T-SQL CASE Clause: How to specify WHEN NULL

Question

I wrote a T-SQL Statement similar like this (the original one looks different but I want to give an easy example here):

SELECT first_name + 
    CASE last_name WHEN null THEN 'Max' ELSE 'Peter' END AS Name
FROM dbo.person

This Statement does not have any syntax errors but the case-clause always chooses the ELSE-part - also if the last_name is null. But Why?

What I want to do is to unite first_name and last_name, but if last_name is null the whole name becomes null:

SELECT first_name +
   CASE last_name WHEN null THEN '' ELSE ' ' + last_name END AS Name 
FROM dbo.person

Do you know where the problem is?

Answer 1

Use the CONCAT function available in SQL Server 2012 onward.

SELECT CONCAT([FirstName], ' , ' , [LastName]) FROM YOURTABLE

Answer 2

I tried casting to a string and testing for a zero-length string and it worked.

CASE 
   WHEN LEN(CAST(field_value AS VARCHAR(MAX))) = 0 THEN 
       DO THIS
END AS field 

Answer 3

You can use IsNull function

select 
    isnull(rtrim(ltrim([FirstName]))+' ','') +
    isnull(rtrim(ltrim([SecondName]))+' ','') +
    isnull(rtrim(ltrim([Surname]))+' ','') +
    isnull(rtrim(ltrim([SecondSurname])),'')
from TableDat

if one column is null you would get an empty char

Compatible with Microsoft SQL Server 2008+

Answer 4

The WHEN part is compared with ==, but you can't really compare with NULL. Try

CASE WHEN last_name is NULL  THEN ... ELSE .. END

instead or COALESCE:

COALESCE(' '+last_name,'')

(' '+last_name is NULL when last_name is NULL, so it should return '' in that case)

Answer 5

The issue is that NULL is not considered to be equal to anything even not to itself, but the strange part is that is also not not equal to itself.

Consider the following statements (which is BTW illegal in SQL Server T-SQL but is valid in My-SQL, however this is what ANSI defines for null, and can be verified even in SQL Server by using case statements etc.)

SELECT NULL = NULL -- Results in NULL

SELECT NULL <> NULL -- Results in NULL

So there is no true/false answer to the question, instead the answer is also null.

This has many implications, for example in

1. CASE statements, in which any null value will always use the ELSE clause unless you use explicitly the WHEN IS NULL condition (NOT the WHEN NULL condition )
2. String concatenation, as
   SELECT a + NULL -- Results in NULL
3. In a WHERE IN or WHERE NOT IN clause, as if you want correct results make sure in the correlated sub-query to filter out any null values.

One can override this behavior in SQL Server by specifying SET ANSI_NULLS OFF, however this is NOT recommended and should not be done as it can cause many issues, simply because deviation of the standard.

(As a side note, in My-SQL there is an option to use a special operator <=> for null comparison.)

In comparison, in general programming languages null is treated is a regular value and is equal to itself, however the is the NAN value which is also not equal to itself, but at least it returns 'false' when comparing it to itself, (and when checking for not equals different programming languages have different implementations).

Note however that in the Basic languages (i.e. VB etc.) there is no 'null' keyword and instead one uses the 'Nothing' keyword, which cannot be used in direct comparison and instead one needs to use 'IS' as in SQL, however it is in fact equal to itself (when using indirect comparisons).

Answer 6

try:

SELECT first_name + ISNULL(' '+last_name, '') AS Name FROM dbo.person

This adds the space to the last name, if it is null, the entire space+last name goes to NULL and you only get a first name, otherwise you get a firts+space+last name.

this will work as long as the default setting for concatenation with null strings is set:

SET CONCAT_NULL_YIELDS_NULL ON 

this shouldn't be a concern since the OFF mode is going away in future versions of SQl Server