三个表中联合查询一个字段的唯一值怎么查?

解1
select ip from test_table1
union
select ip from test_table2
union
select ip from test_table3

解2
select distinct a.ip + b.ip + c.ip from test_table1 a, test_table2 b, test_table3 c

不知道楼主说的加是怎么个加法
字段相加跟记录集相加都列出来

SELECT a.ip b.ip c.ip FROM 表1 as a 表2 as b 表3 as c ORDER BY DESC

//这是取出所有IP 的一个数组

然后写个循环 判断重复IP 应该就可以拉

======================
补充你一个例子.
string myString="agabhbcicdjdekefmf";
string fs="";
for(int i=0;i<myString.Length;i++)
{
string emtpy=myString.Substring(i,1).ToString();
if(fs.Length==0)
{
fs=emtpy;
}
if(fs.IndexOf(emtpy,0)<0)
{
fs+=emtpy;
}
}
Console.WriteLine(fs);

运行结果：agbhcidjekfm

select ip from table1 union select ip from table2 union select ip from table3