CODEWITHSUNDEEP
mysqljoin
Anujith
Anujith

Reputation: 9370

Get the latest data comparing datetime fields from two tables

I have two tables with identical structure like this:

Table : user

+----+--------+--------------+----------------------+
| ID | Name   |    Password  |    LastUpdateTime    |
+----+--------+--------------+----------------------+
|  1 | abu    |    abu123NEW |  2014-06-04 14:55:06 |
|  2 | john   |      john123 |  2014-06-04 14:58:22 |
|  3 | shane  |     shane123 |  2014-06-04 15:02:06 |
|  4 | marie  |     marie123 |  2014-06-04 15:00:06 |
|  5 | mike   |   mike123NEW |  2014-06-04 15:01:32 |
|  6 | kiron  |  kiron123NEW |  2014-06-04 15:05:46 |
+----+--------+--------------+----------------------+

Table : user_k

+----+--------+--------------+----------------------+
| ID | Name   |    Password  |    LastUpdateTime    |
+----+--------+--------------+----------------------+
|  1 | abu    |       abu123 |  2014-06-04 14:53:06 |
|  2 | john   |   john123NEW |  2014-06-04 14:59:48 |
|  3 | shane  |  shane123NEW |  2014-06-04 15:00:06 |
|  4 | marie  |  marie123NEW |  2014-06-04 15:03:17 |
|  5 | mike   |      mike123 |  2014-06-04 15:00:36 |
|  6 | kiron  |     kiron123 |  2014-06-04 15:02:18 |
+----+--------+--------------+----------------------+

Now I need to fetch the latest data (by 'LastUpdateTime') from these two tables, like this:

+----+--------+--------------+----------------------+
| ID | Name   |    Password  |    LastUpdateTime    |
+----+--------+--------------+----------------------+
|  1 | abu    |    abu123NEW |  2014-06-04 14:55:06 |
|  2 | john   |   john123NEW |  2014-06-04 14:59:48 |
|  3 | shane  |     shane123 |  2014-06-04 15:02:06 |
|  4 | marie  |  marie123NEW |  2014-06-04 15:03:17 |
|  5 | mike   |   mike123NEW |  2014-06-04 15:01:32 |
|  6 | kiron  |  kiron123NEW |  2014-06-04 15:05:46 |
+----+--------+--------------+----------------------+

What I have tried:

SELECT uk.* FROM user AS ua, user_k AS uk
WHERE  uk.ID = ua.ID AND uk.LastUpdateTime > ua.LastUpdateTime

UNION

SELECT ua.* FROM user AS ua ,user_k AS uk
WHERE  uk.ID = ua.ID AND uk.LastUpdateTime < ua.LastUpdateTime

I'm not sure if this is the appropriate way. Also I'm concerned about the query performance with large set of data. Is there any better approach for this?

PS: Besides name & password there are a dozen more fields(columns) in both tables. I avoid them just to simplify the question.

Upvotes: 0

Views: 342

Answers (1)

fancyPants
fancyPants

Reputation: 51868

select
u.id,
u.name,
if(u.LastUpdateTime >= k.LastUpdateTime, u.password, k.password) as password,
greatest(u.LastUpdateTime, k.LastUpdateTime) as LastUpdateTime
from
user u
inner join user_k k on u.id = k.id

P.S.: For id 3 the newer password is actually the one from user table, not user_k. Your desired result set needs this adjustment.

Upvotes: 1

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