How to get the count of null values for each column in table

Use jsonb functions:

create table my_table(id int primary key, val numeric, str text, day date);
insert into my_table values
(1, 10, 'a', '2018-01-01'),
(2, 20, 'b', null),
(3, 30, null, null),
(4, null, null, null);

select key as column, count(*) as null_values
from my_table t
cross join jsonb_each_text(to_jsonb(t))
where value is null
group by key;

 column | null_values 
--------+-------------
 val    |           1
 str    |           2
 day    |           3
(3 rows)        

Working example in rextester.

This Query should Create a query to do that:

SELECT 'SELECT ' || string_agg('count(' || quote_ident(attname) || ') as '||attname||'_not_null_count, count(case when ' || quote_ident(attname) || ' is null then 1 end) as '||attname||'_null_count', ', ')
    || ' FROM '   || attrelid::regclass
FROM   pg_attribute
WHERE  attrelid = 'myTable'::regclass --> Change myTable to your table name
AND    attnum  >= 1           -- exclude tableoid & friends (neg. attnum)
AND    attisdropped is FALSE  -- exclude deleted columns
GROUP  BY attrelid;

You can after that transpose columns to rows on excel.

You can have sql query to get that details like below -

select 'col1Name', count(col1Name) from table where col1Name is null
union
select 'col2Name', count(col2Name) from table where col2Name is null
union ...
select 'col20Name', count(col20Name) from table where col20Name is null

If it is oracle, then you can write some dynamic SQL in stored procedure as well.

count(nmuloc) only counts rows where the column nmuloc IS NOT NULL. count(*) counts all rows, regardless of anything being NULL or not. So the difference of them is the count of rows where nmuloc IS NULL.

SELECT count(*) - count(nmuloc1) count_of_nulls_in_nmuloc1,
       ...
       count(*) - count(nmuloc20) count_of_nulls_in_nmuloc20
       FROM elbat;

You can see that in all_tab_cols, once the table is analyzed or gathered stats on that table.

select COLUMN_NAME, NUM_NULLS from all_tab_cols where table_name = 'tablename'