php - SELECT * 和 SELECT value 有什么区别

Question

有什么区别

    $query = "SELECT email FROM users WHERE username='$username' ";
    $result = mysqli_query($db, $query);
    $row = mysqli_fetch_array($result);
    $email = $row['email'];
    $_SESSION['email'] = $email;

和

    $query = "SELECT * FROM users WHERE username='$username' ";
    $result = mysqli_query($db, $query);
    $row = mysqli_fetch_array($result);
    $email = $row['email'];
    $_SESSION['email'] = $email;

它们都返回相同的值，有没有更有效的方法来做到这一点？

Answer 1

select * 将返回表中包含与谓词匹配的行的所有列的值，而 select column_name 将仅返回 中的值column_name 与您的谓词匹配的行的列。