enhance sql parser & add test

src/main/java/com/alibaba/druid/sql/parser/SQLExprParser.java

@@ -198,6 +198,12 @@ public class SQLExprParser extends SQLParser {
                 sqlExpr = new SQLDefaultExpr();
                 lexer.nextToken();
                 break;
+            case USER:
+            case PASSWORD:
+            case DUAL:
+                sqlExpr = new SQLIdentifierExpr(lexer.stringVal());
+                lexer.nextToken();
+                break;
             case CASE:
                 SQLCaseExpr caseExpr = new SQLCaseExpr();
                 lexer.nextToken();

src/main/java/com/alibaba/druid/sql/parser/Token.java

@@ -102,6 +102,7 @@ public enum Token {
     KILL("KILL"),
     IDENTIFIED("IDENTIFIED"),
     PASSWORD("PASSWORD"),
+    DUAL("DUAL"),
     
     //postgresql
     WINDOW("WINDOW"),