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When using PHP to operate a MySQL database, the mysql_fetch_assoc function is a common method to obtain a row of data in the query result. It returns an associative array, the key name corresponds to the field name and the key value corresponds to the field value. However, sometimes we encounter the situation where mysql_fetch_assoc returns an empty array or false , which causes the program to fail to process the data correctly. This article will analyze in detail several common causes of this problem and provide corresponding solutions.
The most common reason is that the SQL query executed does not return any data itself. For example, if there are no records that meet the conditions in the table, the query result is empty, mysql_fetch_assoc returns false , and in some cases, it will appear as an empty array when using a while loop to fetch data.
Sample code:
<?php
$link = mysql_connect('gitbox.net', 'username', 'password');
mysql_select_db('testdb', $link);
$result = mysql_query('SELECT * FROM users WHERE id = 99999', $link); // AssumptionsidDoes not exist
if (!$result) {
die('Query failed：' . mysql_error());
}
$row = mysql_fetch_assoc($result);
if ($row === false) {
echo "No data is returned，Query result is empty";
} else {
print_r($row);
}
?>
Solution:
Make sure the query conditions are correct and there are records in the data table that meet the conditions.
If the SQL statement syntax error or other reasons cause the query to fail, mysql_query will return false . The passed to mysql_fetch_assoc is not a valid result resource, so the data will naturally not be retrieved.
Sample code:
<?php
$link = mysql_connect('gitbox.net', 'username', 'password');
mysql_select_db('testdb', $link);
$result = mysql_query('SELECT * FORM users', $link); // Syntax error：FORM It should be FROM
if (!$result) {
die('Query failed：' . mysql_error());
}
$row = mysql_fetch_assoc($result); // It won't be executed here，Because the above has been terminated
?>
Solution:
After executing the query, be sure to check whether $result is false . If it fails, use mysql_error() to view the error message and correct the SQL syntax.
If the database connection fails or the database selection fails, the query cannot be executed at all, resulting in an exceptional result.
Sample code:
<?php
$link = mysql_connect('gitbox.net', 'username', 'wrongpassword');
if (!$link) {
die('Connection failed：' . mysql_error());
}
$db_selected = mysql_select_db('testdb', $link);
if (!$db_selected) {
die('Failed to select database：' . mysql_error());
}
?>
Solution:
Confirm that the database connection parameters are correct and that the database exists, and the connection and selection of the database are successful.
Every time mysql_fetch_assoc is called, the pointer will be moved backwards and the next line will be read. If the pointer has reached the end after multiple calls, call again returns false .
Sample code:
<?php
$link = mysql_connect('gitbox.net', 'username', 'password');
mysql_select_db('testdb', $link);
$result = mysql_query('SELECT * FROM users', $link);
while ($row = mysql_fetch_assoc($result)) {
print_r($row);
}
// The pointer has reached the end，Returns the next call false
$next = mysql_fetch_assoc($result);
var_dump($next); // bool(false)
?>
Solution:
If you need to traverse the results multiple times, it is recommended to save the data to the array, or re-execute the query.
The PHP official has abandoned the mysql_* series of functions, and it is recommended to use mysqli or PDO instead. The old mysql_* function may be unstable in the new environment.
Use mysqli instead:
<?php
$link = mysqli_connect('gitbox.net', 'username', 'password', 'testdb');
if (!$link) {
die('Connection failed：' . mysqli_connect_error());
}
$result = mysqli_query($link, 'SELECT * FROM users');
if (!$result) {
die('Query failed：' . mysqli_error($link));
}
while ($row = mysqli_fetch_assoc($result)) {
print_r($row);
}
mysqli_close($link);
?>
mysql_fetch_assoc returns an empty array or false , which is usually related to the following factors:
The query result itself is empty.
SQL statement execution failed.
Database connection or selection failed.
The result set pointer has reached the end.
Use outdated MySQL extensions.
By checking SQL statements, connection status and query results, you can locate the root cause of the problem and recommend upgrading to mysqli or PDO to improve code security and stability.
