两个表的对比问题
各位大佬们好，我现在有这样一个问题：
表newMail和表mailAddress.
－－－－－－－－－－－－－－－－－－－－
newMail:
ID mail Flag
1 kyroc@163.com NULL
2 yiyoc@126.com NULL
3 leocha@tom.com X
4 nancy@163.com X
-----------------
mailAddress:
ID mail Tag
1 lyroct@126.com NULL
2 nancy@163.com NULL
3 kyroc@163.com NULL
4 xixi@acer.com X
-----------------
如何才能得到两个表中的所有mail，但不包含重复的mail，单个表中Flag或Tag字段为X的也不包含．
上面两个表中我希望得到的mail为：kyroc@163.com,yiyoc@126.com,lyroct@126.com
我用
SELECT DISTINCT `mail` FROM `newMail` where `Flag` is NULL
UNION
SELECT DISTINCT `mail` FROM `mailAddress` where `Tag` is NULL
会得到kyroc@163.com，yiyoc@126.com，lyroct@126.com，nancy@163.com,多了个nancy@163.com
请问大佬们怎么修改？谢谢了，我一直在线
[解决办法]
按照LZ的意思只有两个MAIL地址。
kyroc@163.com也是重复。
mysql> select * from newmail;
+----+---------------+------+
| id | mail | flag |
+----+---------------+------+
| 1 | kyroc@163.com | NULL |
| 2 | yiyoc@126.com | NULL |
| 3 | leoha@tom.com | x |
| 4 | nancy@163.com | x |
+----+---------------+------+
4 rows in set (0.00 sec)
mysql> select * from mailaddress;
+----+----------------+------+
| id | mail | tag |
+----+----------------+------+
| 1 | lyroct@126.com | NULL |
| 2 | nancy@163.com | NULL |
| 3 | kyroc@163.com | NULL |
| 4 | xixi@acer.com | x |
+----+----------------+------+
4 rows in set (0.00 sec)
mysql> select mail,flag from (select mail,flag from newmail union all select mai
l,tag from mailaddress) t group by mail
-> having count(mail) = 1 and flag is null;
+----------------+------+
| mail | flag |
+----------------+------+
| lyroct@126.com | NULL |
| yiyoc@126.com | NULL |
+----------------+------+
2 rows in set (0.00 sec)
[解决办法]
这样得出来的结果才是楼主要要的结果：
select email,tag, count(email) from
(select email,tag from a
union all
select email,flag from b)
t group by email
having count(email)> =1 and tag is null
结果：
kyroc@163.com,yiyoc@126.com,lyroct@126.com
[解决办法]
看大家这么踊跃，我也来一个。
按照人怎么去做，程序就怎么写的思路，总能写出来的：
SELECT *
FROM (
SELECT mail
FROM newmail
WHERE flag IS NULL
UNION
SELECT mail
FROM mailaddress
WHERE tag IS NULL
)a
WHERE NOT
EXISTS (
SELECT *
FROM (
SELECT mail
FROM newmail
WHERE flag = "x "
UNION
SELECT mail
FROM mailaddress
WHERE tag = "x "
)b
WHERE a.mail = b.mail
)
