词汇：
case sensitive 区分大小写
quote 引号
aggregation 聚合
SQL：Structured Query Language
The language is comprised（包含、组成） of different classes（类） of commands
DML:Data Manipulation Language
- SELECT, INSERT, UPDATE, and DELETE statements
DDL:Data Definition Language
- Schema definitions for tables, indexes, views, and other objects.
DCL:Data Control Language (DCL)
- Security, access controls.
聚合函数
An aggregation function takes in a bag of tuples as its input and then produces a single scalar value as its output. Aggregate functions can (almost) only be used in a SELECT output list.
• AVG(COL): The average of the values in COL
• MIN(COL): The minimum value in COL
• MAX(COL): The maximum value in COL
• COUNT(COL): The number of tuples in the relation