Tip

Finding NULLs in any table

My project manager recently asked me to send him a report in Excel that showed the number of rows with NULLs in each column. The report looked like this:

Plant Code No. Of Recs  PO NUM UNSPSC Code  Commodity code
.........
XXX   100    5  20  30
YYY   200    4  25  34
ZZZ   250   30   1
From this report, he could find out that out of 100 records belonging to the XXX plant, five rows doesn't have a P.O. number, 20 rows don't have a UNSPSC code, 30 rows don't have a commodity code, and so on.

It is very simple to find out the number of rows that have NULLs in one column, but each table has lot of columns and data warehouses have many tables, so it is very difficult to write different SQL statements and combine the results. I came up with following simple solution. It was tested on Oracle 8.1.7 and 9i.

SELECT a.plant_code,
        no_of_recs,
        po_number,
        unspsc_code,
        commodity_code
FROM
( SELECT plant_code, count(1) no_of_recs FROM po_lines GROUP BY plant_code )
A,
( SELECT plant_code, count(1) po_number FROM po_lines where po_number IS
NULL GROUP BY plant_code ) B,
( SELECT plant_code, count(1) unspsc_code FROM po_lines where unspsc_code IS
NULL GROUP BY plant_code ) C,
( SELECT plant_code, count(1) commodity_code FROM po_lines where
commodity_code IS NULL GROUP BY plant_code ) D
WHERE a.plant_code = b.plant_code (+)
   AND a.plant_code = c.plant_code (+)
   AND a.plant_code = d.plant_code (+)
ORDER BY a.plant_code
You

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can use following SQL to generate a FROM clause (which reduces typing). This is real helpful when you have lot of columns in a table:
select '( SELECT plant_code, count(1) '||lower(column_name) ||' FROM
po_lines where '|| lower(column_name) ||
' IS NULL GROUP BY plant_code'||' ) ' ||chr(column_id + 65) || ', '
from user_tab_columns where table_name = 'PO_LINES'
order by column_idp;
You can cut and paste the above SQL's output into a FROM clause. I am now able to give the project manager the number of rows with NULLs in each table within few minutes.

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This was first published in October 2002

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