title: Purpose of '0' before column name
body: <p>I just wanna to know that , what's the <strong>purpose of 0 before column name</strong> , As I'm new to this field so help me understand things clear and in a better way. My working query is given below and the rows are mentioned which doesn't make sense to me. Thanks.</p>
  
  <p><strong>0 sdebit_amount</strong>,</p>
  
  <p><strong>0 scredit_amount</strong>,</p>
  
  <p><strong>0 scredit_amount,</strong></p>
  
  <pre><code>SELECT 
  MONTH AS month,
  ROUND(IFNULL(scredit_amount,0),2) AS purchase,
  ROUND(IFNULL(sdebit_amount,0),2) AS cash_paid
  FROM 
  (SELECT 
  MONTHNAME(sc.scredit_date) as month,
  IFNULL(SUM(sc.scredit_amount),0) AS scredit_amount,
  0 sdebit_amount
  FROM
  sup_credit sc 
  WHERE YEAR(sc.scredit_date) = YEAR(NOW()) 
  GROUP BY MONTHNAME(sc.scredit_date) 
  
  union all
  
  SELECT 
  MONTHNAME(sd.sdebit_date) as month,
  0 scredit_amount,
  IFNULL(SUM(sd.sdebit_amount),0) AS sdebit_amount
  FROM
  sup_debit sd
  WHERE YEAR(sd.sdebit_date) = YEAR(NOW()) 
  GROUP BY MONTHNAME(sd.sdebit_date)
  
  union all
  
  SELECT
   MONTHNAME(tpr.date) as month,
   0 scredit_amount,
   IFNULL(SUM(tpr.sub_total),0) sdebit_amount
   FROM
   tbpurchase_return tpr WHERE tpr.method='CREDIT'
   AND YEAR(tpr.date) = YEAR(NOW())
   GROUP BY MONTH(tpr.date)
   ) ABC
   group by month
  </code></pre>