SQL 进阶视频课程。Udacity: Intro to Relational Databases和 PostgreSQL语法文档。

Udacity: Intro to Relational Databases

The syntax of the select statement with a where clause:

select columns from tables where condition ;

keyword  + columns+  keyword +tables  + row restriction;

Columns are separated by commas，; use * to select all columns.

where 条件查询可以用and or not;  != 代表不等于

We can switch between the expression form (not X) and (not Y) and the form not (X or Y)

例子

select name from users where name !='ihower' and name !="roy";

select name from users where not(name = 'ihower') and not(name = 'roy');

comparison operators

< less than

> greater than

!= not equal

<= less than or equal

SQL uses = instead of == to represent equality.

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用SQL创建表格：例子：

Create table animals (

name text,

species text,

birthdate date

);

⚠️ ：In SQL we always put string and date values inside single quotes.

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SELECT clauses

where :表示