title: How to get start/end date from single date column from a SQL Server table
body: <p>Could you please help me to get a solution for my issue. I am using a SQL Server database, and I want to get end date from the start date + month or day.</p>
  
  <ul>
  <li>if month was not null add it to the start date to get the end date </li>
  <li>if day was not null add it to the start date to get the end date</li>
  </ul>
  
  <p>I have this table:</p>
  
  <pre><code>| ID | startdate |  month |  day  | 
  | 1  | 2019-03-05|   3    | null  |
  | 2  | 2019-03-05|  null  |   30  |
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>| ID | startdate |  month |  day  |  enddate  |
  | 1  | 2019-03-05|   3    | null  |2019-06-05 |
  | 2  | 2019-03-05|  null  |   30  |2019-04-04 |
  </code></pre>
  
  <p>Could you please help me to write a SQL query please?</p>