Question

Example my db structure olddb & newdb

newdb

ID email           Name
-----------------------
1  [e-mail]  John
2  [e-mail] Peter

olddb

ID email           
-------------------
1  [e-mail]  
2  [e-mail] 
3  [e-mail]
4  [e-mail]

Now I want compare olddb with newdb and delete from olddb which email doesn’t have email in newdb

Let me know

DELETE FROM olddb, newdb
USING olddb
    INNER JOIN newdb USING(email) 
WHERE olddb.email <> newdb.email

Answer 1

This will give you a list of all emails from oldtbl which are not on newtbl:

SELECT * 
FROM oldtbl
WHERE id NOT IN (
   SELECT id FROM newtbl
)

Note: oldtbl and newtbl because I found your naming scheme confusing. We are talkind about tables, not databases.

To delete those rows on oldtbl, just change SELECT * with DELETE.