Hey Phil, let's work through this issue step by step—since your SQL query pulls the right data but the PHP output isn't showing up on your site, the problem is almost certainly tied to how you're handling the result set or your conditional logic. Let's break down the fixes and common pitfalls:

Before diving into the IF statement, make sure you're correctly executing the query and capturing errors. It's easy to miss a connection issue or query execution error that silently fails.

For example, using mysqli (adjust if you're using PDO):

// Assume $conn is your established database connection
$categoryId = 1; // Replace with your target category ID
$sql = "SELECT p.Product_ID, p.Name, p.Stock_Level, p.Price, p.Image
        FROM Product p
        INNER JOIN Category c ON p.Category_ID = c.Category_ID
        WHERE c.Category_Name = 'Electronics'"; // Your filter condition

$result = mysqli_query($conn, $sql);

// Always check for query errors first!
if (!$result) {
    die("Query failed: " . mysqli_error($conn));
}

Most issues here come from common logical missteps. Let's cover the top culprits:

1. Checking for results the wrong way #

A common mistake is using if ($result) instead of checking the number of rows. The $result object exists even if no rows are returned, so this condition will always evaluate to true, leading to empty output or broken HTML.

Correct approach:

// Check if there are rows to display
if (mysqli_num_rows($result) > 0) {
    // Loop through results and embed your HTML
    while ($row = mysqli_fetch_assoc($result)) {
        // Insert your specified HTML structure here
        echo '<div class="product-card">';
        echo '<h2>' . htmlspecialchars($row['Name']) . '</h2>';
        echo '<p>Price: $' . number_format($row['Price'], 2) . '</p>';
        echo '<p>Stock: ' . $row['Stock_Level'] . '</p>';
        // Convert blob image to displayable format
        echo '<img src="data:image/jpeg;base64,' . base64_encode($row['Image']) . '" alt="' . htmlspecialchars($row['Name']) . '">';
        echo '</div>';
    }
} else {
    echo '<p>No products found in this category.</p>';
}

2. Confusing assignment (=) with comparison (==) #

If your IF statement uses = instead of == for checks (e.g., if ($row = null)), you're assigning a value instead of comparing it, which will break your logic entirely. Double-check all conditional operators.

3. Forgetting to loop through results #

If you only fetch a row once (outside a while loop), you'll only ever display the first product (or none if you don't fetch at all). The while loop ensures you iterate through every row in the result set.

Always use htmlspecialchars() when outputting database values into HTML—this prevents XSS attacks and fixes cases where special characters (like & or <) break your page layout, making it look like no data is being output.

Putting it all together with your Product table structure:

<?php
// Database connection (adjust credentials to match your setup)
$conn = mysqli_connect("localhost", "username", "password", "your_db");

if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

// Your INNER JOIN query (adjust the JOIN and WHERE clause to your needs)
$sql = "SELECT p.Product_ID, p.Name, p.Stock_Level, p.Price, p.Image
        FROM Product p
        INNER JOIN Admin a ON p.Admin_ID = a.Admin_ID
        WHERE a.Admin_ID = 2"; // Example filter

$result = mysqli_query($conn, $sql);

if (!$result) {
    die("Query error: " . mysqli_error($conn));
}

// Output HTML with data
?>
<div class="product-grid">
    <?php if (mysqli_num_rows($result) > 0): ?>
        <?php while ($row = mysqli_fetch_assoc($result)): ?>
            <div class="product-item">
                <h3><?= htmlspecialchars($row['Name']) ?></h3>
                <div class="product-price">$<?= number_format($row['Price'], 2) ?></div>
                <div class="product-stock">Stock: <?= $row['Stock_Level'] ?></div>
                <img src="data:image/jpeg;base64,<?= base64_encode($row['Image']) ?>" alt="<?= htmlspecialchars($row['Name']) ?>">
            </div>
        <?php endwhile; ?>
    <?php else: ?>
        <p>No products match your selection.</p>
    <?php endif; ?>
</div>
<?php
// Cleanup
mysqli_free_result($result);
mysqli_close($conn);
?>

内容的提问来源于stack exchange，提问作者Phil135