Question

i am getting this error ( see title). How can i correct this:

sql

use adventureworks
go

select si.CustomerID,
'myField' =
            CASE
                 When (Select Top 1 FirstName,LastName  From Person.Contact pc Where si.ContactID = pc.contactid ) is not null Then 
            Cast((Select Top 1 FirstName,LastName  From Person.Contact pc Where si.ContactID = pc.contactid  ) As varchar)
            Else ''
            END
from Sales.Individual si
where si.CustomerID=11000

Answer 1

Modify the query

use adventureworks go  

select si.CustomerID, 'myField' =             
      CASE    When 
          (Select Top 1 FirstName From Person.Contact pc Where si.ContactID = pc.contactid ) is not null 
              Then              
          Cast((Select Top 1 FirstName
                   From Person.Contact pc Where si.ContactID = pc.contactid  ) As varchar)             
              Else ''             
     END from Sales.Individual si where si.CustomerID=11000 

The erro is because you are choosing FirstName and LastName both so to remove erro choose one of them either firstanme or lastname or combine both in one

for example

Select Top 1 FirstName+ ' ' + LastName as fullName
                       From Person.Contact pc Where si.ContactID = pc.

this will resolve your issue

for more deatil check : Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.