Question

Suppose I have a table like:

Tab(MyDate datetime null, MyIs bit null, ...)

Then I have SQL Like

    select Isnull(MyDate, 1), IsNull(MyIs, 999) from Tab;

If MyDate, MyIs value is null in DB, the result is:

    MyDate                   MyIs  
    1900-01-02 00:00:00.000  1

Not the expected value like

MyDate        MyIs  
1             999

So it looks like the return value is based on data type of the column.
How to resolve this issue?

For my testing case as above, I Only got one row from the table.

Answer 1

The way you’re looking for, you don’t.

All the values in the same column must be the same data type. If your data had NOT NULL values in some rows (returning the column’s data-type) and NULL values in other rows (returning a different data type, as per your CASE statement), you would break this requirement.

You could have two extra fields…

select
  MyDate,
  CASE WHEN MyDate IS NULL THEN 1   ELSE 0 END As MyDateWibble,
  MyIs,
  CASE WHEN MyIs   IS NULL THEN 999 ELSE 0 END AS MyIsWibble
FROM
  Tab;

You could CAST() everything to VARCHAR() and return strings instead of DATETIME, BIT or INT.

But, more probably, I’d review the reason you want to do this in the first place and work out an alternative design.