How can I check existence of 1 variable in 1 column in SQL?

Question

I want to know if what user input parameter exists in a column in a table in SQL.

For example: check if "john" exists in column CustomerName in the Customer table.

I have tried exist keyword, but I do not completely understand it. Can someone help with a solution for this?

This is my code:

create procedure getOrderWithCusID
    @customerId char 
as 
    if (@customerId EXISTS(SELECT o.CustomerID FROM Orders o))
    begin
        select * 
        from Orders o 
        where o.CustomerID = @customerId;
    end
    else
        print('This customer does not exist');
    return;
end;

Answer 1

The procedure you're trying to write should look something like:

create procedure getOrderWithCusID
@customerId int 
as 
set nocount, xact_abort on;

if exists (
  select * from Orders
  where CustomerId = @CustomerId
)
begin
  select <only necessary columns>
  from Orders 
  where CustomerID = @customerId;
end;
else
begin
  print Concat('CustomerId ', @customerId, ' has no orders');
end;

Although really it's not necessary to use exists first and hit the table twice - you can simply check if any rows are returned and handle accordingly; also presumably your CustomerId is an int not a char with length 1...

select <columns> from Orders where CustomerId = @CustomerId;

if @@rowcount = 0
begin
  <do this if customerId does not exist in table>
end;