SQL selects the unique field value and carries the following field

Requirement: select the unique value of field a and add field B. If field a is the same as field B, the value of field B appears for the first time.

Table
A B ID
--------------------------------------
1 A 1
1 B 2
2 C 3
2 D 4
3 E 5
3 F 6

Selection result:
A B ID
--------------------------------------
1 A 1
2 C 3
3 E 5

Select *
From Test
Where (ID in
(Select Min (ID)
From Test
Group by ))

ArticleSource: http://jinweijie.blogspot.com/2006/10/codesql.html

Obtain the uniqueness of a field:
Select salary from tablename group by salary having count (*) = 1