want to keep; 

8： 聚集函数 Aggregate Functions (count, sum, avg, max, min)

* aggregate function cannot be used in WHERE clause: solution —> subquery

GROUP BY: combination

HAVING: 过滤row

9: 更新 Updates; 删除 DELETE