表名products，包含列name，price
简单查询：
搜索所有行所有列：
select * from products;
搜索指定列name
select name from products;
搜索指定列并为列取一个假名 名字
select name as 名字 from products;
搜索指定列并为表取一个假名items
select items.name from products as items;
比较查询 > < = >= <= !=
搜索大于1000的价格
select price from product where p;rice > 1000;
逻辑查询and or not
#and select * from products where price>1000 and price <4000; #or select * from products where price > 1000 or price <4000;#not select * from product wheere not price > 1000
范围查询
#between.... and... 闭区间查询连续的 select * from products where price between 1000 and 4000;#in 查询不连续的括号里的值 select * from products where price in (999, 1000,789);
非空判断：
#查询price是空值的 select * from products where price is null; #查询price不是空值的 select * from products where price is not null;