Update records SQL? - sql-server

Question

First when I started this project seemed very simple. Two tables, field tbl1_USERMASTERID in Table 1 should be update from field tbl2_USERMASTERID Table 2. After I looked deeply in Table 2, there is no unique ID that I can use as a key to join these two tables. Only way to match the records from Table 1 and Table 2 is based on FIRST_NAME, LAST_NAME AND DOB. So I have to find records in Table 1 where:
tbl1_FIRST_NAME equals tbl2_FIRST_NAME
AND
tbl1_LAST_NAME equals tbl2_LAST_NAME
AND
tbl1_DOB equals tbl2_DOB
and then update USERMASTERID field. I was afraid that this can cause some duplicates and some users will end up with USERMASTERID that does not belong to them. So if I find more than one record based on first,last name and dob those records would not be updated. I would like just to skip and leave them blank. That way I wouldn't populate invalid USERMASTERID. I'm not sure what is the best way to approach this problem, should I use SQL or ColdFusion (my server side language)? Also how to detect more than one matching record?
Here is what I have so far:
UPDATE Table1 AS tbl1
LEFT OUTER JOIN Table2 AS tbl2
ON tbl1.dob = tbl2.dob
AND tbl1.fname = tbl2.fname
AND tbl1.lname = tbl2.lname
SET tbl1.usermasterid = tbl2.usermasterid
WHERE LTRIM(RTRIM(tbl1.usermasterid)) = ''
Here is query where I tried to detect duplicates:
SELECT DISTINCT
tbl1.FName,
tbl1.LName,
tbl1.dob,
COUNT(*) AS count
FROM Table1 AS tbl1
LEFT OUTER JOIN Table2 AS tbl2
ON tbl1.dob = tbl2.dob
AND tbl1.FName = tbl2.first
AND tbl1.LName = tbl2.last
WHERE LTRIM(RTRIM(tbl1.usermasterid)) = ''
AND LTRIM(RTRIM(tbl1.first)) <> ''
AND LTRIM(RTRIM(tbl1.last)) <> ''
AND LTRIM(RTRIM(tbl1.dob)) <> ''
GROUP BY tbl1.FName,tbl1.LName,tbl1.dob
Some data after I tested query above:
First Last DOB Count
John Cook 2008-07-11 2
Kate Witt 2013-06-05 1
Deb Ruis 2016-01-22 1
Mike Bennet 2007-01-15 1
Kristy Cruz 1997-10-20 1
Colin Jones 2011-10-13 1
Kevin Smith 2010-02-24 1
Corey Bruce 2008-04-11 1
Shawn Maiers 2016-08-28 1
Alenn Fitchner 1998-05-17 1
If anyone have idea how I can prevent/skip updating duplicate records or how to improve this query please let me know. Thank you.

Answer 1

You could check for and avoid duplicate matches using with common_table_expression (Transact-SQL)
along with row_number()., like so:
with cte as (
select
t.fname
, t.lname
, t.dob
, t.usermasterid
, NewUserMasterId = t2.usermasterid
, rn = row_number() over (partition by t.fname, t.lname, t.dob order by t2.usermasterid)
from table1 as t
inner join table2 as t2 on t.dob = t2.dob
and t.fname = t2.fname
and t.lname = t2.lname
and ltrim(rtrim(t.usermasterid)) = ''
)
--/* confirm these are the rows you want updated
select *
from cte as t
where t.NewUserMasterId != ''
and not exists (
select 1
from cte as i
where t.dob = i.dob
and t.fname = i.fname
and t.lname = i.lname
and i.rn>1
);
--*/
/* update those where only 1 usermasterid matches this record
update t
set t.usermasterid = t.NewUserMasterId
from cte as t
where t.NewUserMasterId != ''
and not exists (
select 1
from cte as i
where t.dob = i.dob
and t.fname = i.fname
and t.lname = i.lname
and i.rn>1
);
--*/
I use the cte to extract out the sub query for readability. Per the documentation, a common table expression (cte):
Specifies a temporary named result set, known as a common table expression (CTE). This is derived from a simple query and defined within the execution scope of a single SELECT, INSERT, UPDATE, or DELETE statement.
Using row_number() to assign a number for each row, starting at 1 for each partition of t.fname, t.lname, t.dob. Having those numbered allows us to check for the existence of duplicates with the not exists() clause with ... and i.rn>1

Answer 2

You could use a CTE to filter out the duplicates from Table1 before joining:
; with CTE as (select *
, count(ID) over (partition by LastName, FirstName, DoB) as IDs
from Table1)
update a
set a.ID = b.ID
from Table2 a
left join CTE b
on a.FirstName = b.FirstName
and a.LastName = b.LastName
and a.Dob = b.Dob
and b.IDs = 1
This will work provided there are no exact duplicates (same demographics and same ID) in table 1. If there are exact duplicates, they will also be excluded from the join, but you can filter them out before the CTE to avoid this.

Answer 3

Please try below SQL:
UPDATE Table1 AS tbl1
INNER JOIN Table2 AS tbl2
ON tbl1.dob = tbl2.dob
AND tbl1.fname = tbl2.fname
AND tbl1.lname = tbl2.lname
LEFT JOIN Table2 AS tbl3
ON tbl3.dob = tbl2.dob
AND tbl3.fname = tbl2.fname
AND tbl3.lname = tbl2.lname
AND tbl3.usermasterid <> tbl2.usermasterid
SET tbl1.usermasterid = tbl2.usermasterid
WHERE LTRIM(RTRIM(tbl1.usermasterid)) = ''
AND tbl3.usermasterid is null