Question

I am trying to update all the values in one table that exist in a second table, once for each occurence.

Example feeding_view table:

fishnr | food_name | stock
-------+-----------+-------
1      | f1        | 25
2      | f1        | 25
3      | f2        | 30

Example inventory table:

item_name | stock
----------+-------
f1        | 25
f2        | 30
f3        | 10

Currently I am trying to do this:

UPDATE inventory SET stock = stock - 1 WHERE item_name IN (SELECT food_name FROM feeding_view)

What happens is f1 only gets decremented a single time but I want it to decrement for each each f1 in the feeding_view. So the desired stock for f1 is 23 and for f2 is 29 but instead f1 is only decremented once to 24.

Answer 1

UPDATE inventory AS i
SET stock = stock - COALESCE(
            ( SELECT COUNT(*)
              FROM feeding_view AS f
              WHERE  f.food_name = i.item_name
            ), 0 ) ;

or maybe this, not sure:

UPDATE 
    ( SELECT i.item_name, i.stock, f.cnt
      FROM inventory AS i
        JOIN 
          ( SELECT food_name, COUNT(*) AS cnt
            FROM feeding_view
            GROUP BY food_name
          ) AS f
          ON f.food_name = i.item_name
    ) AS upd 
SET stock = stock - cnt ;