SQL Cookbook：检索记录

Posted 2020-09-16 一个后端狗

1、where子句中，and优先级比or高

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

等同于以下查询：

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

 

2、连接列值：concat

1 select concat(‘title: ‘, title, ‘ description: ‘, description) as info from film\G

 

3、使用case语句

1 select title, 
2 case 
3     when length <= 70 then ‘short‘ 
4     when length > 70 then ‘long‘ 
5 end 
6 as info from film where film_id = 1\G

 

4、随机选择n条数据

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

 

5、查找空值

1 select film_id, title from film where title is null\G

null要使用is

 

6、为空值设定默认值

1 select film_id, coalesce(title, ‘‘) as title from film\G