Why Does `mysqli_num_rows()` Return a Boolean Instead of a Result Set?

PHP & MySQL: Resolving "mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given" Error

The error "mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given" indicates that the input provided to the mysqli_num_rows() function is not a valid result set. This error is often encountered when the input argument is not a result of a successful query execution.

In this specific case, the error occurs in the following line of PHP code:

if (mysqli_num_rows($dbc) == 0) {

Here, the variable $dbc is used as the input to mysqli_num_rows(), but it is returning false because the query executed on $dbc contains an error:

$dbc = mysqli_query($mysqli,"SELECT users.*, profile.*
                                 FROM users 
                                 INNER JOIN contact_info 
                                 ON contact_info.user_id = users.user_id 
                                 WHERE users.user_id=3");

The error in the query is the missing JOIN keyword between the users and profile tables:

SELECT users.*, profile.* --You do not join with profile anywhere.
                                 FROM users 
                                 INNER JOIN contact_info 
                                 ON contact_info.user_id = users.user_id 
                                 WHERE users.user_id=3");

To fix the issue, you should modify the query as follows:

$dbc = mysqli_query($mysqli,"SELECT users.*, profile.*
                                 FROM users 
                                 INNER JOIN profile 
                                 ON contact_info.user_id = users.user_id 
                                 WHERE users.user_id=3");

By correcting the query, the mysqli_num_rows() function will now receive a valid result set and be able to determine whether the number of rows in the result is zero.

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