通过PHP在$ value更改时更新Mysql行

Question

I need some help. Please, note: below lines of codes are only for a main idea example.

I have a php function to count some entries in a mysql table

$query = "SELECT COUNT(my_matches) FROM my_table";
$db->setQuery($query);
$value = $db->loadResult();

Now, I need to update another row of another table with $value

$query = "UPDATE my_table_reviews SET user_rating = (user_rating + '$value') WHERE listing_id = 77");
$db->setQuery($query);
$autorating = $db->loadResult();}

The problem is: how to update row user_rating from my_table_review ONLY WHEN $value change?

In words... if $value is changed so to this (second query). If $value is stable, do nothing.

I'm a little bit confused... any help would be really appreciated!

Answer 1

if $value is changed so to this (second query). If $value is stable, do nothing.

Then try this:

UPDATE my_table_reviews 
SET user_rating = (user_rating + '$value') 
WHERE listing_id = 77
-- update if only the user_rating had changed:
AND user_rating <> user_rating + $value 

Answer 2

If you want to trigger a DB update when a PHP variable changes ($value) then you'll need to abstract all the updates on that variable. eg

class Example {
  var $value = 0;

  function var_set($v) {
    $this->value = $v;
  }

  function var_get() {
    return $this->value;
  }
}

...then instantiate your class and access the variable:

$k = new Example();
$k->var_set(100);
print_r($k);

Once you've got all your accessor/mutator operations going via this abstraction, you can include a check to see if it's changed and use that as the trigger.

  function var_set($v) {
    if ($v != $this->value) {
      $this->value = $v;
      // update the DB
      $query = "UPDATE my_table_reviews SET user_rating = (user_rating + '".$this->value."') WHERE listing_id = 77");
      // ...
    }
  }

If on the other hand you want MySQL to identify if there's been an update, and only apply your update if it's different, then just run the UPDATE statement you proposed initially. MySQL will test to see if there's a change and only apply it if there is. Adding additional logic to force it to detect that change will only slow down the query resolution time. Much better to test in PHP before firing the query at all.