Why can't I use a CASE statement to see if a column exists and not SELECT from it?

Why does something like this not work?

SELECT
CASE 
WHEN NULLIF(COL_LENGTH('Customers', 'Somecol'), '') IS NULL THEN NULL
ELSE Somecol
END AS MyTest
FROM Customers;

I am just checking if the column exists, however, SQL Server complains about Somecol not existing. Is there an alternative to this in a single statement?

The following query uses the same idea as in this amazing answer by ypercube:

SELECT x.*
FROM (SELECT NULL AS SomeCol) AS dummy
CROSS APPLY
(
  SELECT
    ID,
    SomeCol AS MyTest
  FROM dbo.Customers
) AS x;

It works like this:

-
if dbo.Customers has a column named SomeCol, then SomeCol in SomeCol AS MyTest will resolve as dbo.Customers.SomeCol;

-
if the table has no such column, the reference will still be valid, because now it will be resolved as dummy.SomeCol: dummy columns can be referenced in that context.

You can specify multiple "spare" columns that way. The trick is not to use the table alias for such columns (which is a frowned-upon practice in most situations, but in this case omitting the table alias helps you to resolve the issue).

If the table is used in a join and the other table has its own SomeCol, you will probably need to use the above query as a derived table before using it in the join in order to keep the trick working, something like this:

SELECT ...
FROM
(
  SELECT x.*
  FROM (SELECT NULL AS SomeCol) AS dummy
  CROSS APPLY (
    SELECT
      ID,
      SomeCol AS MyTest
    FROM dbo.Customers
  ) AS x
) AS cust
INNER JOIN ...
;

StackExchange Database Administrators Q#66741, answer score: 50