where、and、or
AND: 必须满足and前后所有的条件
select prod_id,prod_price,prod_name from products where vend_id = 1003 order by prod_price;
select prod_id,prod_price,prod_name from products where vend_id = 1003 and prod_price <= 10 order by prod_price;
OR：满足or 前后任意一个条件即可
select vend_id prod_id,prod_price,prod_name from products where vend_id = 1003 or prod_price <= 10 order by prod_price;
where语句中同时有and和or时，优先处理and
select prod_name,prod_price,vend_id from products where prod_price >= 10 and vend_id = 1003 or vend_id = 1002;
select prod_name,prod_price,vend_id from products where prod_price >= 10 and vend_id = 1002 or vend_id = 1003;
sql语句允许使用（）来凸显优先级
select prod_name,prod_price,vend_id from products where prod_price >= 10 and (vend_id = 1003 or vend_id = 1002)
-> order by
-> prod_price;
IN操作符
select prod_name,prod_price,vend_id from products where vend_id in (1003,1002) order by prod_name;
和 select prod_name,prod_price,vend_id from products where vend_id =1003 or vend_id = 1002 order by prod_name;效果相同，但是如果条件多点，用IN更方便；
NOT操作符 作用就是否定它之后的所有语句
select prod_name,prod_price,vend_id from products where vend_id not in (1003,1002) order by prod_name;
选择vend_id不是1002和1003的商品信息
