Keep only one record from left table - sql

Question

I need to get records after inner join without duplicate data from left table.
Parent table :
Parent id parent name
1 Douglas
Child table :
Parent id child name
1 George
1 Michael
With classic Oracle Inner join I get this :
Parent id parent name child name
1 Douglas George
1 Douglas Michael
But I need this result:(i need null values instead of parents names)
Parent id parent name child name
1 Douglas George
Null Null Michael
Of course, there are more parents and children.

Answer 1

Use row_number() or lag() or lead(), like here:
select case when rn = 1 then parent_id end parent_id,
case when rn = 1 then parent_name end parent_name,
child_id, child_name
from (select p.parent_id, p.parent_name, c.child_id, c.child_name,
row_number() over (partition by p.parent_id order by c.child_id) rn
from parents p join children c on p.parent_id = c.parent_id)
rextester demo
PARENT_ID PARENT_NAME CHILD_ID CHILD_NAME
---------- ----------- ---------- ----------
1 Douglas 1 George
2 Michael

Answer 2

You want to do a right join on child id
select p.parentid,p.parentname,c.childname
from parent p
right join child c
on p.parentid=c.childid

Answer 3

One way of doing this is with a CURSOR EXPRESSION:
select parent_id
, parent_name
, cursor ( select child_id
, child_name
from child
where child.parent_id = parent.parent_id )
from parent
How well this renders depends on the client you use to run it. Some handle the output better than others.
Alternatively you can treat it as a display issue. For instance, in SQL*Plus you could use BREAK ON parent_id ON parent_name to suppress repeating values. Find out more.
select parent.parent_id
, parent.parent_name
, child.child_id
, child.child_name
from parent
join child
on child.parent_id = parent.parent_id
order by parent.parent_id, child.child_id;
To make this work you have to have ordering.
"It seems more complicated than I thought. Is there other way without Cursor?"
That's because result sets are meant to be flat not jagged. Tweaking the query to implement display features often results in clunky or verbose SQL.
Talking of which, here's another solution for you:
with cte as (
select parent.parent_id
, parent.parent_name
, child.child_id
, child.child_name
, row_number() over (partition by parent.parent_id
order by child.child_id ) as prn
, row_number() over (order by parent.parent_id , child.child_id ) as rn
from parent
join child
on child.parent_id = parent.parent_id
)
select case when prn = 1 then parent_id else null end as parent_id
, case when prn = 1 then parent_name else null end as parent_name
, child_id
, child_name
from cte
order by rn
This generates two row numbers, one to track the parents, one to sort the entire row set.