Getting data from multiple tables in sql - mysql

Question

Table 1: Invoices (inv_id, inv_value, cust_id)
Table 2: Customers (cust_id, sales_rep)
Table 3: Members (Member_id, member_cateogry, member_type, cust_id)
Note 1: Each Customer Pays multiple Invoices. (One-to-Many Relationship).
Note 2: Each Customer pays for one-or-more members (so more than one member could be related to one customer).
Note 3: Each Member has a category which could be 1 "represents Individual" OR 2 "represents Group".
Note 4: Each Member has a type which could be 1 "represents new" OR 2 "represents renew".
I want to get the TOTAL of the Invoice_value field for customers who's sales_rep = 1 and their member_category = 10 and their members_type = 123
Ex: What is the total amount of Invoices that customers paid IF the Sales_rep for these customers was 1 and the members they paid for were new and Individual members.
I tried:
SELECT Sum(invoices.inv_value) AS total
FROM invoices,
customers,
members
WHERE invoices.cust_id = customers.cust_id
AND members.custid = customers.cust_id
AND members.category = {$category}
AND members_type = {$type}
AND customers.sales_rep = {$id}";
AND
SELECT Sum(invoices.inv_value) AS total
FROM members
INNER JOIN customers
ON members.custid = customers.cust_id
INNER JOIN invoices
ON customers.cust_id = invoices.cust_id
WHERE customers.sales_rep = {$id}
AND members.category = {$category}
AND members.type = {$type}";
But both return double the Invoice value.
ex.: 1 Invoice for $120 in the Invoices table return $240 using these sql queries.
How can I fix this?

Answer 1

This is your query:
SELECT sum(i.inv_value) as total
FROM members m INNER JOIN
customers c
ON m.custid = c.cust_id INNER JOIN
invoices i
ON c.cust_id = i.cust_id
WHERE c.sales_rep = {$id} AND
m.category = {$category} AND
m.type = {$type}";
(Don't use implicit JOIN syntax using commas. It is archaic and less powerful.)
The problem is probably that two members can have the same customer id. You can check this by running:
select m.cust_id, count(*)
from members m
group by m.cust_id
having count(*) > 1;
It is also possible that customer ids are duplicated in customers.
Assuming the duplicates are only in members, change the query to exists:
SELECT sum(i.inv_value) as total
FROM customers c
ON INNER JOIN
invoices i
ON c.cust_id = i.cust_id
WHERE c.sales_rep = {$id} AND
EXISTS (SELECT 1
FROM members m
WHERE m.custid = c.cust_id AND
m.category = {$category} AND
m.type = {$type}
);

Answer 2

It seems like you are taking inv_value from invoices, which has many to one relationship with customers but customer table and members table have one to many relationship.
Say you have below data
Invoice Table
invoices.cust_id invoices.inv_value
custid1 100
Customer table
customer.cust_id
custid1
Members Table
members.cust_id members.category
custid1 1
custid1 2
On join all three tables
customer.cust_id members.cust_id invoices.inv_value members.category
custid1 custid1 100 1
custid1 custid1 100 2
if you notice as custid1 exist in 2 member_category, invoice value is also duplicated.
To solve this, first you can take the distinct records, then summing those distinct records would help you solve your problem as below
Solution
SELECT Sum(invoices.inv_value) AS total
FROM(
SELECT DISTINCT members.custid, invoices.inv_value inv_value
FROM members
INNER JOIN customers
ON members.custid = customers.cust_id
INNER JOIN invoices
ON customers.cust_id = invoices.cust_id
WHERE customers.sales_rep = {$id}
AND members.category = {$category}
AND members.type = {$type});