SQL Query help - Joins

Question

I have a couple of SQL tables which are:

customer(Id, Name, Address, PhoneNumber);
station (Id, City, Country, Location);
car(Id, Reg, Type, Milage);
contract(CustId, StationId, CarId);

I need to use these tables to find all customers who have rented a BMW. I have written out my query as:

SELECT *
FROM CUSTOMER, CAR, CONTRACT
WHERE CUSTOMER.ID = CONTRACT.CUSTID
AND CAR.TYPE = "BMW";

Would that be correct? My thinking was I need to join the tables as there is no way of knowing what customer has rented which car, but I feel like I might be doing something wrong? Any help would be appreciated.

Answer 1

No, your query is not correct -- it is incorrect logically and it is written poorly.

The better way to write the query is to use proper, explicit, standard JOIN syntax:

SELECT *
FROM CONTRACT co JOIN
     CUSTOMER cu
     ON co.CUSTID = cu.ID JOIN
     CAR ca
     ON co.CARID = ca.ID  -- this is a guess
WHERE ca.TYPE = 'BMW';

Notes:

• You are missing the JOIN condition between CONTRACT and CAR (presumably). With explicit JOIN syntax, this is pretty obvious.
• I am guessing at the right JOIN condition for CAR .
• Use single quotes for string constants. That is the SQL standard, although some databases do allow double quotes as well.
• SELECT * is usually to be discouraged. It is generally better practice to list the columns, especially in this case when the same column name might appear in different tables.

Answer 2

SELECT co.Id as ContractId , Name, Address, PhoneNumber,
cu.Id as CustId , City, Country, Location,
ca.Id as CarId, Reg, Type, Milage,
FROM CONTRACT co JOIN
CUSTOMER cu
ON co.CUSTID = cu.ID JOIN
CAR ca
ON co.CARID = ca.ID
STATION st
ON co.StationId = st.ID 
WHERE ca.TYPE = 'BMW';

You need to mention Column names explicitly because there will be name conflict for field Id which is common across many tables

PS: Just modifying query from Gordon Linoff answer on the same question

Answer 3

This should help: (You do not need to worry about Station as per your requirement)

SELECT distinct cus.*
FROM CONTRACT con 
JOIN CUSTOMER cus ON con.CUSTID = cus.ID 
JOIN CAR car ON co.CARID = car.ID 
WHERE car.TYPE = 'BMW';