Question

I have a user table with following fields

id           int
name         varchar
dob          date/time

Now, I have another table called user_map table, with 2 fields

oldid        int
newid        int

user_map table is used for storing user id mapping, field ‘oldid’ is user’s old id (which is current user id from user table), and ‘newid’ is user id going to be assigned

I need a single update query, which can update user id for all users in user table

How should it work? Well, for each id in user table, there will be only 1 record matching in user_map table.

id from user table matches oldid from user_map table, and it should be replaced by newid

For example (ignore dob field please)

user table

       id                   name                     dob
        1                    aaa                     0000-00-00 00:00:00
        2                    bbb                     0000-00-00 00:00:00
        3                    ccc                     0000-00-00 00:00:00

user_map table

      oldid       newid
        1           6   
        2           7   
        3           8   

After query, user table should look like

user table

       id                   name                     dob
        6                    aaa                     0000-00-00 00:00:00
        7                    bbb                     0000-00-00 00:00:00
        8                    ccc                     0000-00-00 00:00:00

how can I achieve this? is it possible in 1 query only?

Answer 1

Check following query

update `user table` a, `user_map` b  set a.id=b.newid where a.id=b.oldid

this query is enough to update main table