How do I return my records grouped by NULL and NOT NULL?

Question

I have a table that has a processed_timestamp column -- if a record has been processed then that field contains the datetime it was processed, otherwise it is null.

Answer 1

Try the following, it's vendor-neutral:

select
    'null    ' as type,
    count(*)   as quant
    from       tbl
    where      tmstmp is null
union all
select
    'not null' as type,
    count(*)   as quant
    from       tbl
    where      tmstmp is not null

After having our local DB2 guru look at this, he concurs: none of the solutions presented to date (including this one) can avoid a full table scan (of the table if timestamp is not indexed, or of the indexotherwise). They all scan every record in the table exactly once.

All the CASE/IF/NVL2() solutions do a null-to-string conversion for each row, introducing unnecessary load on the DBMS. This solution does not have that problem.

Answer 2

In Oracle

SELECT COUNT(*), COUNT(TIME_STAMP_COLUMN)
FROM TABLE;

count(*) returns the count of all rows

count(column_name) returns the number of rows which are not NULL, so

SELECT COUNT(*) - COUNT(TIME_STAMP_COLUMN) NUL_COUNT,
                  COUNT(TIME_STAMP_COLUMN) NON_NUL_COUNT
FROM TABLE

ought to do the job.

If the column is indexed, you might end up with some sort of range scan and avoid actually reading the table.

Answer 3

I personally like Pax's solution, but if you absolutely require only one row returned (as I had recently), In MS SQL Server 2005/2008 you can "stack" the two queries using a CTE

with NullRows (countOf)
AS
(
    SELECT count(*) 
    FORM table 
    WHERE [processed_timestamp] IS NOT NULL
)
SELECT count(*) AS nulls, countOf
FROM table, NullRows
WHERE [processed_timestamp] IS NULL
GROUP BY countOf

Hope this helps

Answer 4

[T-SQL]:

select [case], count(*) tally
from (
  select 
  case when [processed_timestamp] is null then 'null'
  else 'not null'
  end [case]
  from myTable
) a 

And you can add into the case statement whatever other values you'd like to form a partition, e.g. today, yesterday, between noon and 2pm, after 6pm on a Thursday.

Answer 5

Oracle:

group by nvl2(field, 'NOT NULL', 'NULL')

Answer 6

If it's oracle then you can do:

select decode(field,NULL,'NULL','NOT NULL'), count(*)
from table
group by decode(field,NULL,'NULL','NOT NULL');

I'm sure that other DBs allow for similar trick.