select two columns base on different condition in mysql

Question

<?php
    $inventory_in = $db->get_one("SELECT COALESCE(SUM(Qty), 0) 
                    FROM inventory Where id=12 AND inv_type = 'in' LIMIT 1");

    $inventory_out = $db->get_one("SELECT COALESCE(SUM(Qty), 0) 
                     FROM inventory Where id=12 AND inv_type = 'out' LIMIT 1");

    $inventory = $inventory_in - $inventory_out;
?>

Possible to combine two queries into one ?

Thank you.

Answer 1

Assuming you want both values output you can use SUM/CASE

SELECT
      COALESCE(SUM(CASE WHEN inv_type = 'in' THEN Qty ELSE 0 END ), 0) as  Sum1,
      COALESCE(SUM(CASE WHEN inv_type = 'out' THEN Qty ELSE 0 END ), 0) as  Sum2
FROM inventory 
WHERE id=12 
LIMIT 1

You may want to add and inv_type in ('in', 'out') to your where clause but its not required.

Answer 2

How about a conditional sum?

SELECT 
    SUM(CASE WHEN inv_type = 'in' THEN 1 ELSE 0 END) InSum
    SUM(CASE WHEN inv_type = 'out' THEN 1 ELSE 0 END) OutSum
WHERE
    id = 12

Or if you just want the result

SELECT 
    SUM(CASE inv_type WHEN 'in' THEN 1 WHEN 'out' THEN -1 ELSE 0 END) TotalSum
WHERE
    id = 12

Answer 3

SELECT inv_type, COALESCE(SUM(Qty), 0) 
FROM inventory
WHERE id=12 AND inv_type IN ('in', 'out')
GROUP BY inv_type