How to convert nested query to join

Question

Hello All
I wrote a query, at first i find the name of the highest mountain,then find its provience and then list of mountains in this porvience.please help me to convert it to a join or generally a standard form.thank you in advance.
select mountain from geo_mountain where province=
(select province from geo_mountain where mountain=(select name from mountain where height=(select max(height) from mountain)))
mountain: name,height,...
geo_mountain: mountain,country,province

Answer 1

Hi,
Welcome to the forum!
Here's one way to do it:
WITH     got_r_num     AS
     SELECT     name
     ,     RANK () OVER (ORDER BY  height  DESC)     AS r_num
     FROM     mountain
SELECT  p.mountain
FROM      got_r_num     r
JOIN     geo_mountain  n      ON  n.name       = r.name
JOIN     geo_mountain  p  ON  p.proivince  = n.province
WHERE     r.r_num        = 1
;A slightly different approach, closer to what you posted, involves a 4-way join:
(1) In a sub-query (let's call it got_max_height) find just the maximum height from mountain
(2) Join got_max_height to mountain, to get all mountains with that height. (There may be a tie)
(3) Join this to geo_mountain to get the province(s)
(4) Join this to another copy of geo_mountain to get all mountains in the same province(s). This step is just like the self-join in the query I posted.
I hope this answers your question.
If not, post a little sample data (CREATE TABLE and INSERT statements, relevant columns only) for all the tables involved, and the results you want from that data.
Point out where the query above is producing the wrong results, and explain, using specific examples, how you get the right results from the given data in those places.
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