Get SQL Server result using subquery - sql

Question

I have a Members table like this:
PersonID FirstName Address City date
---------------------------------------------------------
3 Rasanga Skagen 21 South 2019-01-05
and a Persons table:
PersonID FirstName Address City date
-------------------------------------------------------
3 Rasanga Skagen 21 South 2019-01-06
1 Tom B. Skagen 21 Colombo 2018-01-07
2 Tom B. Skagen 21 Colombo 2019-01-05
I want to get Persons that do not exists in Members table using the FirstName column. For that I'm using this query:
SELECT *
FROM Persons p
WHERE NOT EXISTS (SELECT * FROM Members m WHERE m.FirstName = p.FirstName)
When I execute above query I'm getting same FirstName and 2 records but my requirement is if there's 2 records for same name retrieve latest record using date column. Therefore above scenario it should be Tom B. with 2018-01-07 record. If both records have same date should retrieve 1 record from 2 records.
Can somebody explain how to do this?

Answer 1

You can use the left join and checking the Members.PersonId is null.
create table Members(PersonID int
, FirstName varchar(20)
, Address varchar(50)
, City varchar(50)
, Dtdate date)
insert into Members values
(3, 'Rasanga', 'Skagen 21', 'South', '2019-01-05')
Create table Persons(PersonID int
, FirstName varchar(20)
, Address varchar(50)
, City varchar(50)
, Dtdate date)
insert into Persons values
(3, 'Rasanga', 'Skagen 21', 'South', '2019-01-06'),
(1, 'Tom B.', 'Skagen 21', 'Colombo', '2018-01-07'),
(2, 'Tom B.', 'Skagen 21', 'Colombo', '2019-01-05')
Select Persons.* from Persons
left join Members on Persons.PersonID = Members.PersonID
where Members.PersonId is null
Demo
Using the not exists you can check as shown below.
SELECT Persons.*
FROM Persons
WHERE NOT EXISTS (SELECT 1
FROM Members
WHERE Persons.PersonID = Members.PersonID)
Using the in operator
SELECT * FROM Persons
WHERE PersonID NOT IN (
SELECT PersonID FROM Members
)
To get the unique records based on the first name and date you can use the following query using ROW_NUMBER() function.
;WITH cte
AS (
SELECT Persons.*
,ROW_NUMBER() OVER (
PARTITION BY Persons.FirstName ORDER BY Persons.Dtdate DESC
) AS RN
FROM Persons
LEFT JOIN Members ON Persons.PersonID = Members.PersonID
WHERE Members.PersonId IS NULL )
SELECT *
FROM CTE
WHERE RN = 1
Output
PersonID FirstName Address City Dtdate RN
----------------------------------------------------------
2 Tom B. Skagen 21 Colombo 2019-01-05 1

Answer 2

You could use a window function as
SELECT T.PersonID,
T.FirstName,
T.Address,
T.City,
T.[Date]
FROM
(
SELECT *, ROW_NUMBER() OVER(PARTITION BY FirstName ORDER BY [Date] DESC) RN
FROM Persons
) T
WHERE NOT EXISTS
(
SELECT 1
FROM Members
WHERE FirstName = T.FirstName
) AND T.RN = 1;
Here is a db<>fiddle