Why Does LAST_INSERT_ID() Return 0 in MySQL?

Troubleshooting LAST_INSERT_ID() Returning 0 in MySQL

When attempting to retrieve the ID of the most recently inserted row using LAST_INSERT_ID(), some users encounter a perplexing issue where it consistently returns 0. This can occur even after inserting rows using various methods, including specifying NULL, 0, or omitting the ID value.

Cause and Solution

The issue often arises due to MySQL's PHP configuration setting, PersistentConnections, being set to FALSE. When PersistentConnections is disabled, a new connection is established for each query, resulting in a unique CONNECTION_ID every time. Consequently, the LAST_INSERT_ID() function is unable to track insert operations across multiple connections and returns 0.

To resolve this issue, examine MySQL's PHP configuration file for the PersistentConnections setting and ensure it is set to TRUE. This will allow MySQL to maintain persistent connections, preserving the CONNECTION_ID across multiple queries and enabling LAST_INSERT_ID() to accurately retrieve the most recent ID.

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