MySQL WHERE語句與交叉表比較

Question

下面的MySQL語句給了我三次相同的結果，因爲我需要從不同的表中選擇所有數據進行比較。我如何告訴MySQL我只需要其中的一個條目？

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 

輸出：

ID 
2 
2 
2 

Answer 1

SELECT distinct(watchedItems.id) FROM ... 

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

「DISTINCT指定去除從結果集的重複行的」。

Answer 2

你需要一些聯接：

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 

因爲有在bidGroups表三排你都拿到副本。