complex query in postgresql - postgresql

Question

Hello and warm greetings to you all,
I am having some challenges writing a complex query in posgresql. the fact is i'm having problem writing this particular query period, and your help is kindly needed. lets get to it.
here are my tables
[products_tb] [client_tb] [sales_tb]
+--------------+ +-------------+ +-------------+--------------+-----+
| product_name | | client_name | | client_name | product_name | qty |
+--------------+ +-------------+ +-------------+--------------+-----+
| shoe | | john | | john | shoe | 20 |
+--------------+ +-------------+ +-------------+--------------+-----+
| belt | | bob | | john | belt | 9 |
+--------------+ +-------------+ +-------------+--------------+-----+
| kim | | bob | shoe | 2 |
+-------------+ +-------------+--------------+-----+
| bob | belt | 98 |
+-------------+--------------+-----+
| kim | shoe | 46 |
+-------------+--------------+-----+
| kim | belt | 3 |
+-------------+--------------+-----+
[query output] # this output will be displayed using php and html
+----------+-------+------+-----+-----+
| products | Total | john | bob | kim |
+----------+-------+------+-----+-----+
| shoe | 68 | 20 | 2 | 46 |
+----------+-------+------+-----+-----+
| belt | 110 | 9 | 98 | 3 |
+----------+-------+------+-----+-----+
I am trying to write a query which will allow me to produce the table [query output]. Any help will be much appreciated.
Thank you very much.

Answer 1

A version without using cross_tab:
select p.product_name, sum(s.qty),
sum(CASE WHEN c.client_name='john' THEN s.qty END) as john,
sum(CASE WHEN c.client_name='bob' THEN s.qty END) as bob,
sum(CASE WHEN c.client_name='kim' THEN s.qty END) as kim
from products_tb AS p
JOIN sales_tb AS s ON p.product_name=s.product_name
JOIN clients_tb AS c ON c.client_name=s.client_name
GROUP BY p.product_name;
http://sqlfiddle.com/#!12/afc9a/10

Answer 2

Here's one way of doing it using standard SQL:
SELECT st.product_name AS products,
SUM(st.qty) AS Total,
(SELECT SUM(st_john.qty)
FROM sales_tb st_john
WHERE client_name = 'john' AND st_john.product_name = st.product_name) AS john,
(SELECT SUM(st_bob.qty)
FROM sales_tb st_bob
WHERE client_name = 'bob' AND st_bob.product_name = st.product_name) AS bob,
(SELECT SUM(st_kim.qty)
FROM sales_tb st_kim
WHERE client_name = 'kim' AND st_kim.product_name = st.product_name) AS kim
FROM sales_tb st
GROUP BY product_name
See SQL Fiddle Demo