如果嵌套查询什么都不返回，请忽略WHERE子句？

Question

如果我的内连接没有返回任何内容，如何忽略第二个WHERE子句？

SELECT MAX( DATE ) , URL
FROM History
WHERE UID =  '$uid'
AND DATE < ( SELECT MAX( DATE ) FROM History WHERE URL =  '$url' AND UID =  '$uid' ) 
GROUP BY URL
ORDER BY DATE DESC 
LIMIT 1

基本上，如果我的内部联接不返回任何查询应该是：

SELECT MAX( DATE ) , URL
FROM History
WHERE UID =  '$uid'
GROUP BY URL
ORDER BY DATE DESC 
LIMIT 1