title: Concat with if else in mysql
body: <p>I have a table named phone , which contains 3 columns </p>
  
  <pre><code>  ======================
    | TEL1 | TEL2  | TEL3 |
    =======================
    | 123  | 455   | 3434 |
    =======================
    |      | 455   | 3434 |
    =======================
    | 675  |       |      |
    =======================
    | 123  |       |      |
    =======================
    | 123  | 455   | 3434 |
  </code></pre>
  
  <p>I want to concatenate these columns into  one column 1 using mysql query which results like below table</p>
  
  <pre><code> +---------------------+
   |   Telephone         |
   +---------------------+
   |   123/455/3435      |
   +---------------------+
   |   455/3435          |
   +---------------------+
   |    675              |
   +---------------------+
   |   123               |
   +---------------------+
   |   123/455/3435      |
   +---------------------+
  </code></pre>
  
  <p>Not the result like below which I am getting using CONCAT_WS('/',<code>TEL1</code>, <code>TEL2</code>, <code>TEL3</code>) function </p>
  
  <pre><code> +---------------------+
   |   Telephone         |
   +---------------------+
   |   123/455/3435      |
   +---------------------+
   |   455/3435          |
   +---------------------+
   |    675//            |
   +---------------------+
   |   123//             |
   +---------------------+
   |   123/455/3435      |
   +---------------------+
  </code></pre>
  
  <p>Please Help me write the query for above kind of result</p>