选择表达式

条件表达式
   switch (op.ToLower())
            {
                case "add":
                    return " + ";
                case "bitwiseand":
                    return " & ";
                case "bitwisenot":
                    return " ~ ";
                case "bitwiseor":
                    return " | ";
                case "bitwisexor":
                    return " ^ ";
                case "divide":
                    return " / ";
                case "equal":
                    return " = ";
                case "greater":
                    return " > ";
                case "greaterorequal":
                    return " >= ";
                case "isnull":
                    return " is null ";
                case "isnotnull":
                    return " is not null ";
                case "less":
                    return " < ";
                case "lessorequal":
                    return " <= ";
                case "like":
                    return " like ";
                case "startwith":
                    return " like ";
                case "endwith":
                    return " like ";
                case "modulo":
                    return " % ";
                case "multiply":
                    return " * ";
                case "notequal":
                    return " <> ";
                case "subtract":
                    return " - ";
                case "and":
                    return " and ";
                case "or":
                    return " or ";
                case "in":
                    return " in ";
                case "notin":
                    return " not in ";
                default:
                    return " = ";