Question

When you CREATE TABLE using CASE expression to create a computed column, you do not explicitly define the data type of this column:

CREATE TABLE OrderDetail
( OrderID INT
, ProductID INT
, Qty INT
, OrderDate DATETIME
, ShipDate DATETIME
, STATUS AS CASE
       WHEN shipdate is NULL AND orderdate < DATEADD( dd, -7, GETDATE()) THEN 3 
       WHEN shipdate is NOT NULL THEN 2 
       ELSE 1
   end
 )
GO

How SQL Server decides the data type of this column?

Answer 1

For a CASE expression it is the branch with the highest datatype precedence. In your example all three branches are literals that are in the range that will be interpreted as integers so the type of the column will be int

You can use sql_variant_property to determine what datatype a literal expression is as per my answer here. e.g. 2147483648 is interpreted as numeric(10,0) rather than bigint.

In the case in the question SQL Server recognises that the resultant column will be NOT NULL but often for calculated expressions it is necessary to wrap the expression in an ISNULL to get that effect.