Why am I Getting a 'Commands out of Sync' Error in MySQLi?

Why is MySQLi giving a "Commands out of sync" error?

The MySQL client doesn't allow you to execute a new query while there are still rows to be fetched from an in-progress query. Refer to the MySQL doc on common errors for more information on "Commands out of sync".

Solution:

Use mysqli_store_result() to pre-fetch all rows from the outer query. This will buffer them in the MySQL client, so the server believes that your app has fetched the entire result set. You can then execute more queries, even within a loop of fetching rows from the now-buffered outer result set.

Alternatively, use mysqli_result::fetch_all(), which returns the full result set as a PHP array. You can then iterate over that array.

Special Case for Stored Procedures:

Stored procedures can return multiple result sets, each with its own set of rows. Therefore, it's necessary to use mysqli_multi_query() and loop until mysqli_next_result() no longer returns additional result sets. This is required to satisfy the MySQL protocol, even if your stored procedure only has a single result set.

Note:

Nested queries are often used for representing hierarchies in data, but an alternative method with more straightforward querying might be preferable. Consider storing your data differently for easier querying.

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