Identifying duplicates within a table: looking for query advice - tsql

Question

So I am trying to identify duplicated contact records within an account, and looking for the best way to do this. There is a an account table, and a contact table. Below is the query I've come up with to give me what I need, but I feel like there is probably a better/more efficient way to do this, so looking for any feedback/advice. Thanks in advance!
SELECT * FROM sysdba.CONTACT a WITH(NOLOCK)
WHERE EXISTS
(
SELECT ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL FROM sysdba.CONTACT b WITH(NOLOCK)
GROUP BY ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL
HAVING COUNT(*) > 1
AND a.ACCOUNTID = b.ACCOUNTID AND a.FIRSTNAME = b.FIRSTNAME AND a.LASTNAME = b.LASTNAME AND a.EMAIL = b.EMAIL
)
ORDER BY ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL
Here is another way I can do this, but having to use DISTINCT seems ugly..
SELECT DISTINCT a.CONTACTID, a.FIRSTNAME, a.LASTNAME, a.EMAIL FROM sysdba.CONTACT a WITH(NOLOCK)
JOIN sysdba.CONTACT b WITH(NOLOCK)
ON a.ACCOUNTID = b.ACCOUNTID AND a.FIRSTNAME = b.FIRSTNAME AND a.LASTNAME = b.LASTNAME AND a.EMAIL = b.EMAIL AND a.CONTACTID != b.CONTACTID
ORDER BY a.CONTACTID, a.FIRSTNAME, a.LASTNAME, a.EMAIL
When checking the execution plans for both, the first query is 37% compared to 63% in the second query, which is surprising, as I've always though (apparently wrong) that using joins is quicker than relying on a where clause.

Answer 1

Quite common practice, when you trying to identify duplicates, is to use windowed aggregate functions, such as COUNT() OVER (...) and ROW_NUMBER() OVER (...).
Below is the query that should return you groups of records, where there are more than one CONTACTID for the same ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL combination. In other words this query returns records, having duplicates, along with their duplicates:
;WITH cteCONTACT
AS (
SELECT ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL, CONTACTID,
CNT = COUNT(*) OVER (PARTITION BY ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL)
FROM sysdba.CONTACT
)
SELECT ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL, CONTACTID
FROM cteCONTACT
WHERE CNT > 1;
And the following query should return duplicates only, without records that they duplicates are:
;WITH cteCONTACT
AS (
SELECT ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL, CONTACTID,
NUM = ROW_NUMBER() OVER (
PARTITION BY ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL
ORDER BY CONTACTID)
FROM sysdba.CONTACT
)
SELECT ACCOUNTID, FIRSTNAME, LASTNAME, EMAIL, CONTACTID
FROM cteCONTACT
WHERE NUM > 1;