Why Am I Getting a 'Commands out of sync' Error in MySQLi When Retrieving Hierarchical Data?

MySQLi Commands Out of Sync Error during Hierarchy Retrieval

One may encounter the "Commands out of sync" error when attempting to execute multiple MySQLi queries, especially during the retrieval of hierarchical data. This article delves into the issue, explaining its cause and providing potential solutions.

Root Cause

The MySQL client restricts the execution of new queries while rows remain unfetched from an in-progress query. This error stems from the client's protocol, ensuring that all rows are retrieved sequentially before proceeding to new queries.

Possible Solutions

To resolve this issue, several options are available:

1. Using mysqli_store_result()

This method pre-fetches all rows from the outer query into a client buffer. The MySQL server acknowledges the full retrieval of the results, allowing subsequent queries to be executed freely.

2. Using mysqli_result::fetch_all()

Similar to mysqli_store_result(), this method retrieves the entire result set as a PHP array, enabling efficient looping and fetching of data.

3. Using mysqli_multi_query() for Stored Procedures

Stored procedures have the potential to return multiple result sets, each containing its own rows. mysqli_multi_query() should be used to iterate through these result sets, handling them sequentially and adhering to the MySQL protocol.

Alternative Data Storage

To avoid the drawbacks of nested queries in hierarchical data retrieval, consider restructuring the data for more straightforward querying. Techniques such as adjacency lists ornested sets can significantly simplify hierarchies.

Custom Hack for CodeIgnitor 3.0.3

For users of CodeIgnitor 3.0.3, a workaround involves modifying line 262 of the mysqli_driver.php file. By adding @mysqli_next_result() to the _execute() method, the 'command out ofsync' error can be mitigated.

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