title: Variable set by user when running query, Teradata
body: <p>I have a query that checks if at least one row exist in the three tables A,B and C where I want to do the check that theDate=myDate. I want to be able to see if the tables contain data for a certain date (myDate). 
  I want the variable myDate to be chosen by the user when running the query. How do I do that? FYI: I am using Teradata:</p>
  
  <p>The query:</p>
  
  <pre><code>SELECT 'A' AS "Table", CASE WHEN COUNT(*) = 1 THEN 'Y' ELSE 'N' END AS "True?"
  WHERE EXISTS (SELECT * FROM A WHERE theDate=myDate) 
  
  UNION ALL
  
  SELECT 'B', CASE WHEN COUNT(*) = 1 THEN 'Y' ELSE 'N' END
  WHERE EXISTS (SELECT * FROM B WHERE theDate=myDate) 
  
  UNION ALL
  
  SELECT 'C', CASE WHEN COUNT(*) = 1 THEN 'Y' ELSE 'N' END
  WHERE EXISTS (SELECT * FROM C WHERE theDate=myDate)
  </code></pre>