SQL query involving multiple tables

Question

I have a scenario in SQL where For each post there are associated tags. A user is one who makes post and he belongs to a particular location. The location corresponds to a particular country.
I create tables as
CREATE TABLE post_table
post_id VARCHAR(20),
user_id VARCHAR(20),
PRIMARY KEY(post_id)
CREATE TABLE tags_table
   post_id VARCHAR(20),
   tags VARCHAR(20),
   PRIMARY KEY(tags, post_id),
   FOREIGN KEY(post_id) REFERENCES post_table(post_id) ON DELETE CASCADE
CREATE TABLE location
location_id VARCHAR(20),
country VARCHAR(20),
PRIMARY KEY (location_id)
CREATE TABLE user_info
user_id VARCHAR(20),
location_id VARCHAR(30),
PRIMARY KEY (user_id),
FOREIGN KEY(location_id) REFERENCES location(location_id) ON DELETE CASCADE
);Now, I insert values in these tables as
INSERT INTO post_table VALUES( 'p1', 'u1' );
INSERT INTO post_table VALUES( 'p2', 'u1' );
INSERT INTO post_table VALUES( 'p3', 'u2' );
INSERT INTO post_table VALUES( 'p4', 'u3' );
INSERT INTO post_table VALUES( 'p5', 'u2' );
INSERT INTO tags_table VALUES( 'p1', 'US Open' );
INSERT INTO tags_table VALUES( 'p1', 'Real good' );
INSERT INTO tags_table VALUES( 'p1', 'Madrid' );
INSERT INTO tags_table VALUES( 'p2', 'Madrid' );
INSERT INTO tags_table VALUES( 'p3', 'Rossoneri' );
INSERT INTO tags_table VALUES( 'p4', 'Milan' );
INSERT INTO tags_table VALUES( 'p4', 'Los Angeles' );
INSERT INTO tags_table VALUES( 'p5', 'Rossoneri' );
INSERT INTO location VALUES( 'loc1', 'Spain');
INSERT INTO location VALUES( 'loc2', 'England');
INSERT INTO user_info VALUES( 'u1', 'loc1' );
INSERT INTO user_info VALUES( 'u2', 'loc2' );
INSERT INTO user_info VALUES( 'u3', 'loc1' );Now I have to fire a query For each country, display the most seen tag(s)
So for this data the result should be like
Country              MostTags
Spain                Madrid
England              RossoneriPlease help me write this query guys.

Answer 1

Hi,
Something like this
select      country
     , tags from
             select      l.country
               , t.tags
               , dense_rank() over (     partition by l.country
                              order by count(*) desc) drn
          from      location l
               , user_info u
               , post_table p
               , tags_table t
          where      l.location_id = u.location_id
               and p.user_id=u.user_id
               and p.post_id=t.post_id
          group by l.country
               , t.tags
where drn <=1   -- rank depends on like 1,2 or 3 popular tags for each countryRegards
Anurag