CASE without Else expression

The Case expression syntax is:

CASE ..... WHEN ....... THEN ........ [ ...more WHEN ... THEN expressions ] 

     [ ELSE ......... ]  END 

ELSE is not obligatory expression.

And - If this argument is omitted and no comparison operation evaluates to TRUE, CASE returns NULL.

CASE without the Else expression can cause to insertion of NULL values!

For Example:

DECLARE @TestTable (TestColumn int not null)

INSERT INTO @TestTable select 1 union select 2 union select 3

-- case without else expression

UPDATE@TestTable 

SET TestColumn = ( CASE WHEN TestColumn = 2 THEN 22 END )

-- Result: table values are: NULL, 22, NULL

-- if we want to update only the '2' rows:

UPDATE@TestTable 

SET TestColumn = ( CASE WHEN TestColumn = 2 THEN 22 ELSE TestColumn END )

-- Result: table values are: 1, 22, 3

http://msdn.microsoft.com/en-us/library/ms181765.aspx