为什么我的程序不起作用？

Question

I can't figure out why :(

I have to update the table products when I call the the procedure with the parameters(reference_requested, quantity_requested) and "return" (I don't how to do it, i read about return and params in, out and inout, but i didn't understand very well) the value of units sold

delimiter &&
create procedure compra (@reference_requested varchar, @quantity_requested int)
begin
  if quantity >= quantity_requested then
    update products
    set quantity = quantity - quantity_requested
    where reference_requested = reference;
    return quantity_requested;
  else
    return quantity;
    update products
    set quantity = 0;
    where reference_requested = reference;
  end if;
end
&&

Answer 1

Edit: As was pointed out in the comments, you cannot return a value (or use a return statement) in a procedure. If you're looking to do that, then you should use "create function" and not "create procedure" and additionally specify a return data type.

In your "else" the "return quantity" statement should be last, right before "end if" or the update will not run.

Answer 2

• It is important to indicate the difference between 9.4. User-Defined Variables (@quantity_requested) and routine parameters (quantity_requested) 13.1.15. CREATE PROCEDURE and CREATE FUNCTION Syntax , are different variables.

  Try:

   ...
   create procedure compra(reference_requested varchar(50), quantity_requested int)
   ...

• Does the statement:

   ...
   where reference_requested = reference;
   ...

reference_requested refers to the parameter or column in the table?. Avoid the names of the parameters/variables are equal to the columns of the tables, this can lead to problems.

• The RETURN :

   ...
   return quantity;
   ...

is used in functions for stored procedure is not used.

Answer 3

1. To return a value from a stored procedure use an OUT parameter.
2. Besides issues that have been already mentioned you can't just arbitrarily address quantity column in products table. It should be used in a context of some statement (eg SELECT ).

Now a streamlined version of your procedure might look something like

DELIMITER $$
CREATE PROCEDURE compra
(
  IN  _ref_requested VARCHAR(32), 
  IN  _qty_requested INT, 
  OUT _result INT
)
BEGIN
  DECLARE qty INT DEFAULT 0;

  SET qty = 
  (
    SELECT quantity
      FROM products
     WHERE reference = _ref_requested
     LIMIT 1
  );

  SET qty = IF(qty > _qty_requested, _qty_requested, qty);

  UPDATE products
     SET quantity = quantity - qty
   WHERE reference = _ref_requested;

  SET _result = qty;
END$$
DELIMITER ;

To be able to get the value of an OUT parameter you have to use a user variable

SET @result = NULL;
CALL compra('REF001', 25, @result);
SELECT @result;

Here is SQLFiddle demo