SQL Datetime Not Equal To Is Not Working - mysql

Question

I have a database consisting of a Customer, Product, and Transaction table.
I'm trying to write an SQL statement to list all customers' names and SSN's for those customers who have made no transaction in the year 2000.
The TransactionDate column in the Transaction table is a Date/Time data type (e.g. 2000-12-18 00:00:00).
This is the SQL code I've written:
SELECT DISTINCT CustomerName, Customer.CustomerSSN
FROM Customer, Transaction
WHERE Customer.CustomerSSN=Transaction.CustomerSSN
AND YEAR(TransactionDate)<>2000;
The not equal to symbol (<>) seems to not be working for some reason. When I change it to an equal sign, it does return the correct result...
Any advice is appreciated.

Answer 1

I'd change the approach.
The following query doesn't need distinct or GROUP BY because none of the customer records are joined to multiple transaction records.
It also works for customers who have never made Any transactions.
Finally, it uses >= AND < rather than YEAR()=2000. This enable an index seek rather than a full scan (assuming that you have an approriate index on the transactions table).
SELECT
CustomerName,
CustomerSSN
FROM
Customer
WHERE
NOT EXISTS (
SELECT *
FROM Transaction
WHERE CustomerSSN = Customer.CustomerSSN
AND TransactionDate >= '20000101'
AND TransactionDate < '20010101'
)

Answer 2

SELECT DISTINCT
Customer.CustomerName,
Customer.CustomerSSN
FROM Customer
LEFT JOIN Transaction
ON Customer.CustomerSSN=Transaction.CustomerSSN
AND YEAR(TransactionDate) = 2000
WHERE Transaction.TransactionDate IS NULL
This query joins transactions onto customers, however joins specifically Transactions from the year 2000. Any customers which have no patching record from Transactions therefore had no transaction in that year. Therefore you are looking for Transaction.TransactionDate IS NULL
In your own query, you are simply finding any customers who had transactions in a year that was not 2000, however some may have had transactions within the year 2000 also.

Answer 3

SELECT CustomerName, CustomerSSN
FROM Customer
WHERE CustomerSSN NOT IN (
SELECT CustomerSSN
FROM Transaction
WHERE Year(TransactionDate)=2000);

Answer 4

I know its solved, but still wanted to post this as an additional answer here (may be helpful to others).
An Alternate way of fixing this is to use NULLIF operator which is a least modification to the original query and I presume it to be a better replacement if <> doesn't work.
SELECT DISTINCT CustomerName, Customer.CustomerSSN
FROM Customer, Transaction
WHERE Customer.CustomerSSN=Transaction.CustomerSSN
AND (NULLIF(YEAR(TransactionDate), 2000) IS NOT NULL)