private static void validateCheckExprAST(ASTNode checkExpr) throws SemanticException {
    if (checkExpr == null) {
      return;
    }
    if (checkExpr.getType() == HiveParser.TOK_SUBQUERY_EXPR) {
      throw new SemanticException(
          ErrorMsg.INVALID_CSTR_SYNTAX.getMsg("Subqueries are not allowed in Check Constraints"));
    }
    for (int i = 0; i < checkExpr.getChildCount(); i++) {
      validateCheckExprAST((ASTNode)checkExpr.getChild(i));
    }
  }
Validate and get the default value from the AST.
@param node AST node corresponding to default value
@return retrieve the default value and return it as string
ql/src/java/org/apache/hadoop/hive/ql/ddl/table/constraint/ConstraintsUtils.java