mysql中in 做条件匹配 带逗号的字符串 数据异常解决

在数据库中表doctor中cancer字段存放着以逗号分隔的外键id

select cancer from doctor where id=1;

数据如下
[table]
|cancer
|1,2,3
[/table]
我需要在表cancer_type中匹配刚刚的外键,
于是用FIND_IN_SET函数:

select * from cancer_type c where FIND_IN_SET(c.cancer_id, (select cancer from doctor where id=1) )

数据如下
[table]
|cancer_id|cancer_name|cancer_prefix
|1|肺癌|Lung
|2|结直肠癌|Col
|3|胃癌|Sto
[/table]

用in来匹配却不行

select * from cancer_type where cancer_id in (select cancer from doctor where id=1)

数据如下:
[table]
|cancer_id|cancer_name|cancer_prefix
|1|肺癌|Lung
[/table]