How to count null columns in a row? - sql

Question

How to count all the columns in a table that have a null value?
The table having a large number of columns and the method should iterate over the columns in a dynamic manner.
In any given row (selected by an identifier), count the null cells.
Select count(number of null value cells) where id=1 from table
e.g:
I have a table consisting of 200 columns I want to know how many null
cells does the row with id=1 have

Answer 1

Basically, you need to check every column in a selected row if it's null or not.
Since you have 200+ columns in your table, manual approach seems tedious, so you can automate it a little bit and construct the query dynamically by querying user_tab_columns:
-- set up
create table t1(
rid number primary key,
c1 varchar2(17),
c2 date,
c3 timestamp,
c4 number
);
insert into t1
valueS(1, 'string', null, systimestamp, null);
commit ;
-- going to use DECODE function - doesnt require type consistency.
select 'decode('||column_name||', null, 1, 0)+' as res
from user_tab_columns
where table_name = 'T1'
Result:
RES
------------------------------
decode(RID, null, 1, 0)+
decode(C1, null, 1, 0)+
decode(C2, null, 1, 0)+
decode(C3, null, 1, 0)+
decode(C4, null, 1, 0)+
And final query:
select decode(C4, null, 1, 0)+
decode(C3, null, 1, 0)+
decode(C2, null, 1, 0)+
decode(C1, null, 1, 0)+
decode(RID, null, 1, 0) as num_of_nulls
from t1
where rid = 1
Result:
NUM_OF_NULLS
--------------
2

Answer 2

Try this. You can pass any ID and get the number of columns with NULL value.
CREATE TABLE TEST (ID NUMBER, B VARCHAR2(20), C NUMBER, D VARCHAR2(200));
INSERT INTO TEST VALUES (1,NULL,NULL,'XX');
SELECT COUNT(NULL_COLS)
FROM (
SELECT
to_number(extractvalue(xmltype(dbms_xmlgen.getxml('SELECT CASE WHEN '||COLUMN_NAME||' IS NULL THEN 0 ELSE NULL END COL_VAL FROM '||TABLE_NAME||' WHERE ID=&VALUE')),'/ROWSET/ROW/COL_VAL')) NULL_COLS
FROM USER_TAB_COLUMNS
WHERE TABLE_NAME='TEST');