SELECT dbo.tb1.username AS u1, dbo.tb2.username AS u2,
dbo.tb3.username AS u3
FROM dbo.tb1 CROSS JOIN
dbo.tb2 CROSS JOIN
dbo.tb3以上视图代码,如果tb1、tb2或tb3中有一个表是空的话,那么就查不到任何记录,请问该怎么修改才可以避免这种问题出现?
dbo.tb3.username AS u3
FROM dbo.tb1 CROSS JOIN
dbo.tb2 CROSS JOIN
dbo.tb3以上视图代码,如果tb1、tb2或tb3中有一个表是空的话,那么就查不到任何记录,请问该怎么修改才可以避免这种问题出现?
SELECT dbo.tb1.username AS u1, dbo.tb2.username AS u2,
dbo.tb3.username AS u3
FROM dbo.tb1 CROSS JOIN
dbo.tb2 LEFT JOIN
dbo.tb3
列表包含NULL,否则,整个结果集行包含基表的数据值
dbo.tb3.username AS u3
FROM dbo.tb1 full outer JOIN
dbo.tb2 full outer JOIN
dbo.tb3这样就是全外连接,速度比较慢.呵呵,你想,要把所有表的所有行都关联一遍