CODEWITHSUNDEEP
sqloracle-databasejoincartesian-product
dokgu
dokgu

Reputation: 6070

SQL JOIN Multiplies Result Set - Cartesian Product

I have a base table and I want to join 2 more tables to get all the data I need. Upon running my query I get a Cartesian product because 2 of the tables (the main table and the other table I am joining) is being joined by a non-unique key.

Here's an example:

Invoice (i)

id    order_id       name             comment
-----------------------------------------------------------------
1     500            Males            Mice
2     500            Females          Mice
3     500            Courier Fee      Within City

Order (o)

id    order_number
------------------------------
500   AN5246516264

Order Items (oi)

id    order_id       strain_id    species_id      comments
-----------------------------------------------------------------
1100  500            858          9876            Mice
1101  500            858          9876            Mice
1102  500            NULL         NULL            Within City

Using the tables defined above here's my query:

Query

SELECT
    i.name, i.comment,
    o.order_number,
    oi.strain_id, oi.species_id
FROM invoice i
LEFT JOIN order o
    ON i.order_id = o.id
LEFT JOIN order_items oi
    ON o.id = oi.order_id

After running the query I get a Cartesian product as shown below (not necessarily in order):

name         comment        order_number     strain_id    species_id
-----------------------------------------------------------------------
Males        Mice           AN5246516264     858          9876 ---> I want this row
Males        Mice           AN5246516264     858          9876
Males        Within City    AN5246516264     NULL         NULL
Females      Mice           AN5246516264     858          9876
Females      Mice           AN5246516264     858          9876 ---> I want this row
Females      Within City    AN5246516264     NULL         NULL
Courier Fee  Mice           AN5246516264     858          9876
Courier Fee  Mice           AN5246516264     858          9876
Courier Fee  Within City    AN5246516264     NULL         NULL ---> I want this row

I understand what's happening here and I know why it results in a Cartesian product but I don't know how to solve my problem.

I just want to append the order_number, strain_id and the species_id to the Invoice (i) table.

And yes the table structure is built somewhat in a weird way. I do believe the database designer was intoxicated when he made the tables but that's not something I can change now.

Any suggestions?

EDIT

I've looked at all 3 tables and I can't find any other columns to join them with. I'm left with the order_id.

Upvotes: 0

Views: 601

Answers (1)

Ponder Stibbons
Ponder Stibbons

Reputation: 14858

You could number rows in tables i and oi at first using function row_number() and then use it as part of joining condition:

with i as  (select row_number() over (partition by order_id order by id) rn, i.*  
              from invoice i),
     oi as (select row_number() over (partition by order_id order by id) rn, oi.* 
              from order_items oi)
select i.name, i.comments, o.order_number, oi.strain_id, oi.species_id
  from i left join orders o on i.order_id = o.id
  left join oi on oi.order_id = o.id and oi.rn = i.rn

Test data and output:

create table invoice (id number(4), order_id number(4), 
  name varchar2(15), comments varchar2(20));
insert into invoice values (1, 500, 'Males', 'Mice');
insert into invoice values (2, 500, 'Females' ,'Mice');
insert into invoice values (3, 500, 'Courier Fee', 'Within City');
--
create table orders (id number(4), order_number varchar2(15));
insert into orders values (500, 'AN5246516264');
--
create table order_items(id number(5), order_id number(4), 
  strain_id number(5), species_id number(5), comments varchar2(20));
insert into order_items values(1100, 500, 858,  9876, 'Mice');
insert into order_items values(1101, 500, 858,  9876, 'Mice');
insert into order_items values(1102, 500, NULL, NULL, 'Within City');

Output:

NAME            COMMENTS             ORDER_NUMBER    STRAIN_ID SPECIES_ID
--------------- -------------------- --------------- --------- ----------
Males           Mice                 AN5246516264          858       9876
Females         Mice                 AN5246516264          858       9876
Courier Fee     Within City          AN5246516264

Upvotes: 1

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