php中的sql语句

Question

$username = $_POST['username'];
$result = mysqli_query($conn,"SELECT * FROM user Where username = '".username."'");

$id = $_GET['id'];
$result = mysqli_query($conn,"SELECT body FROM user Where id = $id");

以上这两串代码中，第一条 中单引号，双引号，和点的用处是什么？还有为什么不像下面的那个id一样直接写上去呢？