FULL JOIN in MySQL connectiong 6 tables - mysql

Question

I have a question about FULL JOIN at MySql. I know that alternative is UNION but I have dificulties to combine them all.
I guess it would be already enough to get answer on 4 tables as 5th and 6th are same as in first 4.
Tables: Bill, Service, BS, Item, BI, Buyer
BS connect more Services to Bill and BI connect more Items to Bill. Buyer is 1:1 relation with Bill.
Tables example:
Bill:
----------------------
id | number | Buyer_id
1 | 12014 | 3
2 | 22014 | 2
3 | 32014 | 5
Services:
----------------------
id | cost
1 | 2
2 | 7
3 | 1
4 | 12
BS:
----------------------
id | Bill_id | Services_id
1 | 1 | 3
2 | 1 | 4
3 | 2 | 2
4 | 3 | 1
5 | 3 | 2
6 | 3 | 3
7 | 3 | 4
Item:
----------------------
id | cost
1 | 34
2 | 77
3 | 2
4 | 15
5 | 13
BI:
----------------------
id | Bill_id | Items_id
1 | 1 | 5
2 | 2 | 3
3 | 3 | 2
Buyer:
----------------------
id | name
1 | John
2 | Mary
3 | Dave
4 | Carl
5 | Jack
So far the closest I got to was that I used following SQL:
SELECT *
FROM Bill b
LEFT JOIN BI ON BI.Bill_id = b.id
LEFT JOIN BS ON BS.Bill_id = b.id
LEFT JOIN Item i ON i.id = BI.Item_id
LEFT JOIN Services s ON s.id = BS.Services_id
LEFT JOIN Buyer ON Buyer.id = b.Buyer_id
WHERE b.number = '12014'
Result gives me 2 Services and 1 duplicated Item but I want one Item and one NULL item (as only one item is attached to that bill.
Result that I get (just ids as it's shorter):
b.id | s.id | BS.id | i.id | BI.id | Buyer.id
1 | 3 | 1 | 1 | 5 | 3
1 | 4 | 2 | 1 | 5 | 3
And desired result in table:
b.id | s.id | BS.id | i.id | BI.id | Buyer.id
1 | 3 | 1 | 1 | 5 | 3
1 | 4 | 2 | NULL | NULL | 3 (or NULL, doesn't really matter)
I tried with others as well but I got even more rows than 2 (note that two are expected or if Bill.number=32014 then 4 rows).
Thank you!

Answer 1

You are taking the wrong approach.
The result of a query to a relational database is a relation - you may treat it as a rectangular matrix with rows and columns. Every row represents something (at least it should), every column represents an attribute and every cell represents the thing's attribute's value.
| [attr 1] | [attr 2]
-----------+---------------+-------------------------
[thing 1] | value | some value
[thing 2] | value | other value
[thing 3] | a value | yeah, a value
Now here is what you are trying to produce:
| [attr 1] | [attr 2] | | [other attr 3]
-----------+---------------+------------------+---------------------+-------------------
[thing 1] | value | some value | [other thing 1] | a value
[thing 2] | value | other value |
See? Attempting to return two relations with a single query. Not rectangular anymore, huh? Items and services are independent here, but you are trying to put them into a single row. Don't go this way, here are three queries for you:
-- Get bill/buyer details (1 row)
SELECT b.id, Buyer.id
FROM Bill b
LEFT JOIN Buyer ON Buyer.id = b.Buyer_id
WHERE b.number = '12014';
-- Get billed items (1 row per item)
SELECT BI.id, i.id
FROM Bill b
JOIN BI ON BI.Bill_id = b.id
JOIN Item i ON i.id = BI.Item_id
WHERE b.number = '12014';
-- Get billed services (1 row per service)
SELECT BS.id, s.id
FROM Bill b
JOIN BS ON BS.Bill_id = b.id
JOIN Services s ON s.id = BS.Services_id
WHERE b.number = '12014';
Note that item and services queries don't use left joins. You would like to return 0 rows if there are no items/services on the bill.
Then handle the results of them one by one in your application.
Edit:
Sometimes two (or more) entities share some common characteristics, for example in your application you could treat services and items as bill lines. In this case, this could be valid to retrieve all of them in a single query, but only this way using union:
-- Get bill lines (items and services)
SELECT BI.id AS bill_item_id, i.id AS item_id, NULL as bill_service_id, NULL as service_id
FROM Bill b
JOIN BI ON BI.Bill_id = b.id
JOIN Item i ON i.id = BI.Item_id
WHERE b.number = '12014';
UNION ALL
SELECT NULL AS bill_item_id, NULL AS item_id, BS.id as bill_service_id, s.id as service_id
FROM Bill b
JOIN BS ON BS.Bill_id = b.id
JOIN Services s ON s.id = BS.Services_id
WHERE b.number = '12014';
Which will return a result similar to what you originally expected:
BI.id | i.id | BS.id | s.id
5 | 1 | NULL | NULL
NULL | NULL | 1 | 3
NULL | NULL | 2 | 4
Note that:
each item and service is an individual bill line represented by a record. Don't try to artificially "compress" data across rows or columns
it's not the case in your schema, but most often there are also some shared attributes, like line id, quantity ordered or amount paid.