public void setLeftAlias(String leftAlias) {
    if ( this.leftAlias != null && !this.leftAlias.equals(leftAlias) ) {
      this.leftAlias = null;
    } else {
      this.leftAlias = leftAlias;
    }
  }
set left alias for the join expression.

@param leftAlias
         String
ql/src/java/org/apache/hadoop/hive/ql/parse/QBJoinTree.java