Select a column if other column is null

问题:

I need to select a field called ProgramID from a table and if the ProgramID is NULL then I need to select the value in the InterimProgramID from the same table and alias it as ProgramID.

How can I make a conditional SELECT statement to do this?

回答1:

You need the ISNULL/function operator.

Select ISNULL(a, b)

b gets selected if a is null.

Also, you can use the WHEN/THEN select option, lookup in BOL. Essentially: its c switch/case block meets SQL.

回答2:

You can use either the ISNULL function or the COALESCE function. They both do pretty much the same thing, however ISNULL only takes two parameters and COALESCE takes multiple parameters (returning the first non-null it encounters). Both try the first param, then the second, (and COALESCE continues on)

DECLARE @IAMNULL VARCHAR
DECLARE @IAMNOTNULL VARCHAR
SET @IAMNOTNULL = 'NOT NULL'

SELECT ISNULL(@IAMNULL, @IAMNOTNULL)
--Output: 'NOT NULL'

DECLARE @IAMNULLALSO VARCHAR

SELECT COALESCE(@IAMNULL, @IAMNULLALSO, @IAMNOTNULL)
--Output: 'NOT NULL'

回答3:

  select COALESCE ( ProgramID , InterimProgramID ) as 'ProgramID' 

回答4:

SELECT ProgramID
  FROM a_table
 WHERE ProgramID IS NOT NULL
UNION
SELECT InterimProgramID AS ProgramID
  FROM a_table
 WHERE ProgramID IS NULL;

回答5:

You can also use IFNULL function

select IFNULL(ProgramId,interimId) as ProgramId

回答6:

Coalesce('zzz-' + ProgramID, InterimID) as programID will still ignore ProgramID even if you have a pretext value. It's a cool little function