Php PDO rowCount() return wrong result

Question

I have a function for getting path in a tree structure data.

According to my table in DB, root should get no result when I query sub_id='root_id'

When I just pass the root the rowCount return correct result which is 0. However when I pass a node from lower rank(3rd), in the end of the recursive which is root, rowCount return 1?

PS

I use Mysql as DB

This is my table

main_id | sub_id
----------------    
    1   |    2    
    1   |    3    
    2   |    4    
    3   |    5

The code:

$stmt = $conn->prepare("Select * from table where sub_id= ? ");  

function get_path($stmt,$node,&$map){

    $res=$stmt->execute(array($node));

    if(!$res){ throw new Exception( implode(' ',$stmt->errorInfo()),1); }

        echo $node.' found '.$stmt->rowCount().'<br>';

        if($stmt->rowCount()==0){ //root
            $map[]=$node;
        }else{

            foreach($stmt->fetchAll(PDO::FETCH_ASSOC) AS $row){
                $map[]=$node;
                $upper_node=$row['main_id'];
                get_path($stmt,$upper_node,$map);
        }
    }

}

If I just pass get_path($stmt,1,$map); (the root)

the output:

1 found 0

but when I for example pass 4 into it the output become:

4 found 1
2 found 1
1 found 1 <= it should found 0

Why?

Community · Accepted Answer · 2023-11-17 19:30:35Z

6

You shouldn't rely on PDOStatement::rowCount() to get the number of rows affected by a SELECT statement, see PHP manual, PDOStatement::rowCount:

DOStatement::rowCount() returns the number of rows affected by the last DELETE, INSERT, or UPDATE statement executed by the corresponding PDOStatement object.

If the last SQL statement executed by the associated PDOStatement was a SELECT statement, some databases may return the number of rows returned by that statement. However, this behaviour is not guaranteed for all databases and should not be relied on for portable applications.

[...]

Example #2 Counting rows returned by a SELECT statement

For most databases, PDOStatement::rowCount() does not return the number of rows affected by a SELECT statement. Instead, use PDO::query() to issue a SELECT COUNT(*) statement with the same predicates as your intended SELECT statement, then use PDOStatement::fetchColumn() to retrieve the number of rows that will be returned. Your application can then perform the correct action.

edited Nov 17, 2023 at 19:30

CommunityBot

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answered Jul 30, 2014 at 11:45

VMai

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3 Comments

Anna Over a year ago

I found many discussion said rowCount only can work normal with MySQL, so I thought it would work, but I gusess it does not. Many thanks

Anna Over a year ago

Can I use PDOStatement::execute() to do the Select Count(*) statement? Then use the fetchColumn()? will this be reliable too? Because I hope to use prepare statement.

VMai Over a year ago

Yes, this will be reliable.

Arth · Accepted Answer · 2014-07-30 11:43:22Z

1

It is probably something to do with this from PDO rowCount Docs:

"If the last SQL statement executed by the associated PDOStatement was a SELECT statement, some databases may return the number of rows returned by that statement. However, this behaviour is not guaranteed for all databases and should not be relied on for portable applications."

In other words rowCount only really works with DML, and I believe this is the case in MySQL.

You have to count the fetched results to get the guaranteed row count.

edited Jul 30, 2014 at 11:43

answered Jul 30, 2014 at 10:50

Arth

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2 Comments

Anna Over a year ago

this is a recursive function, pass 4 into the function will trace up the tree.

Arth Over a year ago

@Anna Sorry, I misunderstood your qu.. thanks for making it clearer.

FreeSoftwareServers · Accepted Answer · 2017-02-28 00:24:21Z

1

Eg of @VMai 's Correct Answer:

$query = $db->prepare("

  SELECT COUNT(*) FROM table
  WHERE
       whatever
 ");

$query->execute();

$result = $query->fetchColumn();

if ($result > 5)
{
echo "Result is greater than 5";
}
else
{
echo "Result is less than 5";
}
?>

Note: You will need to run the "real" SQL Query again, except without COUNT(*).

answered Feb 28, 2017 at 0:24

FreeSoftwareServers

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