How do I check if a Sql server string is null or empty

I want to check for data, but ignore it if it's null or empty. Currently the query is as follows...

Select              
Coalesce(listing.OfferText, company.OfferText, '') As Offer_Text,         
from tbl_directorylisting listing  
 Inner Join tbl_companymaster company            
  On listing.company_id= company.company_id      

But I want to get company.OfferText if listing.Offertext is an empty string, as well as if it's null.

What's the best performing solution?

I think this:

SELECT 
  ISNULL(NULLIF(listing.Offer_Text, ''), company.Offer_Text) AS Offer_Text
FROM ...

is the most elegant solution.

And to break it down a bit in pseudo code:

// a) NULLIF:
if (listing.Offer_Text == '')
  temp := null;
else
  temp := listing.Offer_Text; // may now be null or non-null, but not ''
// b) ISNULL:
if (temp is null)
  result := true;
else
  result := false;

SELECT
   CASE WHEN LEN(listing.OfferText) > 0 THEN listing.OfferText 
        ELSE COALESCE(Company.OfferText, '') END 
   AS Offer_Text,

... 

In this example, if listing.OfferText is NULL, the LEN() function should also return NULL, but that's still not > 0.

Update

I've learned some things in the 5 1/2 years since posting this, and do it much differently now:

COALESCE(NULLIF(listing.OfferText,''), Company.OfferText, '')

This is similar to the accepted answer, but it also has a fallback in case Company.OfferText is also null. None of the other current answers using NULLIF() also do this.

Select              
CASE
    WHEN listing.OfferText is null or listing.OfferText = '' THEN company.OfferText
    ELSE COALESCE(Company.OfferText, '')
END As Offer_Text,         
from tbl_directorylisting listing  
 Inner Join tbl_companymaster company            
  On listing.company_id= company.company_id

Here is another solution:

SELECT Isnull(Nullif(listing.offertext, ''), company.offertext) AS offer_text, 
FROM   tbl_directorylisting listing 
       INNER JOIN tbl_companymaster company 
         ON listing.company_id = company.company_id

You can use ISNULL and check the answer against the known output:

SELECT case when ISNULL(col1, '') = '' then '' else col1 END AS COL1 FROM TEST

In SQL Server 2012 you have IIF, e.g you can use it like

SELECT IIF(field IS NULL, 1, 0) AS IsNull

The same way you can check if field is empty.

Use the LEN function to check for null or empty values. You can just use LEN(@SomeVarcharParm) > 0. This will return false if the value is NULL, '', or ' '. This is because LEN(NULL) returns NULL and NULL > 0 returns false. Also, LEN(' ') returns 0. See for yourself run:

SELECT 
 CASE WHEN NULL > 0 THEN 'NULL > 0 = true' ELSE 'NULL > 0 = false' END,
 CASE WHEN LEN(NULL) > 0 THEN 'LEN(NULL) = true' ELSE 'LEN(NULL) = false' END,
 CASE WHEN LEN('') > 0 THEN 'LEN('''') > 0 = true' ELSE 'LEN('''') > 0 = false' END,
 CASE WHEN LEN(' ') > 0 THEN 'LEN('' '') > 0 = true' ELSE 'LEN('' '') > 0 = false' END,
 CASE WHEN LEN(' test ') > 0 THEN 'LEN('' test '') > 0 = true' ELSE 'LEN('' test '') > 0 = false' END

I know this is an old thread but I just saw one of the earlier posts above and it is not correct.

If you are using LEN(...) to determine whether the field is NULL or EMPTY then you need to use it as follows:

...WHEN LEN(ISNULL(MyField, '')) < 1 THEN NewValue...

this syntax :

SELECT *
FROM tbl_directorylisting listing
WHERE (civilite_etudiant IS NULL)

worked for me in Microsoft SQL Server 2008 (SP3)

Select              
Coalesce(NullIf(listing.OfferText, ''), NullIf(company.OfferText, ''), '') As Offer_Text,         
from tbl_directorylisting listing  
 Inner Join tbl_companymaster company            
  On listing.company_id= company.company_id

This simple combination of COALESCE and NULLIF should do the trick:

SELECT             
  Coalesce(NULLIF(listing.OfferText, ''), company.OfferText) As Offer_Text
...

Note: Add another empty string as the last COALESCE argument if you want the statement to return an empty string instead of NULL if both values are NULL.

Here's a solution, but I don't know if it's the best....

Select              
Coalesce(Case When Len(listing.Offer_Text) = 0 Then Null Else listing.Offer_Text End, company.Offer_Text, '') As Offer_Text,         
from tbl_directorylisting listing  
 Inner Join tbl_companymaster company            
  On listing.company_id= company.company_id

SELECT              
	COALESCE(listing.OfferText, 'company.OfferText') AS Offer_Text,         
FROM 
	tbl_directorylisting listing  
	INNER JOIN tbl_companymaster company ON listing.company_id= company.company_id

To check if variable is null or empty use this:

IF LEN(ISNULL(@var, '')) = 0
    -- Is empty or NULL
ELSE
    -- Is not empty and is not NULL

To prevent the records with Empty or Null value in SQL result

we can simply add ..... WHERE Column_name != '' or 'null'

[Column_name] IS NULL OR LEN(RTRIM(LTRIM([Column_name]))) = 0

[Column_name] > ' ' excludes Nulls and empty strings. There is a space between the single quotes.

When dealing with VARCHAR/NVARCHAR data most other examples treat white-space the same as empty string which is equal to C# function IsNullOrWhiteSpace.

This version respects white-space and works the same as the C# function IsNullOrEmpty:

IIF(ISNULL(DATALENGTH(val), 0) = 0, whenTrueValue, whenFalseValue)

Simple test:

SELECT
    '"' + val + '"' AS [StrValue],
    IIF(ISNULL(DATALENGTH(val), 0) = 0, 'TRUE', 'FALSE') AS IsNullOrEmpty
FROM ( VALUES 
    (NULL), 
    (''), 
    (' '), 
    ('a'), 
    ('a ')
) S (val)

This caters for spaces as well.

(len(rtrim(ltrim(isnull(MyField,'')))) !=0