带有交叉表比较的MySQL WHERE语句

Question

下面的MySQL语句给了我三倍相同的结果，因为我需要从不同的表中选择所有数据来进行比较。我怎么能告诉MySQL我只需要其中一个条目？

SELECT watchedItems.id FROM watchedItems, globalItems, bidGroups 
WHERE (watchedItems.bidGroupID IS NULL OR (watchedItems.bidGroupID IS NOT NULL AND bidGroups.bidGroupQty > 0))
AND watchedItems.aid = globalItems.aid
AND watchedItems.maxPrice > globalItems.currentPrice

输出：

ID
2
2
2

Answer 1

SELECT distinct(watchedItems.id) FROM ...

http://dev.mysql.com/doc/refman/5.0/en/select.html

“DISTINCT指定从结果集中删除重复的行。”

Answer 2

你需要一些联接：

SELECT watchedItems.id
FROM watchedItems
left join globalItems on watchedItems.bidGroupID = bidGroups.bidGroupID 
left join bidGroups on watchedItems.aid = globalItems.aid
WHERE (watchedItems.bidGroupID IS NULL OR bidGroups.bidGroupQty > 0)
AND watchedItems.maxPrice > globalItems.currentPrice

您获得了重复项，因为bidGroups表中有三行。