求POS小票上同时存在A,B两个商品的解法

原问题
https://www.modb.pro/issue/9491

有一个tag表:

id tag
1 a,b,c
2 a,c,d
3 a

如何查询 tag 里包含 a 和 c 的？ 也就是第一行和第二行

with cte as
(
select 1 as id,'a,b,c' as tag union all
select 2 as id,'a,c,d' as tag union all
select 3 as id,'a,d,e' as tag union all
select 4 as id,'c,d,e' as tag union all
select 5 as id,'a' as tag 
)

select * from cte
where id in
(
    select id from (
        SELECT id,
        SUBSTRING_INDEX(SUBSTRING_INDEX(tag,',',help_topic_id+1),',',-1) AS num
        FROM
        mysql.help_topic ,cte
        WHERE
        help_topic_id < LENGTH(tag)-LENGTH(REPLACE(tag,',',''))+1
    ) as t
    where num ='a' or num = 'c'
    group by id
    having count(*) >=2
)

结果
id tag
1 a,b,c
2 a,c,d