Why Are My Database Queries Returning Zero Rows When Data Should Exist?

Troubleshooting Database Row Matching Errors

When attempting to retrieve row counts to determine the existence of specific data in a database, users may encounter unexpected results where the query returns 0 rows despite the expected presence of matching records. This article explores potential reasons for such discrepancies and provides guidance on resolving these issues.

SQL Errors

Ensure that the query is executed without errors. A "no such table/database" error indicates incorrect connection credentials. Verify that the database and table names are correct and that the user has appropriate access privileges.

Invalid Conditions

Review the query conditions. Contradictory conditions, such as WHERE col=1 AND col=2, will prevent any rows from being returned. Simplify the conditions to identify the necessary clauses and refine them subsequently.

Data Inconsistencies

Inspect the data involved in the query. Confirm that the variables used exist and contain valid values. Non-printable characters or converted symbols in the data (e.g., linefeeds, HTML entities) can result in mismatches, particularly when inputs are dynamically generated. Utilize rawurlencode() to reveal non-Latin characters for easier verification.

Debugging Techniques

• Enable full error reporting for PDO and PHP to identify potential typographical or spelling errors.
• Scrutinize both the database data and the input to locate differences. Utilize urlencode() to expose non-printable characters in both sources.
• Verify the connection credentials and established database connection. Ensure that the query is targeting the intended database and that the user has adequate access permissions.
• Consider character set and encoding discrepancies. Reference the checklist provided in the referenced answer for comprehensive troubleshooting.

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