SQL3:

1. Using comments

use--(two dashes) to comment out everything to the right of them on a given line

leave comments across multiple lines using /* to begin the comment and */ to close it(以/*开始，以*/结束)

可以用comments对query中可能产生的错误进行标注，如在SELECT中任一list前加--（two dashes）可使该list不产生results，以检查是否该list出现了错误（debug）

Basic SQL结束

二. Intermediate SQL

1. SQL Aggregate Functions

Arithmetic operators only perform operations across rows. Aggragate functions are used to perform operations across entire columns.

COUNT: SELECT COUNT (*) 或 (1)

不包含空值（not include null value）

SUM: only used on columns containing numerical values;

only aggragate vertically;

treats nulls as 0

MIN/MAX: SQL aggregation functions that return the lowest and highest values in a particular column.

MIN will return the lowest number, earliest date, or non-numerical value as close alphabetically to "A" as possible.

MAX returns the highest number, the latest date, or the non-numerical value closest alphabetically to "Z"

AVG: only be used on numerical columns and ignores nulls completely

2. GROUP BY: aggregate only part of a table, separate data into groups, which can be aggregated independently of one another

GROUP by multiple columns- separate column names with commas.

Using GROUP BY with ORDER BY(把GROUP BY的结果进行排序整理)

最后可加LIMIT限制显示数据结果

注：GROUP BY出现的部分必须要出现在SELECT中（SELECT要与GROUP BY的内容一致）

3. HAVING: to filter a query that has been aggregated.（作用类似于WHERE，但WHERE doesn't allow you to filter on aggregate columns)，一般用在GROUP BY之后

WHERE在aggregate之前使用，HAVING在aggregate之后使用

Query clause order: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY, LIMIT

4. CASE WHEN... THEN: SQL's way of handling if/then logic

always goes in the SELECT clause; every CASE statement must end with the END statement; the ELSE statement is optional, and provides a way to capture values not specified in the WHEN/THEN statements.

常见：CASE WHEN... THEN... (ELSE) END