php - Having issue with matching rows in the database

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Solution:

First of all, you have to embrace the fact: if your query found no rows, it means there is no match, even if you can swear the data is all right. When the query returns no rows, then there are no rows to match the condition. So you have to find out - why. But first of all you need to make sure that your query is all right:

Problems caused by SQL errors

First of all you need to make sure that your query actually runs without errors as "no result" could mean an error in the query. Refer to these answers for the details: pdo and mysqli.

Problems caused by the condition

Check your conditions. There are mutual exclusive conditions, such asWHERE col=1 AND col=2. It will never return any rows. Try to simplify the condition until it starts returning some rows, and then refine the conditions to get the desired result.


But all right, there are no errors, conditions are correct, and you can swear there is data in the table to match your query. Still, there are some pitfalls:

Problems caused by the data

Most likely there are some converted or non-printable characters in the input data (or database). For example, there could be a linefeed character or a peculiarly encoded symbol, or some characters such as< and> converted into HTML entities. As a result, the query contains<[email protected]> will never match a text&lt;[email protected]&gt;.

The problem is, this is only a guess, and nobody can tell you what the actual issue is, because it is your database, your input data and only you can find the issue.

I wrote an article that explains how to debug your PDO issues.

To debug a particular issue, you need

  • make sure the full error reporting is enabled for both PDO and PHP. It really helps, showing you occasional typographic errors, spelling errors and the such
  • scrutinize both the data in the database and the input to find the difference.bin2hex() function would help, revealing all non-printable and converted characters, in both database and the input.

Problems caused by the connection credentials

Another frequent issue is when you have several databases and connect to the wrong one that doesn't have the data requested. This issue is similar to this one, so just follow the same routine, only checking not the list of tables but the data rows.

Problems caused by character set/encoding

It's a rare case, but just to to be sure, follow the checklist from this great answer

An irrelevant but important note

On a side note, but very important nevertheless: your prepared statement is a cargo cult code that protects nothing. Here is how it must be:

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

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