How to set a default row for a query that returns no rows?

Question

I need to know how to return a default row if no rows exist in a table. What would be the best way to do this? I\'m only returning a single column from this particular table

Answer 1

One approach for Oracle:

SELECT val
FROM myTable
UNION ALL
SELECT 'DEFAULT'
FROM dual
WHERE NOT EXISTS (SELECT * FROM myTable)

Or alternatively in Oracle:

SELECT NVL(MIN(val), 'DEFAULT')
FROM myTable

Or alternatively in SqlServer:

SELECT ISNULL(MIN(val), 'DEFAULT')
FROM myTable

These use the fact that MIN() returns NULL when there are no rows.

Answer 2

I figured it out, and it should also work for other systems too. It's a variation of WW's answer.

select rate 
from d_payment_index
where fy = 2007
  and payment_year = 2008
  and program_id = 18
union
select 0 as rate 
from d_payment_index 
where not exists( select rate 
                  from d_payment_index
                  where fy = 2007
                    and payment_year = 2008
                    and program_id = 18 )

Answer 3

One table scan method using a left join from defaults to actuals:

CREATE TABLE [stackoverflow-285666] (k int, val varchar(255))

INSERT  INTO [stackoverflow-285666]
VALUES  (1, '1-1')
INSERT  INTO [stackoverflow-285666]
VALUES  (1, '1-2')
INSERT  INTO [stackoverflow-285666]
VALUES  (1, '1-3')
INSERT  INTO [stackoverflow-285666]
VALUES  (2, '2-1')
INSERT  INTO [stackoverflow-285666]
VALUES  (2, '2-2')

DECLARE @k AS int
SET @k = 0

WHILE @k < 3
    BEGIN
        SELECT  @k AS k
               ,COALESCE(ActualValue, DefaultValue) AS [Value]
        FROM    (
                 SELECT 'DefaultValue' AS DefaultValue
                ) AS Defaults
        LEFT JOIN (
                   SELECT   val AS ActualValue
                   FROM     [stackoverflow-285666]
                   WHERE    k = @k
                  ) AS [Values]
                ON 1 = 1

        SET @k = @k + 1
    END

DROP TABLE [stackoverflow-285666]

Gives output:

k           Value
----------- ------------
0           DefaultValue

k           Value
----------- ------------
1           1-1
1           1-2
1           1-3

k           Value
----------- ------------
2           2-1
2           2-2

Answer 4

This would be eliminate the select query from running twice and be better for performance:

Declare @rate int

select 
    @rate = rate 
from 
    d_payment_index
where 
    fy = 2007
    and payment_year = 2008
    and program_id = 18

IF @@rowcount = 0
    Set @rate = 0

Select @rate 'rate'

Answer 5

How about this:

SELECT DEF.Rate, ACTUAL.Rate, COALESCE(ACTUAL.Rate, DEF.Rate) AS UseThisRate
FROM 
  (SELECT 0) DEF (Rate) -- This is your default rate
LEFT JOIN (
  select rate 
  from d_payment_index
  --WHERE 1=2   -- Uncomment this line to simulate a missing value

  --...HERE IF YOUR ACTUAL WHERE CLAUSE. Removed for testing purposes...
  --where fy = 2007
  -- and payment_year = 2008
  --  and program_id = 18
) ACTUAL (Rate) ON 1=1

Results

Valid Rate Exists

Rate        Rate        UseThisRate
----------- ----------- -----------
0           1           1

Default Rate Used

Rate        Rate        UseThisRate
----------- ----------- -----------
0           NULL        0

Test DDL

CREATE TABLE d_payment_index (rate int NOT NULL)
INSERT INTO d_payment_index VALUES (1)

Answer 6

If your base query is expected to return only one row, then you could use this trick:

select NVL( MIN(rate), 0 ) AS rate 
from d_payment_index
where fy = 2007
  and payment_year = 2008
  and program_id = 18

(Oracle code, not sure if NVL is the right function for SQL Server.)