SELECT pub_name
FROM publishers
WHERE EXISTS
(SELECT *
FROM titles
WHERE pub_id = publishers.pub_id
AND type = 'business')
可以写成
SELECT pub_name
FROM publishers p,
titles t
where t.pub_id = p.pub_id
and t.type = 'business'
这样的效率高
FROM publishers
WHERE EXISTS
(SELECT *
FROM titles
WHERE pub_id = publishers.pub_id
AND type = 'business')
可以写成
SELECT pub_name
FROM publishers p,
titles t
where t.pub_id = p.pub_id
and t.type = 'business'
这样的效率高
对于exists的子查询中,当符合子查询的条件成立时,主查询就返回这一行。如此循环,知道主查询完毕。