Join multiple tables to return only one result for each record from main table - sql

Question

Currently I have three tables I am joining. I have data that was migrated from one system(old) to another system(new). I need to compare this data to ensure matches but also mismatches. I have three tables. One has the list of accounts being moved. The two systems have differnt ID types so this first table is a list of all IDs for the two tables and each account that was moved. So this is my base population.
ID1 ID2
ABC 123
ABC 123
ABC 123
DEF 456
DEF 456
DEF 456
I then have table 2 which is all the data from the old system.
ID Fname Lname
ABC John Smith
ABC Tom Smith
ABC Kate Smith
DEF Jason Thomas
DEF Ruby Thomas
DEF Alex Johnson
Then table 3 is all the data found in the new system.
ID Fname Lname
123 John Smith
123 Tom Smith
123 Kate Smith
456 Jason Thomas
456 Ruby Thomas
Right now when I join these tables on the ID I get a lot more rows than I need.
When I do my join I receive this:
ID Fname_old Lname_old ID2 Fname_new Lname_new
ABC John Smith 123 John Smith
ABC John Smith 123 Tom Smith
ABC John Smith 123 Kate Smith
I am trying to join them where it only returns the row that matches, and if it can't find a match I should still get the ID from the ID file and the data from table 2(old data) as this is the data that was sent to the new system.
ID1 ID2 Fname_old Lname_old Fname_new Lname_new
ABC 123 John Smith John Smith
ABC 123 Tom Smith Tom Smith
ABC 123 Kate Smith Kate Smith
DEF 456 Jason Thomas Jason Thomas
DEF 456 Ruby Thomas Ruby Thomas
DEF 456 Alex Johnson
The code I am using is:
Select a.ID1, a.ID2, b.fname as fname_old, b.lnam as lname_old,
c.fname as fname_new, c.lname as lname_new
from table1 a
left join table2 b
on a.ID1 = b.ID
left join table3 c
on a.ID2 = c.ID

Answer 1

If its just duplicate rows in your first table you could try distincting them in a derived table like below:
Select a.ID1, a.ID2, b.fname as fname_old, b.lnam as lname_old,
c.fname as fname_new, c.lname as lname_new
from (SELECT DISTINCT ID1, ID2 FROM table1) a
left join table2 b
on a.ID1 = b.ID
left join table3 c
on a.ID2 = c.ID

Answer 2

You are joining them on ID columns.
ID columns are usually UNIQUE while you have multiple identical IDs and specify join on those IDs.
Since you need to compare data, i suggest you lookup MATCH and how it works as that seems to be closer to what you are looking for here.

Answer 3

You can get a match using row_number():
Select a.ID1, a.ID2, b.fname as fname_old, b.lnam as lname_old,
c.fname as fname_new, c.lname as lname_new
from (select a.*,
row_number() over (partition by id order by id) as seqnum
from table1 a
) a left join
(select b.*,
row_number() over (partition by id order by id) as seqnum
from table2 b
) b
on a.ID1 = b.ID and a.seqnum = b.seqnum
(select c.*,
row_number() over (partition by id order by id) as seqnum
from table3 c
) c
on a.ID2 = c.ID and a.seqnum = c.seqnum;
Note: This does not preserve the "ordering" of the original values, so any rows can be matched with any other. Why? SQL tables represent unordered sets.
If there is an ordering in the tables, you can use that in the order by clauses to get a match consistent with the ordering.

Answer 4

If you have a compare chance for name and last name this code will work.
select DISTINCT a.ID1, a.ID2, b.fname as fname_old, b.lname as lname_old, c.fname as
fname_new, c.lname as lname_new from table2 b
left join table1 a on a.ID1=b.ID
left join table3 c on a.ID2=c.ID and b.Fname=c.Fname and b.Lname=c.Lname
My Result :
ID1 ID2 fname_old lname_old fname_new lname_new
ABC 123 John Smith John Smith
ABC 123 Kate Smith Kate Smith
ABC 123 Tom Smith Tom Smith
DEF 456 Alex Johnson NULL NULL
DEF 456 Jason Thomas Jason Thomas
DEF 456 Ruby Thomas Ruby Thomas

Answer 5

You say that this is data transferred to two systems. So you expect all data to match. You could hence reduce the query to only find data that doesn't match, if any.
Here is a SQL standard compliant query. You tagged your request with hive. I don't know about hive, so you may have to adjust the query.
select
t2.id as id1,
t3.id as id2,
t2.fname as fname_old,
t2.lname as lname_old,
t3.fname as fname_new,
t3.lname as lname_new
from table2 t2
full outer join t3
on t3.fname = t2.fname
and t3.lname = t2.lname
and exists (select null from table1 t1 where t1.id1 = t2.id and t1.id2 = t3.id)
where t2.id is null or t3.id is null;
This is a full anti join. It returns all rows that have no exact match in the other table. It doesn't, however guesstimate which deviating rows may be pairs. You will get a result like this:
ID1 | ID2 | Fname_old | Lname_old | Fname_new | Lname_new
----+-----+-----------+-----------+-----------+----------
DEF | | Alex | Johnson | |
GHI | | Jone | Miller | |
GHI | | Maxx | Miller | |
GHI | | Fritz | Miller | |
| 789 | | | Joan | Miller
| 789 | | | Max | Miller
| 799 | | | Fritz | Miller
As you see, you would have to examine this result manually. But ideally the query shouldn't return any row at all, which would just prove that everything went as expected and nobody (system or person) messed with the data :-)