SQL best alternative to a LEFT JOIN and WHERE statement? - sql-server

Question

Basically I have two tables. Customer and Purchase table. My Problem is the Purchase Table is very large and causing performance issues, and I'm trying to keep my code organized into relevant CTE's.
I'm trying to pull all the purchase records for those who purchased a Guitar Type A or have no purchases.
I want to filter out any customer who didn't buy a GuitarType A but still keep customer who didn't buy anything.
Here's my code:
WITH Purchases AS
(
SELECT
, CustID
, GuitarType
FROM
Purchase
WHERE
GuitarType = 'A'
)
,
RelevantCustomers AS
(
SELECT
P.Custid
, P.PurchaseDate
, C.CustType
FROM
Customer
)
SELECT
Custid
, GuitarType
, PurchaseDate
FROM
Purchases AS p
INNER JOIN
RelevantCustomers AS rc ON p.CustId= rc.CustId
Customer:
+--------+-------------+----------+
| CustId | CreatedDate | CustType |
+--------+-------------+----------+
| 1 | 01/01/2017 | A |
+--------+-------------+----------+
| 2 | 01/01/2018 | B |
+--------+-------------+----------+
| 4 | 01/01/2018 | C |
+--------+-------------+----------+
Purchase
+----------+--------------+------------+
| GuitarId | PurchaseDate | GuitarType |
+----------+--------------+------------+
| 1 | 04/01/2018 | A |
+----------+--------------+------------+
| 1 | 05/01/2018 | A |
+----------+--------------+------------+
| 1 | 06/01/2018 | C |
+----------+--------------+------------+
| 2 | 06/01/2018 | A |
+----------+--------------+------------+
| 2 | 06/01/2018 | B |
+----------+--------------+------------+
| 2 | 06/01/2018 | A |
+----------+--------------+------------+
If I use INNER JOIN then it will only return those who bought Guitar Type A. If I use LEFT then it will include all Customers.
One alternative is to move the "Where GuitarType = 'A' down to the where clause and do a LEFT JOIN but this will cause my code to be unorganized and potentially some performance issues.

Answer 1

This might do it
SELECT rc.Custid, p.GuitarType, p.PurchaseDate
FROM RelevantCustomers rc
LEFT JOIN Purchases p
ON p.CustId = rc.CustId
LEFT JOIN Purchases pn
ON pn.CustId = rc.CustId
AND p.GuitarType != 'A'
WHERE (p.GuitarType = 'A' OR p.CustID IS NULL)
and pn.CustID is null

Answer 2

You appear to want:
SELECT rc.Custid, p.GuitarType, p.PurchaseDate
FROM RelevantCustomers rc LEFT JOIN
Purchases p
ON p.CustId = rc.CustId
WHERE p.GuitarType = 'A' OR p.GuitarType IS NULL;
For performance, you want an index on Purchases(CustId).