Query to pick rows of matching columns of table B else table A

Question

Hi All,
I have a urgent requirement which can be explained from below example.
Suppose I have two tables : A and B with same table schema as mentioned below:
A (Trading ID, Name, Phone Number, Mobile no, City)
B (Trading ID, Name, Phone Number, Mobile no, City)
Now I have to write a select query with the following conditions:
If the value of column "TradingID" in Table A exists in columns "TradingID" in table B then select the columns - (Name, Phone Number, Mobile no, City) from table B. Otherwise select the columns - (Name, Phone Number, Mobile no, City)
from table A.
I need a single select query with optimized solution.
Thanks in Advance.!!!
Please mark it as an answer/helpful if you find it as useful. Thanks, Satya Prakash Jugran

Answer 1

Please try this:
create table A (TradingID int, Name nvarchar(128), PhoneNumber int, Mobileno int, City nvarchar(128))
go
create table B (TradingID int, Name nvarchar(128), PhoneNumber int, Mobileno int, City nvarchar(128))
go
insert dbo.A (TradingID)
values ( 1 ), (2 )
insert dbo.B (TradingID, Name)
values ( 1, 'test01' ), ( 3, 'test02' )
go
with cte as
( select x.TradingID
from dbo.B as x
join dbo.A as y
on x.TradingID = y.TradingID )
select *
from B
where exists ( select *
from cte
where TradingID = B.TradingID )
union all
select *
from A
where exists ( select *
from cte
where TradingID <> A.TradingID )