SQL Cookbook：检索记录

1、where子句中，and优先级比or高

select * from film where film_id < 10 or title like '%ARK' and length = 50G

等同于以下查询：

select * from film where film_id < 10 or (title like '%ARK' and length = 50)G

2、连接列值：concat

select concat('title: ', title, ' description: ', description) as info from filmG

3、使用case语句

select title, 
case 
    when length <= 70 then 'short' 
    when length > 70 then 'long' 
end 
as info from film where film_id = 1G

4、随机选择n条数据

select film_id, title from film order by rand() limit 5G

5、查找空值

select film_id, title from film where title is nullG

null要使用is

6、为空值设定默认值

select film_id, coalesce(title, '') as title from filmG