MySQL用数字辅助表实现复杂的列变行_MySQL
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MySQL用数字辅助表实现复杂的列变行
某天，QQ上一个圈子内的网友发来一个求助信息 ：
PropID name Amount3 元宝 22 乐豆 3
能否用一条sql 写成输出，就是复制对应Amount 的条数 ：
PropID name3 元宝3 元宝2 乐豆2 乐豆2 乐豆
研究了下，用普通的group by sum if case when之类的搞不定，得换种思路，一时hang住了，就与网友Mr.W说起，他提供了数字辅助表的思路。
OK，搞定了，SQL如下：
select t1.propid,t1.name from tb t1,(select 1 as num union select 2 as num union select 3 as num ) t2 where t1.amount>= t2.num order by 1;
中间的,
(select 1 as num union select 2 as num union select 3 as num ) t2
最好你另外建一个num单列辅助表表，从1自增到N，给个几万就好，然后关联查方便一些自连接的sql判断，select貌似也没循环生成数字的功能。如下表nums
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