title: Creating new columns based on existing column storing a string
body: <p>Given a table with users' data and their purchase history, stored as a string - I'd like to create using SQL new columns (column per product) that we'll indicate whether the customer purchased this product.
  Current table:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>user_number</th>
  <th>products</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>12</td>
  <td>(4,2,8,10,20)</td>
  </tr>
  <tr>
  <td>13</td>
  <td>(9,3,8,15)</td>
  </tr>
  <tr>
  <td>52</td>
  <td>(8,14)</td>
  </tr>
  <tr>
  <td>4</td>
  <td>(10)</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>Desired table:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>user_number</th>
  <th>2</th>
  <th>3</th>
  <th>4</th>
  <th>8</th>
  <th>9</th>
  <th>10</th>
  <th>14</th>
  <th>15</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>12</td>
  <td>1</td>
  <td>0</td>
  <td>1</td>
  <td>1</td>
  <td>0</td>
  <td>1</td>
  <td>0</td>
  <td>0</td>
  </tr>
  <tr>
  <td>13</td>
  <td>0</td>
  <td>1</td>
  <td>0</td>
  <td>1</td>
  <td>1</td>
  <td>0</td>
  <td>0</td>
  <td>1</td>
  </tr>
  <tr>
  <td>52</td>
  <td>0</td>
  <td>0</td>
  <td>0</td>
  <td>1</td>
  <td>0</td>
  <td>0</td>
  <td>1</td>
  <td>0</td>
  </tr>
  <tr>
  <td>4</td>
  <td>0</td>
  <td>0</td>
  <td>0</td>
  <td>0</td>
  <td>0</td>
  <td>1</td>
  <td>0</td>
  <td>0</td>
  </tr>
  </tbody>
  </table>
  </div>