-- mysql  值为空则返回 0  : mysql--> ifnull  oracle --> nvl

select category.id as c_id,  category.category_id , category.category_name ,  ifnull(rule.rule_id,0) is_selected 
 
from 
goods_category category

LEFT JOIN integral_relation_category  rule 

on category.category_id = rule.category_id

-- 为空 返回0  不为空 则返回1   : is not null :   true-->1  false--> 0 
 select category.id as c_id,  category.category_id , category.category_name ,
(rule.rule_id is not null) is_selected 
 
from 
goods_category category

LEFT JOIN integral_relation_category  rule 

on category.category_id = rule.category_id

-- 为空 返回0  不为空 返回1 :  if (column is null ,0 ,1)
 select category.id as c_id,  category.category_id , category.category_name ,
if(rule.rule_id is null,0,1) is_selected 
 
from 
goods_category category

LEFT JOIN integral_relation_category  rule 

on category.category_id = rule.category_id

查询结果如下: