2.1 - 2.2 Basics of Filtering with SQL
2.1用 where 子句（clause）
SELECT column1, column2, columnN
FROM table_name
WHERE [condition]；运算符
| Operator | Description | |
|---|---|---|
| = | Equal | |
| > | Greater than | |
| < | Less than | |
| >= | Greater than or equal | |
| <= | Less than or equal | |
| <> | Not equal. Note: In some versions of SQL this operator may be written as != | |
| BETWEEN | Between a certain range | |
| LIKE | Search for a pattern | |
IS NULL | Null value |
查找不匹配项，筛选出除了Alice mutton以外的东西
2.2 用IN OR NOT 三个operator
2.2.1 用IN()
要用()将需要筛选的值框起来生成list。list里面只能用str，都需要用到"
SELECT * FROM Customers
WHERE City IN ('Paris','London');2.2.2 用 OR
如果满足第一个条件，就不会评估第二个条件（如果想要两个条件都满足需要用到 AND ）
SELECT column1, column2, ...
FROM table_name
WHERE condition1 OR condition2 OR condition3 ...; 对比OR 优先用IN：1不用考虑顺序 2运行速度更快 3可以在子查询中用另一个选择语句（后面有）
2.2.3 用 AND
SELECT column1, column2, ...
FROM table_name
WHERE condition1 AND condition2 AND condition3 ...; ！！sql会先运行AND 然后运行 OR ，如果想将OR单独运行要用到()
2.2.4 用 NOT
SELECT column1, column2, ...
FROM table_name
WHERE NOT condition; 2.3 通配符wildcards
| Symbol | Description | Example |
|---|---|---|
| % | Represents zero or more characters | bl% finds bl, black, blue, and blob |
| _ | Represents a single character | h_t finds hot, hat, and hit |
| [] | Represents any single character within the brackets | h[oa]t finds hot and hat, but not hit |
| ^ | Represents any character not in the brackets | h[^oa]t finds hit, but not hot and hat |
| - | Represents any single character within the specified range | c[a-b]t finds cat and cbt |
%wildcard不能匹配null
2.3.2 LIKE 运算符
SELECT * FROM Customers
WHERE City LIKE 'ber%'; 2.4 ORDER BY 排序
SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;
ORDER BY 2,3
--直接按照2、3列顺序排序ASC ascending升序
DESC descending降序
2.5 数字运算 + - * /
2.6 聚合函数 aggregate functions ： AVG() COUNT() MIN() MAX() SUM()
计算平均值 AVG()
了解表中有多少记录 COUNT()
最大值最小值
2.7 分组GROUP BY
NULL 会被分组成null类别，这样就不会丢失部分数据
having count ，确保只选择≥2的订单。
select, from, where, group by, having
