Column in field list is ambiguous error

Question

i am using jdbc in my program with a query statement that retrieve holdingsID by querying different tables. when i run the program it has an error saying
Column HoldingsID in field list is ambiguous, what does this mean? How can i solve it?
here is my code, included is my sql select statement please check it.
        String key=request.getParameter("key");  
          Connection conn = null;
          String[][] a=new String[10][3];          
          int r=0;
          String qstr="";
          ResultSet rs=null;          
          try{     
          Class.forName("com.mysql.jdbc.Driver").newInstance();
          conn = DriverManager.getConnection("jdbc:mysql://localhost/scitemp?user=scinetadmin&password=A1kl@taN");
          Statement stmt=conn.createStatement();
            qstr="SELECT DISTINCT HoldingsID FROM tblHoldings, tblHoldingsAuthorName," +
                     "tblHoldingsSubject, tblHoldingsPublisherName"+
                  " WHERE (tblHoldings.Title LIKE "+"'"+"%"+key+"%"+"'"+")"+" OR "+
                 "(tblHoldings.Contents LIKE "+"'"+"%"+key+"%"+"'"+")"+" OR " +
                 "(tblHoldingsAuthorName.AuthorName LIKE "+"'"+"%"+key+"%"+"'"+")"+" OR " +
                "(tblHoldingsSubject.SubjectHeadings LIKE "+"'"+"%"+key+"%"+"'"+")"+" OR " +
               "(tblHoldingsPublisherName.PublicationDate = "+"'"+key+"'"+")"+
                     " ORDER BY HoldingsID";                              
          rs  = stmt.executeQuery(qstr);
          while ( rs.next() && r <10) {                     
                    a[r][0]=rs.getString("HoldingsID");     
                    a[r][1]="1";
                    a[r][2]="SILMS";  
                    r++;      
          }catch(Exception e){out.println("error:"+e.getMessage());}-----
thanks in advance for your help

Answer 1

Actually, this is probably going to be a bit more efficient on large scale data. (I'm guessing, if it matters then test...)
SELECT DISTINCT HoldingsID FROM
  SELECT HoldingsID
  FROM tblHoldings
  WHERE tblHoldings.Title LIKE '%key%'
       OR tblHoldings.Contents LIKE '%key%'
UNION ALL
  SELECT HoldingsID
  FROM tblHoldingsAuthorName
  WHERE tblHoldingsAuthorName.AuthorName LIKE '%key%'
UNION ALL
  SELECT HoldingsID
  FROM tblHoldingsSubject
  WHERE tblHoldingsSubject.SubjectHeadings LIKE  '%key%'
UNION ALL
  SELECT HoldingsID
  FROM tblHoldingsPublisherName
  WHERE tblHoldingsPublisherName.PublicationDate = 'key'
ORDER BY HoldingsIDThe diffence is when the sort(s) to create distinct rows is done. In the first version, UNION must produce only distinct values, so three sorts must be performed, one to create each UNION intermediate result. In the 2nd version, UNION ALL doesn't produce
distinct values, we delay the sorting until the last minute and sort all candidates once, instead of one sub-set once, one sub-set twice and two sub-sets 3 times. If this is a school project, the difference probably doesn't matter.
By the way, many databases implement the DISTINCT keyword in such a way that it creates sorted result sets, sorted by the resulting column values. As far as I know, this is not guaranteed by the SQL standard, so the additional ORDER BY clause is still needed for rigourous portability to databases that don't work that way. If that's not a concern and speed is, in many situations you can drop the ORDER BY; however for small result sets the difference is nearly invisible, and even for moderately large result sets, sorting an already sorted result set is usually an Order(n) in-memory operation. For really big results, the database will be swapping results to disk as it sorts and this sort of optimization matters then. This also applies to the use of GROUP BY.