How to use NULL or empty string in SQL

Question

I would like to know how to use NULL and an empty string at the same time in a WHERE clause in SQL Server. I need to find records that have either null values o

Answer 1

SELECT * FROM DBO.AGENDA
WHERE 
  --IF @DT_START IS NULL OR EMPTY
  ( ISNULL( @DT_START,'' ) = '' AND DT_START IS NOT NULL ) -- GET ALL DATE
  OR --ELSE
  ( DT_START >= @DT_START ) --FILTER

-- MORE FILTER

SELECT * FROM DBO.AGENDA
WHERE 
  ( ( ISNULL( @DT_START,'' ) = '' AND DT_START IS NOT NULL ) OR ( DT_START >= @DT_START ) ) 
  AND
  DT_END < GETDATE()

Answer 2

This is ugly MSSQL:

CASE WHEN LTRIM(RTRIM(ISNULL([Address1], ''))) <> '' THEN [Address2] ELSE '' END

Answer 3

by this function:

ALTER FUNCTION [dbo].[isnull](@input nvarchar(50),@ret int = 0)
RETURNS int
AS
BEGIN

    return (case when @input='' then @ret when @input is null then @ret else @input end)

END

and use this:

dbo.isnull(value,0)

Answer 4

You could use isnull function to get both null and empty values of a text field:

SELECT * FROM myTable
WHERE isnull(my_nullable_text_field,'') = ''

Answer 5

Select *
From Table
Where (col is null or col = '')

Or

Select *
From Table
Where IsNull(col, '') = ''

Answer 6

--setup
IF OBJECT_ID('tempdb..#T') IS NOT NULL DROP TABLE #T;
CREATE TABLE #T(ID INT NOT NULL IDENTITY(1,1) PRIMARY KEY, NAME VARCHAR(10))
INSERT INTO #T (Name) VALUES('JOHN'),(''),(NULL);
SELECT * FROM #T
 1  JOHN
 2  -- is empty string
 3  NULL

You can examine '' as NULL by converting it to NULL using NULLIF

--here you set '' to null
UPDATE #T SET NAME = NULLIF(NAME,'')
SELECT * FROM #T 
 1  JOHN
 2  NULL
 3  NULL

or you can examine NULL as '' using SELECT ISNULL(NULL,'')

-- here you set NULL to ''
UPDATE #T SET NAME = ISNULL(NULL,'') WHERE NAME IS NULL
SELECT * FROM #T
1   JOHN
2   -- is empty string
3   -- is empty string

--clean up
DROP TABLE #T