Count(*) Includes Null Values

COUNT(*) Includes Null Values?

If you have this table

Table1:

 Field1    Field2    Field3
 ---------------------------
   1         1         1
  NULL      NULL      NULL
   2         2        NULL
   1         3         1

Then

 SELECT COUNT(*), COUNT(Field1), COUNT(Field2), COUNT(DISTINCT Field3)
 FROM Table1

Output Is:

 COUNT(*) = 4; -- count all rows, even null/duplicates

 -- count only rows without null values on that field
 COUNT(Field1) = COUNT(Field2) = 3

 COUNT(Field3) = 2 
 COUNT(DISTINCT Field3) = 1 -- Ignore duplicates

Why aren't nulls counted in COUNT(columnname)

COUNT counts values, since null is not a value it does not get counted.

If you want to count all null values you could do something like this:

SELECT COUNT(ID) as NotNull, SUM(CASE WHEN ID IS NULL then 1 else 0 end) as NullCount

How to count null values in postgresql?

Use count(*):

select count(*) from train where "column" is NULL;

count() with any other argument counts the non-NULL values, so there are none if "column" is NULL.

How to include count of NULL values in a temp table without changing the NULL data to 0?

Depending on your DBMS, you could convert null to 0.

SQL Server:

select 
        LoanId,
        Constraints_Count = count(isnull(ConstraintId,0))
    into #Test
    from LoanExample
    group by LoanId

Oracle:

select 
        LoanId,
        Constraints_Count = count(nvl(ConstraintId,0))
    into #Test
    from LoanExample
    group by LoanId

Other:

select 
        LoanId,
        Constraints_Count = count(case when ConstraintId is null then 0 else ConstraintId end))
    into #Test
    from LoanExample
    group by LoanId

NULL value count in group by

It's pretty simple:

count(<expression>) counts the number of values. Like most aggregate functions, it removes null values before doing the actual aggregation.

count(*) is a special case that counts the number of rows (regardless of any null).

count (no matter if * or <expression>) never returns null (unlike most other aggregate functions). In case no rows are aggregated, the result is 0.

Now, you have done a group by on an nullable column. group by put's null values into the same group. That means, the group for nbr null has two rows. If you now apply count(nbr), the null values are removed before aggregation, giving you 0 as result.

If you would do count(id), there would be no null value to be removed, giving you 2.

This is standard SQL behavior and honored by pretty much every database.

One of the common use-cases is to emulate the filter clause in databases that don't support it natively: http://modern-sql.com/feature/filter#conforming-alternatives

The exceptions (aggregate functions that don't remove null prior to aggregation) are functions like json_arrayagg, json_objectagg, array_agg and the like.

how can i count null values or non-null values with range of date using SQL

SELECT SUM(CASE WHEN FIELDNAME IS NULL THEN 1 ELSE 0 END) [CountOfNull],
       SUM(CASE WHEN FIELDNAME IS NOT NULL THEN 1 ELSE 0 END) [CountOfNotNull]
FROM DATABASE..TABLE WHERE FIELDNAME IS NULL OR (FIELDNAME BETWEEN 'DATE' AND 'DATE') 

T-SQL COUNT(*) counts NULLS but I don't want to count NULLS in row count

Some SQL implementations (I think this is also proper ANSI standard but don't know that for sure) exhibit a different behaviour for COUNT(*) VS COUNT(field).

The former will include NULLs, the latter will exclude them.

Count non-null values from multiple columns at once without manual entry in SQL

Consider below approach (no knowledge of column names is required at all - with exception of user)

select column, countif(value != 'null') nulls_count
from your_table t,
unnest(array( 
  select as struct trim(arr[offset(0)], '"') column, trim(arr[offset(1)], '"') value
  from unnest(split(trim(to_json_string(t), '{}'))) kv,
  unnest([struct(split(kv, ':') as arr)])
  where trim(arr[offset(0)], '"') != 'user'
)) rec
group by column          

if applied to sample data in your question - output is

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