PostgreSQL searching the query result based on defined conditions - sql

Question

I am a beginner and I am working on a quite big query which returns about 50k of rows.
I've spent a big number of hours trying to figure out the issue and it looks that there is a gap in my knowledge and I would be really greatful if you could help me.
In order to show you the main idea I decided to simplify and split the data. I am presenting the relevant tables here:
*company*
+----+----------+---------------+----------------+
| ID | name | classificaton | special_number |
+----+----------+---------------+----------------+
| 1 | companyX | 309 | 242 |
+----+----------+---------------+----------------+
*branch*
+----+---------------+-------+
| ID | name | color |
+----+---------------+-------+
| 1 | environmental | green |
| 2 | navy | blue |
+----+---------------+-------+
*company_branch*
+------------+-----------+
| ID_company | ID_branch |
+------------+-----------+
| 1 | 1 |
| 1 | 2 |
+------------+-----------+
Ok as we have all the needed data presented I need to create a query which will select all the companies along with the main color of the branches they are working in.
A companyX can belong to more than one branch but I need to show only the main branch which can be calculated based on the three conditions below:
*if classification = 309 and special_number is even then show the relevant color and go the next company (ignore the next conditions)
*if classification = 209 and special_number is even then show the relevant color and go the next company (ignore the next condition)
*else show as grey
I created a query like that: (I know that the case phrase is not correct but I am keeping it as it shows in a better way what I am trying to accomplish)
SELECT c.ID, c.name, b.color, c.classification, c.special_number,
CASE
WHEN c.classification = 309 AND c.special_number % 2 = 0 THEN b.color
WHEN c.classification = 209 AND c.special_number % 2 = 0 THEN b.color
ELSE 'grey'
END AS 'case'
FROM company c INNER JOIN company_branch cb ON c.ID = cb.ID_company
INNER JOIN branch b ON b.ID = cb.ID_branch
then I get the following result
*result*
+----+----------+-------+----------------+----------------+------+
| ID | name | color | classification | special_number | case |
+----+----------+-------+----------------+----------------+------+
| 1 | companyX | green | 309 | 242 | green|
| 1 | companyX | blue | 309 | 242 | blue |
+----+----------+-------+----------------+----------------+------+
The problem is that if any company belongs to more than one branch then I always get many colors... what I would like to get is a list of companies with only one color of the main branch they are working in.
can you guys help the newbie ?

Answer 1

I think you want something like this:
SELECT DISTINCT ON (c.id) c.ID, c.name,
COALESCE(b.color, 'grey') as color
c.classification, c.special_number,
FROM company c LEFT JOIN
company_branch cb
ON c.ID = cb.ID_company LEFT JOIN
branch b
ON b.ID = cb.ID_branch
ORDER BY c.Id,
(CASE WHEN c.classification = 309 AND c.special_number % 2 = 0 THEN 1
WHEN c.classification = 209 AND c.special_number % 2 = 0 THEN 2
ELSE 3
END);
DISTINCT ON is a (useful) Postgres extension that returns one row per item(s) in parentheses. The returned row is based on the GROUP BY. The first key(s) in the ORDER BY need to be the item(s). The following specifies what "first" means.
I switched the joins to being outer joins, so all companies are in the result set.