Getting duplicate rows after two JOINS and a UNION command - sql

Question

I have 3 tables in the following way:
Client with a client id
Cards with a card id and a FK with client id
Transactions with a transaction id and a FK with Card id
Every client can have more than a card, and a card can make more than a transaction.
I want to do the following: Obtain the 3 clients with the minor number of transactions.
My problem: my query returns a client 2 times.
SELECT TOP 3
ClientId, Name, LastName, QtyTrans
FROM
(SELECT
ClientId, Name, LastName, 0 AS QtyTrans
FROM
Client
JOIN
Card ON ClientId = IdClient
WHERE
CardId NOT IN (SELECT CardId FROM Transaction)
GROUP BY
ClientId, Name, LastName
UNION
SELECT
ClientId, Name, LastName, COUNT(TransId) AS QtyTrans
FROM
Client
JOIN
Card c ON ClientId = IdClient
JOIN
Transaction t ON c.CardId = t.CardId
GROUP BY
ClientId, Name, LastName
) TotalTrans
ORDER BY
QtyTrans
When the client has transactions with every card, just returns one row, but if one of the cards has no transactions returns one row with the count 0 and another one with the sum of the other cards transactions.
Result:
11 Martin Camejo 0,
7 Matias Barrenechea 0,
7 Matias Barrenechea 2
Any help?

Answer 1

I want to do the following: Obtain the 3 clients with the minor number of transactions.
I don't see what UNION has to do with this question. You just want LEFT JOIN and GROUP BY:
SELECT TOP (3) cl.ClientId, cl.Name, cl.LastName,
COUNT(t.CardId) AS QtyTrans
FROM Client cl LEFT JOIN
Card c
ON cl.ClientId = c.IdClient LEFT JOIN -- don't know which table has which column
Transaction t
ON t.CardId = c.CardId
GROUP BY cl.ClientId, cl.Name, cl.LastName
ORDER BY QtyTrans ASC;