CountOperation.java

package database.table.operations;

import database.table.internals.Row;

/**
 * Author: Koushik Sen (ksen@cs.berkeley.edu)
 * Date: 8/22/12
 * Time: 2:00 AM
 */
public class CountOperation extends StandardOperation {
  final private static Object dummy = new Object();

  public CountOperation(int tableIndex, String columnName) {
    super(tableIndex, columnName);
  }

  public CountOperation(String columnName) {
    super(columnName);
  }

  public CountOperation() {
    super(null);
  }

  public Object apply(Object old, Row[] rows) {
    Object nu;
    if (columnName == null) nu = dummy;
    else nu = rows[tableIndex].get(columnName);
    if (old == null && nu == null) return 0;
    if (old == null) return 1;
    if (nu == null) return old;
    return (Integer) old + 1;
  }

  @Override
  protected Object operation(Object aggregate, Object current) {
    throw new RuntimeException("Should not be called.");
  }

  @Override
  public String name() {
    if (columnName == null) return "COUNT(*)";
    else return "COUNT(" + columnName + ")";
  }
}