Question

I want to join 2 databases together and work out the sum of each one with the same ID from both tables, if you get where im coming from…

I have one database with ID, product_name, product_description
and the other has ID, stock amount, date_added

i have joined the databases together using the JOIN function and it displays them correctly, but for each entry into the database it adds another row when i add stock and shows another row at the frontend. I would like it to group all of the same id’s into one row.

I have tried this:

$result = mysql_query("SELECT * FROM site_products JOIN site_trans ON site_products.product_id = site_trans.trans_product GROUP BY site_products.product_id");

it groups the same id’s together into one row, but the stock amount just displays the last amount added, not the total amount.

i was thinking about adding the SUM() to this but im not sure where it would go in the query.

Answer 1

From sqlcourse2.com

The GROUP BY clause will gather all of the rows together that contain data in the
specified column(s) and will allow aggregate functions to be performed on the 
one or more columns.

You can read more about GROUP BY with aggregate functions usage in following links:

• w3schools.com
• SQL: GROUP BY Clause
• GROUP BY clause
• MySQL GROUP BY – Aggregate Functions

Now I think it will be simple to understand the following query which solves your problem:

SELECT 
    site_products.product_id, 
    SUM(stock amount) AS total_amount
    FROM site_products JOIN site_trans 
            ON site_products.product_id = site_trans.trans_product 
    GROUP BY site_products.product_id