MySQL temporary variable does not work when executed from PHP script

Question

When executing the following query in MySQL the rows are updated correctly.

UPDATE market SET price = 
CASE 
WHEN 
    @order_price:=(
        @market_price:=(IF(
            market_id = 0, 
            (SELECT value FROM fiat WHERE id = 6), 
            (SELECT value FROM fiat WHERE id = 5)
        ) 
        + 
        (
            (order_percentage_dynamic/100) * @market_price
        ))
    ) < IFNULL(exchange_rate_max, 999999999) AND @market_price > IFNULL(exchange_rate_min, -999999999)
THEN @market_price 
WHEN @order_price > exchange_rate_max AND exchange_rate_max IS NOT NULL THEN exchange_rate_max
WHEN @order_price < exchange_rate_min AND exchange_rate_min IS NOT NULL THEN exchange_rate_min 
END 
WHERE bdynamicorder = true;

Executing that same query in a PHP script updates all rows but with NULL value.

if ($update_stmt = $mysqli->prepare($query)) {

    $update_stmt->execute();

}

Any ideas?

Thanks!

Answer 1

I'm pretty sure that the problem is the lack of initialization of the variables. Consider this expression:

@market_price := (IF(market_id = 0, 
                     (SELECT value FROM fiat WHERE id = 6), 
                     (SELECT value FROM fiat WHERE id = 5)
                    ) + 
                    (order_percentage_dynamic/100) * @market_price
                 )

If @market_price has not been set, then this will return NULL -- when either argument to an arithmetic operator is NULL, the result is NULL.

One solution uses additional logic such as coalesce():

@market_price := IF(market_id = 0, 
                    (SELECT value FROM fiat WHERE id = 6), 
                    (SELECT value FROM fiat WHERE id = 5)
                   ) + 
                   (order_percentage_dynamic/100) * coalesce(@market_price, 0);

Alternatively, you can initialize the values using a join:

UPDATE market CROSS JOIN
       (select @market_price := 0, @order_price := 0) vars
    SET price =  . . .

My guess is that when you run this on the database, the variables have already been set to non-NULL values.