joining multiple tables on common attributes with disjoint rows - sql

Question

I have multiples tables which need to be joined on multiple common attributes such the different attributes can be shown in a single table.
table1
+--------+---------+-------+--------+
| make | model | year | kms |
+--------+---------+-------+--------+
| toyota | corolla | 1999 | 25000 |
| toyota | camry | 2002 | 50000 |
+--------+---------+-------+--------+
table2
+--------+---------+-------+---------+
| make | model | year | mileage |
+--------+---------+-------+---------+
| toyota | corolla | 1999 | 20 |
| toyota | qualis | 2004 | 25 |
+--------+---------+-------+---------+
table3
+--------+----------+-------+-------+
| make | model | year | color |
+--------+----------+-------+-------+
| toyota | camry | 2002 | blue |
| toyota | rav4 | 2006 | green |
+--------+----------+-------+-------+
I'm doing the following to join the results
select
* from table1 as a
full join table2 as b
using (make, model, year)
full join table3 as c
using (make, model, year)
What I need is a table like below.
+--------+---------+-------+-------+----------+--------+
| make | model | year | kms | mileage | color |
+--------+---------+-------+-------+----------+--------+
| toyota | corolla | 1999 | 25000 | 20 | |
| toyota | camry | 2002 | 50000 | | blue |
| toyota | qualis | 2004 | | 25 | |
| toyota | rav4 | 2006 | | | green |
+--------+---------+-------+-------+----------+--------+
However I get results with the make,model,year duplicated with some empty value for some rows.
How do I go about getting the required. Note that, for the real data set I'm working with, there are 5 common attributes per table and around 20-40 different attributes per table.

Answer 1

Duplicates could be due to the full join you are using. Change those to left joins as given below:
SELECT
A.MAKE, A.MODEL, A.YEAR, T1.KMS, T2.MILEAGE, T3.COLOR
FROM
(SELECT MAKE, MODEL, YEAR FROM TABLE1 UNION
SELECT MAKE, MODEL, YEAR FROM TABLE2 UNION
SELECT MAKE, MODEL, YEAR FROM TABLE3) A
LEFT JOIN TABLE1 T1
ON T1.MAKE = A.MAKE AND T1.MODEL = A.MODEL AND T1.YEAR = A.YEAR
LEFT JOIN TABLE2 T2
ON T2.MAKE = A.MAKE AND T2.MODEL = A.MODEL AND T2.YEAR = A.YEAR
LEFT JOIN TABLE3 T3
ON T3.MAKE = A.MAKE AND T3.MODEL = A.MODEL AND T3.YEAR = A.YEAR;;