SQL OVER (Partiton by) - Handle nulls - sql

Question

I have a following scenario:
Table Employees:
First Name | Last Name | Department | Salary
-----------|-----------|------------|---------
John | Doe | Finance | 20
John | Doe | R&D | 20
John | null | Finance | 20
John | long | Finance | 20
and I want 1 row for each (First Name,Last Name),
unless we have a null in the last name, and then i want just 1 row with (First Name,null)
for the above example the result is:
First Name | Last Name | Department | Salary
-----------|-----------|------------|---------
John | null | Finance | 20
but if i didn't have that record then the result should have been:
First Name | Last Name | Department | Salary
-----------|-----------|------------|---------
John | Doe | R&D | 20
John | long | Finance | 20
I guess the answer involves some Partition By-s, but I'm not sure where.
Right now I came to this:
SELECT FirstName,LastName, DEPARTMENT,Salary,RK FROM
(
select * from
SELECT EXT.*,
ROW_NUMBER() OVER(PARTITION BY EXT.FirstName,EXT.LastName
ORDER BY rownum ASC) AS RK
FROM Employees EXT
)
WHERE RK = 1 ;
Thanks !

Answer 1

Your problem is in the PARTITION clause. You want every first name where there is a surname unless at least one surname with that first name is NULL, in which case you want only those first names that have a NULL surname.
The answer here is to use RANK() instead of ROW_NUMBER(). RANK() does not create a consecutive list; instead rows with equal values get the same rank.
select firstname, lastname, department, salary, rk
from ( select a.*
, rank() over ( partition by firstname
order by case when lastname is null then 0
else 1
end
) as rnk
from employees a
)
where rnk = 1
This works by making the existence of a surname relevant rather than the surname itself.
Two more points:
You had a nested select without parenthesis. This won't work.
There's no point ordering by ROWNUM. By definition rownum returns rows in the order returned by the statement, which means the rows will always be in the order of the ROWNUM.

Answer 2

something like this:
SQL> create table person
2 (
3 fname varchar2(10),
4 lname varchar2(10),
5 dept varchar2(10),
6 sal number
7 );
Table created.
SQL> insert into person values ('John', 'Doe', 'Finance', 20);
1 row created.
SQL> insert into person values ('John', 'Doe', 'R&D', 20);
1 row created.
SQL> insert into person values ('John', '', 'Finance', 20);
1 row created.
SQL> insert into person values ('John', 'Long', 'Finance', 20);
1 row created.
SQL> insert into person values ('Paul', 'Doe', 'R&D', 30);
1 row created.
SQL> insert into person values ('Paul', 'Doe', 'Finance', 30);
1 row created.
SQL> insert into person values ('Paul', 'Long', 'Finance', 30);
1 row created.
SQL> select fname, lname, dept, sal
2 from (select fname, lname, dept, sal,has_null,
3 row_number() over(partition by fname,
4 case when has_null = 'N' then lname else null end
5 order by lname desc nulls first) rn
6 from (select fname, lname,
7 nvl(max(case when lname is null then 'Y'
8 end) over(partition by fname), 'N') has_null, dept, sal
9 from person))
10 where rn = 1;
FNAME LNAME DEPT SAL
---------- ---------- ---------- ----------
John Finance 20
Paul Doe R&D 30
Paul Long Finance 30

Answer 3

That query does the (same) trick, but preforms better.
SELECT fname,
lname,
dept,
sal
FROM (SELECT fname,
lname,
dept,
sal,
First_value(lname)
OVER(
partition BY fname
ORDER BY lname nulls first) null_domain,
Row_number()
OVER (
partition BY fname, lname
ORDER BY fname) r
FROM person)
WHERE ( ( null_domain IS NULL
AND lname IS NULL )
OR null_domain IS NOT NULL )
AND r = 1;