Why Does My PDO Prepared Statement Fail with 'SQLSTATE[HY093]: Invalid parameter number' When Inserting Multiple Records?

Troubleshooting SQL Error When Inserting Multiple Records

In a recent development effort, a code snippet designed for inserting multiple records into a database using a PDO prepared statement encountered an error:

SQLSTATE[HY093]: Invalid parameter number: parameter was not defined

Despite the apparent equivalence in the number of values to be inserted (count($matches)) and the number of placeholders (count($values)), the query execution failed.

Root Cause

The error stemmed from a misconception about the initialization of the $values array. Unlike $matches, which initially contained an empty array, $values was populated with uninitialized values. As a result, $values contained a count greater than $matches, leading to the parameter mismatch error.

Resolution

To resolve this issue, it is crucial to explicitly initialize $values as an empty array before entering the loop:

$matches = array('1');
$count = count($matches);
$values = [];
for($i = 0; $i < $count; ++$i) {
    $values[] = '(?)';
}

With this modification, the counts of $matches and $values will match, allowing the query to execute successfully.

Additional Considerations

In addition, it is advisable to ensure that the hash column in the target table has a unique index to handle duplicate key conflicts. This can be achieved by adding the following line after the INSERT statement:

ON DUPLICATE KEY UPDATE hash=values(hash)

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