Count the Null columns in a row in SQL

Question

I was wondering about the possibility to count the null columns of row in SQL, I have a table Customer that has nullable values, simply I want a query that return an int of

Answer 1

There isn't a straightforward way of doing so like there would be with counting rows. Basically, you have to enumerate all the columns that might be null in one expression.

So for a table with possibly null columns a, b, c, you could do this:

SELECT key_column, COALESCE(a,0) + COALESCE(b,0) + COALESCE(c,0) null_col_count
  FROM my_table

Answer 2

You don't state RDBMS. For SQL Server 2008...

SELECT CustomerId,
       (SELECT COUNT(*) - COUNT(C)
        FROM   (VALUES(CAST(Col1 AS SQL_VARIANT)),
                      (Col2),
                      /*....*/
                      (Col9),
                      (Col10)) T(C)) AS NumberOfNulls
FROM   Customer  

Answer 3

Depending on what you want to do, and if you ignore mavens, and if you use SQL Server 2012, you could to it another way. .

The total number of candidate columns ("slots") must be known. 1. Select all the known "slots" column by column (they're known). 2. Unpivot that result to get a table with one row per original column. This works because the null columns don't unpivot, and you know all the column names. 3. Count(*) the result to get the number of non-nulls; subtract from that to get your answer.

Like this, for 4 "seats" in a car

select 'empty seats' = 4 - count(*)
from 
(
    select carId, seat1,seat2,seat3,seat4 from cars where carId = @carId
) carSpec
unpivot (FieldValue FOR seat in ([seat1],[seat2],[seat3],[seat4])) AS results

This is useful if you may need to do more later than just count the number of non-null columns, as it gives you a way to manipulate the columns as a set too.

Answer 4

Unfortunately, in a standard SQL statement you will have to enter each column you want to test, to test all programatically you could use T-SQL. A word of warning though, ensure you are working with genuine NULLS, you can have blank stored values that the database will not recognise as a true NULL (I know this sounds strange).

You can avoid this by capturing the blank values and the NULLS in a statement like this:

CASE WHEN col1 & '' = '' THEN 1 ELSE 0 END

Or in some databases such as Oracle (not sure if there are any others) you would use:

CASE WHEN col1 || '' = '' THEN 1 ELSE 0 END

Answer 5

This will give you the number of columns which are not null. you can apply this appropriately

SELECT   ISNULL(COUNT(col1),'') + ISNULL(COUNT(col2),'') +ISNULL(COUNT(col3),'') 
   FROM TABLENAME
   WHERE  ID=1

Answer 6

For ORACLE-DBMS only.

You can use the NVL2 function:

NVL2( string1, value_if_not_null, value_if_null )

Here is a select with a similiar approach as Michael Berkowski suggested:

SELECT (NVL2(col1, 0, 1) 
        + NVL2(col2, 0, 1)
        + NVL2(col3, 0, 1)
        ...
        ...
        + NVL2(col10, 0, 1)
        ) AS sum_of_nulls
FROM table
WHERE Customer=some_cust_id

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A more generic approach would be to write a PL/SQL-block and use dynamic SQL. You have to build a SELECT string with the NVL2 method from above for every column in the all_tab_columns of a specific table.