title: Modify a column to NULL - Oracle
body: <p>I have a table named <code>CUSTOMER</code>, with few columns. One of them is <code>Customer_ID</code>.</p>
  
  <p>Initially <code>Customer_ID</code> column <code>WILL NOT</code> accept <code>NULL</code> values.</p>
  
  <p>I've made some changes from code level, so that <code>Customer_ID</code> column will accept <code>NULL</code> values by default.</p>
  
  <p>Now my requirement is that, I need to again make this column to accept <code>NULL</code> values.</p>
  
  <p>For this I've added executing the below query:</p>
  
  <pre><code>ALTER TABLE Customer MODIFY Customer_ID nvarchar2(20) NULL
  </code></pre>
  
  <p>I'm getting the following error:</p>
  
  <pre><code>ORA-01451 error, the column already allows null entries so
  therefore cannot be modified
  </code></pre>
  
  <p>This is because already I've made the <code>Customer_ID</code> column to accept <code>NULL</code> values.</p>
  
  <p>Is there a way to check if the column will accept <code>NULL</code> values before executing the above query...??</p>