Delete and Merge Records in SQL Server - sql

Question

I have a table as following.
id | firstname| lastname | email | homephone
-------------------------------------------------------
1 | aaa | bbb | xxx#yyy.com | 12344444
2 | aaa | bbb | null | null
3 | ccc | ddd | zzz#fff.com | null
4 | ccc | ddd | null | 34343322
The issue is I want to keep only 1 record since these are considered as duplicates and merge the nulls so that the table appears as follows
1 aaa | bbb | xxx#yyy.com | 12344444
3 ccc | ddd | zzz#fff.com | 3433322
So far I have managed to get the duplicates using the following the code
Select
max(a.id) as original id, b.id as DuplicateId,
a.firstname, b.firstname as dup_fname,
a.lastname, b.lastname as dup_lname,
a.email, b.email
From
tbl_xxx a
join
tbl_xxx b on a.firstname = b.firstname
and a.lastname = b.lastname
and a.email is null
and a.homephone is null
and b.email is null
and b.homephone is null
and v.id < v2.id
Group by
b.id, a.firstname, b.firstname, a.lastname,
b.lastname, a.homephone, b.homephone
My merge query looks like this
update tbl_xxx
SET
email = email ,
phone = phone
where
firstname = firstname
and lastname = lastname
and email is null
and phone is null
Eventually I will get distinct rows.
Is my approach correct? kindly suggest how can I make my query more efficient

Answer 1

update tbl_tmpdupes3 SET
email = email ,
phone = phone ,
where
firstname=firstname
and lastname=lastname
and email is null
and homephone is null
will do absolutely nothing, since the query is not comparing the table against itself, but each row against itself. Using an update won't work either, you'll still have duplicates. What you want is to remove the duplicate data entirely, AFTER doing an update comparing the table to itself. So, basically we run one query that ensures all information in duplicates that is not null is copied to the originals, then we delete the higher value pk.
One way of tackling the problem update one column at a time:
update tbl_xxx SET tbl_xxx.email = tmp.email
FROM (SELECT tbl_xxx.firstname,tbl_xxx.lastname,tbl_xxx.email FROM tbl_xxx
WHERE NOT tbl_xxx.email IS NULL LIMIT 1)
AS tmp ON tmp.firstname = tbl_xxx.firstname AND tmp.lastname = tbl_xxx.lastname
WHERE tbl_xxx.email IS NULL;
update tbl_xxx SET tbl_xxx.phone = tmp.phone
FROM (SELECT tbl_xxx.firstname,tbl_xxx.lastname,tbl_xxx.phone FROM tbl_xxx
WHERE NOT tbl_xxx.phone IS NULL LIMIT 1)
AS tmp ON tmp.firstname = tbl_xxx.firstname AND tmp.lastname = tbl_xxx.lastname
WHERE tbl_xxx.phone is NULL;
The query finds the values of each column for first name and last name, and copies over first value it finds into null fields. So, if the original data was missing, it will add it. It may not be 100% correct if two different people in the DB have the same name, you'll have to take that into consideration.
That said, follow it up with this query, which should only delete the higher-pk row that is identical.
DELETE FROM tbl_xxx WHERE tbl_xxx.id IN (
SELECT max(id) FROM tbl_xxx
GROUP BY tbl_xxx.firstname,tbl_xxx.lastname,tbl_xxx.phone,tbl_xxx.email
HAVING count(tbl_xxx.id) > 1));
Edit: if there are potentially multiple duplicates, you could do:
DELETE FROM tbl_xxx WHERE tbl_xxx.id NOT IN (
SELECT min(id) FROM tbl_xxx
GROUP BY tbl_xxx.firstname,tbl_xxx.lastname,tbl_xxx.phone,tbl_xxx.email);

Answer 2

you can use Merge statement for this. Try this sample.
create table temptable (id int, firstname varchar(50), lastname varchar(50), email varchar(50), homephone varchar(50))
insert into temptable values
(1,'aaa' , 'bbb', 'xxx#yyy.com', 1234444),
(2,'aaa' , 'bbb', null, null),
(3,'ccc' , 'ddd', 'abc#ddey.com', null),
(4,'ccc' , 'ddd', null, 34343322 )
select * from temptable
;with cte as
(
select firstname, lastname
,(select top 1 id from temptable b where b.firstname = a.firstname and b.lastname = a.lastname and ( b.email is not null or b.homephone is not null)) tid
,(select top 1 email from temptable b where b.firstname = a.firstname and b.lastname = a.lastname and b.email is not null ) email
,(select top 1 homephone from temptable b where b.firstname = a.firstname and b.lastname = a.lastname and b.homephone is not null ) homephone
from temptable a
group by firstname , lastname
)
--select * from cte
merge temptable as a
using cte as b
on ( a.id = b.tid )
when matched
then
update set a.email = b.email , a.homephone = b.homephone
when not matched by source then
delete ;
select * from temptable
drop table temptable