I have two table:

A:
id    code
1      A1
2      A1
3      B1
4      B1
5      C1
6      C1

=====================

B:
id    Aid
1      1
2      4

(B doesn't contain the Aid which link to code C1)

Let me explain the overall flow:
I want to make each row in table A have different code(by delete duplicate),and I want to retain the Aid which I can find in table B.If Aid which not be saved in table B,I retain the id bigger one.

so I can not just do something as below:

DELETE FROM A
WHERE  id NOT IN (SELECT MAX(id)
                  FROM   A
                  GROUP  BY code,
) 

I can get each duplicate_code_groups by below sql statement:

SELECT code
FROM   A
GROUP  BY code
HAVING COUNT(*) > 1

Is there some code in sql like

for (var ids in duplicate_code_groups){
    for (var id in ids) {
        if (id in B){
            return id
        }
    }

    return max(ids)
}

and put the return id into a idtable?? I just don't know how to write such code in sql.

then I can do

DELETE FROM A
WHERE id NOT IN idtable