求助：排查代码中mysqli_num_rows函数及相关SQL查询问题

Hey there! It sounds like you're tangled up in code chaos after making a bunch of changes, and suspect mysqli_num_rows() might be part of the problem. Let's walk through the most common pitfalls with this function that could be tripping you up:

• Calling it on a failed query result
  If your SQL query fails (e.g., syntax error, invalid table name), mysqli_query() returns false instead of a valid result set. Trying to pass false to mysqli_num_rows() will throw an error. Always validate your query first:

  $result = mysqli_query($conn, $your_sql);
  if (!$result) {
      // Debug the actual SQL error to fix the root issue
      die("Query failed: " . mysqli_error($conn));
  }
  $row_count = mysqli_num_rows($result);
  

• Using it with non-SELECT queries
  mysqli_num_rows() only works with queries that return a result set—like SELECT, SHOW, or DESCRIBE. If you're running INSERT, UPDATE, or DELETE, you need to use mysqli_affected_rows() instead to get the number of rows changed. Mixing these up is a super common mistake!

• Forgetting to store results with prepared statements
  If you're using mysqli prepared statements, you can't call mysqli_stmt_num_rows() right after executing the statement. You first need to fetch the result from the server to your local script with mysqli_stmt_store_result():

  $stmt = mysqli_prepare($conn, "SELECT * FROM users WHERE email = ?");
  mysqli_stmt_bind_param($stmt, "s", $user_email);
  mysqli_stmt_execute($stmt);
  // Critical step—store the result before checking row count
  mysqli_stmt_store_result($stmt);
  $row_count = mysqli_stmt_num_rows($stmt);
  

  Skip this step, and you'll almost always get 0 even if there are matching rows in the database.

• Result set/connection was closed early
  If you accidentally close your database connection (mysqli_close($conn)) or free the result set (mysqli_free_result($result)) before calling mysqli_num_rows(), the function won't have access to the data it needs to count rows. Double-check the order of your code to make sure this isn't happening.

• Your SQL query actually returns no rows
  Sometimes the issue isn't the function at all—it's your query! If your WHERE clause is too restrictive, or there's no data matching your conditions, mysqli_num_rows() will correctly return 0. Test your raw SQL query directly in a database tool (like phpMyAdmin or MySQL CLI) to confirm if it returns the rows you expect.

If you can share a snippet of your actual code, we can pinpoint the exact issue even faster.

内容的提问来源于stack exchange，提问作者Prasad Supare