title: Add Computed Column with Liquibase
body: <p>How can I add a computed column in liquibase? When I run the following SQL in a sql tag I get the computed value I'm expecting:</p>
  
  <pre><code>ALTER TABLE TableName
  ADD ComputedColumn AS (CASE WHEN DateColumn1 IS NULL OR DateColumn2 IS NULL
  THEN 0
  ELSE DATEDIFF(DAY, DateColumn1, DateColumn2) END)
  </code></pre>
  
  <p>How do I do this without using an sql tag? I am able to get a column created with the following yaml:</p>
  
  <p><code>- addColumn:
        tableName: TableName
        columns:
          - column:
              name: ComputedColumn
              type: tinyint
              constraints:
                nullable: true
              defaultValueComputed:  0
              valueComputed:  DATEDIFF(DAY, DateColumn1, DateColumn2) END
  </code></p>
  
  <p>However the column type is not Computed like when I run the raw SQL.</p>