SQL special select from table - sql

Question

I have a table with about 20 columns and 2000 rows.
Example:
Col1 Col2 Col3 Col4 ...
A01 22 AB 11
A01 22 AX 112
A01 23 A5 11
A02 20 AB AA
A04 21 AB 11
A04 21 AU 11
A04 29 AB BA
A05 21 AB 11
AAA 111 XX 18
AAA 222 GT 1O
...
I need a select which displays all rows and all columns that satisfy the requirement of two columns (Col1 and Col2) based on the following:
if Col1 is unique - show row,
or
if Col1 is not unique show all row only if Col1 and Col2 are same.
From the previos table is after select the result:
Col1 Col2 Col3 Col4 ...
A01 22 AB 11
A01 22 AX 112
A02 20 AB AA
A04 21 AB 11
A04 21 AU 11
A05 21 AB 11
AAA 111 XX 18
...
The new table (your solution) contains data:
Col1 Col2 Col3 Col4 ...
A01 22 AB 11
A01 22 AX 112
A02 20 AB AA
A04 21 AB 11
A04 21 AU 11
A05 21 AB 11
AAA 111 XX 18
...
what I wont see from this is:
Col1 Col2 Col3 Col4 ...
A01 2 AB 11
A02 1 AB AA
A04 2 AB 11
A05 1 AB 11
AAA 1 XX 18
...

Answer 1

In Oracle and MS SQL I would use analytical functions:
select * from
(
select
t.* ,
count(Col1) over (partition by Col1) as count_col1,
count(Col2) over (partition by Col1, Col2) as count_col2
from yourTable t
) t
where count_col1 = 1 or count_col2 > 1;
See this fiddle (Oracle) and this fiddle (MSSQL) as proof.

Answer 2

select *
from table t1
join (select col1
from table
group by col1
having avg(col2)=max(col2)) t2
on t1.col1=t2.col1
Seeing that I didn't look at your example .. and your request is slightly different then the example, Because my query checks that for a col1 all col2 should be the same. It will not display the ones that are the same.
In this case the answer will be
select *
from table1 t1
join (select col1,col2
from table1
group by col1,col2
having count(*)>1
union
select col1,cast(null as varchar)
from table1 group by col1
having count(*)=1) t2
on t1.col1=t2.col1 and t1.col2=isnull(t2.col2,t1.col2)
This is the updated query, and the fiddle for it http://sqlfiddle.com/#!3/e944b/2/0
Ok .. updated one more time:
select *
from table1 t1
join (select col1,col2
from table1
group by col1,col2
having count(*)>1
union
select col1,min(col2)
from table1 group by col1
having count(*)=1 or count(*)=count(distinct col2)) t2
on t1.col1=t2.col1 and t1.col2=t2.col2
and with fiddle http://sqlfiddle.com/#!3/d5437/12/0
This should be enough for the second problem:
select t3.*
from (select distinct col1 from table1)t1
cross apply (select top 1 * from table1 t2 where t1.col1=t2.col1) t3
and the fiddle: http://sqlfiddle.com/#!3/e944b/4/0