SQL Query to Obtain the Oldest People - sql

Question

I am trying to find the oldest customers in my database. I want just their full names and their ages, but my current results are outputting all customers and their ages (not just the oldest). What am I doing wrong here?
SELECT
LTRIM(CONCAT(' ' + Prefix, ' ' + FirstName,
' ' + MiddleName, ' ' + LastName, ', ' + Suffix)),
MAX(DATEDIFF(year, BirthDate, GETDATE()))
FROM
Customers
WHERE
BirthDate is not null
GROUP BY
Prefix, FirstName, MiddleName, LastName, Suffix
ORDER BY
MAX(DATEDIFF(year, e.BirthDate, GETDATE())) desc
Note that there seems to be multiple customers with the same oldest age.

Answer 1

You have not defined what you mean with "oldest customers".
So I will give a few options you could try
to see a list of customers with the oldest on top, use a simple querie like this
SELECT FirstName, LastName, Suffix, BirthDate
FROM Customers
WHERE BirthDate is not null
ORDER BY BirthDate desc
to restrict the result to a number of rows, for example the 10 oldest, use top 10
SELECT top 10
FirstName, LastName, Suffix, BirthDate
FROM Customers
WHERE BirthDate is not null
ORDER BY BirthDate desc
to restrict the result to all customers born after a certain date, add to the where clause
SELECT FirstName, LastName, Suffix, BirthDate
FROM Customers
WHERE BirthDate is not null
and BirtDate < '19920101'
ORDER BY BirthDate desc

Answer 2

The first thing you need to do before you do anything else is define a unique numeric primary key on the Customers table.
ALTER TABLE Customers ADD Cust_Id int IDENTITY(1,1);
ALTER TABLE Customers ADD CONSTRAINT PK_Customers PRIMARY KEY (Cust_Id);
After you've doe that, the following code will give you the "oldest customer (or customers) in your database".
With qry1 As (
SELECT Cust_Id,
DATEDIFF(year, BirthDate, GETDATE()) As Age
FROM Customers
WHERE BirthDate is not null
),
qry2 As (
SELECT Max(Age) As Max_Age
FROM qry1
)
SELECT Customers.Cust_Id,
Customers.Prefix,
Customers.FirstName,
Customers.MiddleName,
Customers.LastName,
Customers.Suffix,
Qry1.Age
FROM Customers
Inner Join Qry1 On Customers.Cust_Id = Qry1.Cust_Id
Inner Join Qry2 On Qry1.Age = Qry2.Max_Age