PostgreSQL 3 Table Join Multiplying - sql

Question

I have 3 tables. The first has the records I want. The other two have categories to be applied to the first table. If the lookup value from table3 is found in the description, I want to return that category. Else, return the category in table2. I think I have that logic correct, but the results are being multiplied. How can I limit the results to just the table1 records I want, but apply the correct category?
Here is my query with an example schema. It should only return the first 6 rows in table1 with whichever category is correct, but it returns 10. http://sqlfiddle.com/#!15/fc6fa/49/0
SELECT table1.product_code, table1.date_signed, table1.description,
CASE
WHEN lower(table1.description) LIKE ('%' || lower(table3.lookup_value) || '%')
THEN table3.category
ELSE table2.category
END
FROM table1
LEFT JOIN table2 ON table2.psc_code = table1.product_code
LEFT JOIN table3 ON table3.psc_code = table1.product_code
WHERE date_signed = '2017-02-01';
create table table1 (
product_code int,
date_signed timestamp,
description varchar(20)
);
insert into table1
(product_code, date_signed, description)
values
(1, '2017-02-01', 'i have a RED car'),
(2, '2017-02-01', 'i have a blue boat'),
(3, '2017-02-01', 'i have a dark cat'),
(1, '2017-02-01', 'i have a green truck'),
(2, '2017-02-01', 'i have a blue rug'),
(3, '2017-02-01', 'i have a dark dog'),
(1, '2017-02-02', 'i REd NO SHOW'),
(2, '2017-02-02', 'i blue NO SHOW'),
(3, '2017-02-02', 'i dark NO SHOW');
create table table2 (
psc_code int,
category varchar(20)
);
insert into table2
(psc_code, category)
values
(1, 'vehicle'),
(2, 'vehicle');
create table table3 (
psc_code int,
lookup_value varchar(20),
category varchar(20)
);
insert into table3
(psc_code, lookup_value, category)
values
(1, 'fox', 'animal'),
(1, 'red', 'color'),
(1, 'box', 'shipping'),
(2, 'cat', 'animal');

Answer 1

You're trying to join 1 to many, and you only want one value.
SELECT table1.product_code, table1.date_signed, table1.description,
CASE
WHEN EXISTS (select 1 from table3
where table3.psc_code = table1.product_code and
lower(table1.description) LIKE ('%' || lower(table3.lookup_value) || '%'))
THEN (select table3.category from table3
where table3.psc_code = table1.product_code and
lower(table1.description) LIKE ('%' || lower(table3.lookup_value) || '%') limit 1)
ELSE (select table2.category
from table2
where table2.psc_code = table1.product_code
limit 1)
END
FROM table1
WHERE date_signed = '2017-02-01';
http://rextester.com/TQIY93378
+--------------+---------------------+----------------------+----------+
| product_code | date_signed | description | category |
+--------------+---------------------+----------------------+----------+
| 1 | 01.02.2017 00:00:00 | i have a RED car | color |
| 2 | 01.02.2017 00:00:00 | i have a blue boat | vehicle |
| 3 | 01.02.2017 00:00:00 | i have a dark cat | NULL |
| 1 | 01.02.2017 00:00:00 | i have a green truck | vehicle |
| 2 | 01.02.2017 00:00:00 | i have a blue rug | vehicle |
| 3 | 01.02.2017 00:00:00 | i have a dark dog | NULL |
+--------------+---------------------+----------------------+----------+

Answer 2

Yep you will get a cartesian product on this one.
Your problem is that you have multiple rows of each product_code that match fro table1. So when you join on table3, you get six records out with an id of 1. The other join conditions don't pose a situation where you have multiple matches on both sides, so that is how you get six product code 1 rows, 2 product code 2 rows and 2 product code 3 rows.
The solution is to join in ways in which the foreign key is targetting a unique row in the target table.
This really ought to be a useful example of why normalization and key awareness matter. Where you break the basic rules of functional dependencies, bad problems ted to multiply.