How to Fix MySQLi's 'Commands out of sync' Error?

Troubleshooting MySQLi's "Commands out of sync" Error

The MySQLi "Commands out of sync" error occurs when executing subsequent queries before fetching all rows from the previous query. To resolve this, ensure you fetch all rows before executing additional queries.

Cause of the Error:

The MySQL client forbids executing new queries while rows remain unfetched from an ongoing query. This is to maintain data integrity and prevent conflicts between multiple result sets.

Possible Solutions:

1. Pre-fetch Rows Using mysqli_store_result() or mysqli_fetch_all()

Using mysqli_store_result():

$result = mysqli_store_result($db);

Using mysqli_fetch_all():

$result = mysqli_query($db, $sql)->fetch_all();

2. Use mysqli_next_result() for Stored Procedures

If you execute a stored procedure, it may return multiple result sets. In such cases, use mysqli_next_result() to iterate through and complete all result sets before executing additional queries.

while ($result = mysqli_next_result($db)) {
    // process the result set
}

3. Alternative Data Storage for Hierarchies

Consider storing hierarchical data differently to simplify queries. Explore various database models, such as adjacency lists or nested set models, to optimize data structure and improve query efficiency.

Additional Note:

For CodeIgnitor users, you may encounter this error due to a known issue in the mysqli driver. To resolve it, modify the driver's _execute() function as follows:

protected function _execute($sql)
{
    $results = $this->conn_id->query($this->_prep_query($sql));
    @mysqli_next_result($this->conn_id); // Fix 'command out of sync' error
    return $results;
}

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