Why Does My PDO Database Row Matching Query Return Zero Even When Matches Exist?

Matching Rows in Database with PDO: Troubleshooting Issues

When attempting to check for duplicate rows in a database using PDO, you may encounter scenarios where the results differ from your expectations. Here are potential problems and solutions to consider:

Problems Caused by SQL Errors

Confirm that your query is executing without errors. PDO may return '0' as the result count even if there are no matches, indicating a query issue.

Problems Caused by the Condition

Review your query conditions for exclusivity. Conditions such as "WHERE col=1 AND col=2" will always return '0'. Simplify the conditions to isolate the issue.

Problems Caused by the Data

• Ensure that the variables involved in the query exist and contain values.
• Check for non-printable or converted characters in the input data. Encode these characters using rawurlencode() to make them visible.

Debugging Tips

• Enable full error reporting for both PDO and PHP.
• Scrutinize the data in the database and the input values for differences. Use urlencode() to reveal non-printable characters.
• Ensure you are connected to the correct database.
• Verify the character set and encoding configurations for the database and PHP.

Example Issue and Resolution

Your query includes a string with HTML entities, such as "SELECT count(*) FROM inbox WHERE ... AND from_email = "abc Offers <[email protected]>"". When executed in phpMyAdmin, this query works, but returns '0' via PDO. The HTML entities ("<" and ">" converted to entities) cause the mismatch. Modifying the query to remove these entities resolves the issue:

$sql = 'SELECT count(*) FROM inbox WHERE uid = ? AND from_email = "abc Offers [email protected]"';
$result = $link->prepare($sql); 
$result->execute([$email_number,$email_f]); 
$number_of_rows = $result->fetchColumn();

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