Self Join SQL Query

Question

a few days ago I asked for a solution to a SQL query i needed to work out, my data looks like:

meta_id post_id     meta_key    meta_value
269     4   _apais               USA
270     4   _aciudad             New york
271     4   _aservicio           Pleasure

...
272     43  _apais               USA
273     43  _aciudad             Chicago
274     43  _aservicio           Fun
...
275     44  _apais               USA
276     44  _aciudad             Miami
277     44  _aservicio           Night Life

278     45  _apais               USA
279     45  _aciudad             Miami
280     45  _aservicio           Sports

What i need to do, is to display all the registers that matches Country with City and Service . Or order all the _aciudad registers by country (and _aservicio As service), something like:

meta_id     post_id     meta_key    meta_value  meta_key    meta_value  meta_key  meta_value
270         7           _apais          USA     _aciudad    New York   _aservicio Pleasure
261         13          _apais          USA     _aciudad    Chicago    _aservicio Fun
276         4           _apais          USA     _aciudad    Miami      _aservicio Sports

@Ravi Gummadi Gave me a solution that looks like this:

SELECT t1.meta_id, 
           t1.post_id, 
           t1.meta_key, 
           t1.meta_value, 
           t2.meta_key, 
           t2.meta_value 
    FROM th_postmeta t1, th_postmeta t2  
    WHERE t1.post_id = t2.post_id 
    AND t1.meta_key = '_apais'
    AND t2.meta_key = '_aciudad'
    ORDER BY t1.meta_key

That query returns this:

meta_id     post_id     meta_key    meta_value  meta_key    meta_value
270         7           _apais          USA     _aciudad    New York
261         13          _apais          USA     _aciudad    Chicago
276         4           _apais          USA     _aciudad    Miami
279         10          _apais          USA     _aciudad    Miami

How can I acchive to show only the records that are not duplicated in the meta_value value (for _aciudad and _aservicio only, _apais can be duplicated)?

Thanks a lot guys!

Answer 1

So you will end up with more than 1 row with the same meta_value. You want to discard some of these rows, even though they have different values for other fields? In SQL Server you might do something like:

SELECT
    ..., x = ROW_NUMBER() OVER(PARTITION BY meta_value ORDER BY meta_id )
WHERE
   x <= 1

This introduces another calculated column (x) with an increasing number for each row (order by) restarting on each meta_value (partition by). The order by clause defines which ones are most important to keep/discard and the where is how many of each you keep.

meta_id     post_id     meta_key    meta_value  meta_key    meta_value   x
261         13          _apais          USA     _aciudad    Chicago      1
270         7           _apais          USA     _aciudad    New York     2
276         4           _apais          USA     _aciudad    Miami        3
279         10          _apais          USA     _aciudad    Miami        4
280         10          _apais          ABC     _aciudad    Miami        1
281         10          _apais          ABC     _aciudad    Miami        2

I realize you tagged it MySql. I don't really use MySQL and hope this just gives you a clue for something new to Google. Perhaps:

ROW_NUMBER() in MySQL