分组后从两个表中减去列（用于库存）

Question

So for my inventory system I have two tables with the same columns names (one for stock produced and one for stock shipped). I figured out how to group the columns by the product and then sum the quantities. So I want to run this query on both tables then subtract the quantity column from each table where the product variables match up.

I use this to add group and sum stock totals (in):

 $query = "SELECT id, type, color, product, SUM(Quantity) AS TotalQuantity FROM inventory GROUP BY id, color, type";

And I use this to group and sum stock shipments (out):

$query = "SELECT id, type, color, product, SUM(Quantity) AS TotalQuantity FROM shipped GROUP BY id, color, type";

So how do I subtract the quantity columns for each of these?

Edit:

I use this for output: (a table)

 echo '<tr><td>'. $row['product'] . '</td><td id="replace">' . $row['type'] . '</td><td>' . $row['color']. '</td><td>'. $row['TotalQuantity'];
 echo "</td></tr>";

Answer 1

This can be done entirely in one query. An INNER JOIN between these will allow you to subtract the quantities. The id, color, product columns are only needed from one of the tables in your SELECT list.

SELECT
  inv.id, 
  inv.color,
  inv.product,
  /* total inventory quantity */
  SUM(inv.Quantity) AS TotalInvQuantity,
  /* total shipped quantity */
  SUM(ship.Quantity) AS TotalShipQuantity,
  /* inventory quantity minus shipped quantity */
  SUM(inv.Quantity) - COALESCE(SUM(ship.Quantity), 0) AS SubtractedQuantity
FROM
  inventory inv
  LEFT JOIN shipped ship ON inv.id = ship.id AND inv.color = ship.color AND inv.product = ship.product
GROUP BY
  inv.id,
  inv.color,
  inv.product

Update after comments

SELECT
  inv.id,
  inv.color,
  inv.product,
  inv.TotalInvQuantity, 
  COALESCE(ship.TotalShipQuantity, 0) AS TotalShipQuantity,
  inv.TotalQuantity - COALESCE(ship.TotalQuantity, 0) AS SubtractedQuantity
FROM (
    SELECT id, product, color, SUM(Quantity) AS TotalInvQuantity
    FROM inventory
    GROUP BY id, product, color
  ) inv
  LEFT JOIN (
    SELECT id, product, color, SUM(Quantity) AS TotalShipQuantity
    FROM inventory
    GROUP BY id, product, color
  ) ship ON 
      inv.id = ship.id 
      AND inv.product = ship.product 
      AND inv.color = ship.color