Joining two joins in PostgreSQL - sql

Question

Background
I've needed to learn some PostgreSQL quickly and from scratch in order to do a data analysis project about car insurance. I have a locally stored PostgreSQL database of fairly decent size (around 8gb worth of data on insurance claims for vehicles like cars and motorcycles), and I've needed to JOIN and UNION ALL a couple of things in order to get the table I need for my statistical models.
The first part of what I've needed to do is this thing, a JOIN inside of a UNION ALL between two tables about car claims and motorcycle claims:
select
l.customer_combined_id,
l.claim_id,
l."Part_Cd",
l.service_date,
h.principal_problem_cd,
h.problem_code_vers_flag
from claims.auto_claims_line_items as l
JOIN claims.auto_claims_general h on l.claim_id = h.claim_id
UNION ALL
select
l.customer_combined_id,
l.claim_id,
l."Part_Cd",
l.entry_date as service_date,
NULL as principal_problem_cd,
NULL as problem_code_vers_flag
from claims.motorcycle_claims_line_items as l
This yields a table that looks like this (column names abbreviated for aesthetics):
cust_comb_id| claim_id | "Part_Cd" | svc_date | prin_prob_cd | prob_cd_vers_flg |
------------+----------+-----------+----------+--------------+------------------|
| | | | | |
As you can see, the car claims have some columns that the motorcycle claims don't have. This is fine -- I've filled those in as NULL in order to get the UNION ALL to work. Now the car claims table is nicely stacked on top of the motorcycle claims table. So far, so good.
The second part of what I've done so far is this other thing, which concerns data about car and motorcycle insurance policyholders ("customers"):
select m.customer_dob,
m.customer_id,
m.customer_gender_cd,
m.customer_zip_cd,
c.customer_combined_id
from customer."Customer" m
JOIN customer.customer_combined_crosswalk c on m.customer_id = c.customer_id
The result of which looks like this:
dob | customer_id | gender_cd | zip_cd | cust_comb_id |
----+-------------+-----------+--------+--------------|
| | | | |
The Problem
I've figured out two halves of my data manipulation, but I don't know how to put these halves together, so to speak. I want (I think) to left join these two things on cust_comb_id, but I'm not sure how to write it. I want to keep everything in the first part (the claim data) and bring in data from the second part (the policyholders / customers) when cust_comb_id matches, and give null values if it doesn't. Here's a visual of what I'm looking for:
cust_comb_id| claim_id | "Part_Cd" | svc_date | prin_prob_cd | prob_cd_vers_flg |dob | cust_id | gender_cd | zip_cd |
------------+----------+-----------+----------+--------------+------------------|----+---------+-----------+--------+
| | | | | | | | | |
What I've tried
I've tried to use subqueries to join these joins, but I keep getting errors. Edit:
Here's a concrete example of something I've tried:
select *
from
(select
l.customer_combined_id,
l.claim_id,
l."Part_Cd",
l.service_date,
h.principal_problem_cd,
h.problem_code_vers_flag
from claims.auto_claims_line_items as l
JOIN claims.auto_claims_general h on l.claim_id = h.claim_id
UNION ALL
select
l.customer_combined_id,
l.claim_id,
l."Part_Cd",
l.entry_date as service_date,
NULL as principal_problem_cd,
NULL as problem_code_vers_flag
from claims.motorcycle_claims_line_items as l) as cl
LEFT JOIN
select m.customer_dob,
m.customer_id,
m.customer_gender_cd,
m.customer_zip_cd,
c.customer_combined_id
from customer."Customer" m
JOIN customer.customer_combined_crosswalk c on m.customer_id = c.customer_id
This yields the error ERROR: syntax error at or near "select".
Any help is much appreciated.
[Note: customer_combined_id and customer_id are two different things: the combined id is unique, and made to account for when a customer switches from one insurance plan - where they have one customer_id - to another, where they're given a new one.]

Answer 1

So it was a syntax issue.
OP already had all needed parts:
Part I and Part II subqueries were already implemented
it was defined how to join them
The only problem was a struggle with syntax.
I suppose this form would be the most readable:
WITH PartI AS(
select
l.customer_combined_id,
l.claim_id,
l."Part_Cd",
l.service_date,
h.principal_problem_cd,
h.problem_code_vers_flag
from claims.auto_claims_line_items as l
JOIN claims.auto_claims_general h on l.claim_id = h.claim_id
UNION ALL
select
l.customer_combined_id,
l.claim_id,
l."Part_Cd",
l.entry_date as service_date,
NULL as principal_problem_cd,
NULL as problem_code_vers_flag
from claims.motorcycle_claims_line_items as l
),
PartII AS (
select customer_dob,
customer_id,
customer_gender_cd,
customer_zip_cd,
customer_combined_id
from customer."Customer" m
JOIN customer.customer_combined_crosswalk c on m.customer_id = c.customer_id
)
SELECT
*
FROM
PartI P1
LEFT JOIN PartII P2
ON P1.customer_combined_id = P2.customer_combined_id;
https://www.db-fiddle.com/f/msAtD89dn4DndMtxukkgkP/2

Answer 2

Alex Yu's answer is better, but I wanted to post this because a) it also works and b) shows a neat use for views in SQL.
Take the first part, and make a view of it by adding a single line of CREATE OR REPLACE VIEW before the first select:
CREATE OR REPLACE VIEW clms AS
select
l.customer_combined_id,
l.claim_id,
l."Part_Cd",
l.service_date,
h.principal_problem_cd,
h.problem_code_vers_flag
from claims.auto_claims_line_items as l
JOIN claims.auto_claims_general h on l.claim_id = h.claim_id
UNION ALL
select
l.customer_combined_id,
l.claim_id,
l."Part_Cd",
l.entry_date as service_date,
NULL as principal_problem_cd,
NULL as problem_code_vers_flag
from claims.motorcycle_claims_line_items as l
Next, do the same for the second part:
CREATE OR REPLACE VIEW cstmr AS
select m.customer_dob,
m.customer_id,
m.customer_gender_cd,
m.customer_zip_cd,
c.customer_combined_id
from customer."Customer" m
JOIN customer.customer_combined_crosswalk c on m.customer_id = c.customer_id
Finally, do a SQL 101-level simple join of the two views:
select *
from clms
join cstmr m on clms.customer_combined_id = customer_combined_id
I bumped into this answer after posting the problem and was happy to find a (somewhat) elegant solution myself.