mysql create view - filter duplicated rows

Question

My users are splited in user tables over 5 databases. Looks like this:

db1.user
id  token  app  lang  last_update
XX  1      a    en    11:00
XX  2      a    en    12:00


db2.user
id  token  app  lang  last_update
XX  1      b    en    12:00
XX  2      a    en    16:00

db3.user
…

Now i need all users in one view, it should look like this (token and app are together the primary key):

view db.user_all
id  token  app  lang  last_update
XX  1      a    en    11:00
XX  1      b    en    12:00
XX  2      a    en    16:00

I tried to union everything:

create view `db`.`user_all` AS 
SELECT * FROM `db1`.`user` union 
SELECT * FROM `db2`.`user`

The result looks like this:

view db.user_all
id  token  app  lang  last_update
XX  1      a    en    11:00
XX  2      a    en    12:00
XX  1      b    en    12:00
XX  2      a    en    16:00

My sql statement deletes the duplicated rows, but if the last_update is different i got a duplicated user in the result view. How to solve this? I need all data, also the newest last_update.

Answer 1

Try this:

CREATE VIEW user_intermediate AS
SELECT * FROM db1 UNION ALL SELECT * FROM db2
ORDER BY last_update DESC
;

CREATE VIEW user_all AS
SELECT *
FROM user_intermediate
GROUP BY app, token
;

Assuming App and Token make the user key, the user_all view should have the distinct users with the latest last_update across both tables.

Answer 2

Try using a DISTINCT along with a MAX, so something like this:

SELECT DISTINCT id,  token,  app,  lang,  max(last_update) FROM (
    SELECT * FROM `db1`.`user` union 
    SELECT * FROM `db2`.`user`
)
GROUP BY id, token, app, lang

you may need to use an alias after the last bracket.

---

update

Apparently, MYSQL cannot have subqueries in views so you'll have to create this as a stored procedure instead http://dev.mysql.com/doc/refman/5.0/en/create-procedure.html