What is the proper syntax for a cross-table SQL query?

Question

Right now, I have

SELECT gp_id FROM gp.keywords 
WHERE keyword_id = 15 
AND (SELECT practice_link FROM gp.practices 
     WHERE practice_link IS NOT NULL 
     AND id = gp_id)

This does not provide a syntax error, however for values where it should return row(s), it just returns 0 rows.

What I'm trying to do is get the gp_id from gp.keywords where the the keywords table keyword_id column is a specific value and the practice_link is the practices table corresponds to the gp_id that I have, which is stored in the id column of that table.

Answer 1

SELECT k.gp_id
FROM gp.keywords k, gp.practices p
WHERE 
   p.id = k.gp_id.AND
   k.keyword_id = 15 AND
   p.practice_link is not null

Answer 2

SELECT g.gp_id, p.practice_link FROM gp.keywords g, gp.practices p 
WHERE
g.keyword_id = 15 AND p.practice_link IS NOT NULL AND p.id = g.gp_id

Answer 3

I'm not even sure that is valid SQL, so I'm surprised it is working at all:

SELECT gp_id
FROM gp.keywords
WHERE keyword_id = 15
    AND (SELECT practice_link FROM gp.practices WHERE practice_link IS NOT NULL AND id = gp_id)

How about this instead:

SELECT kw.gp_id, p.practice_link
FROM gp.keywords AS kw
INNER JOIN gp.practices AS p
    ON p.id = kw.gp_id
WHERE kw.keyword_id = 15

I would steer clear of implicit joins as in the other examples. It only leads to tears later.