跨表SQL查询的正确语法是什么?

我甚至不确定这是否是有效的SQL,所以我很惊讶它能正常工作:

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)

改成这个怎么样:

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

我会像其他例子一样避开隐式连接。它只会导致后来的眼泪。

select k.gp_id 
from gp.keywords as k,
     gp.practices as p
where
keyword_id=15
and practice_link is not null
and p.id=k.gp_id

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

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