improved sql parser.

src/main/java/com/alibaba/druid/sql/dialect/db2/visitor/DB2OutputVisitor.java

@@ -43,9 +43,6 @@ public class DB2OutputVisitor extends SQLASTOutputVisitor implements DB2ASTVisit
         if (x.isForReadOnly()) {
             println();
             print0(ucase ? "FOR READ ONLY" : "for read only");
-        } else if (x.isForUpdate()) {
-            println();
-            print0(ucase ? "FOR UPDATE" : "for update");
         }
 
         if (x.getIsolation() != null) {

src/main/java/com/alibaba/druid/sql/dialect/oracle/parser/OracleStatementParser.java

@@ -1695,7 +1695,7 @@ public class OracleStatementParser extends SQLStatementParser {
         if (lexer.token() == Token.ALL) {
             lexer.nextToken();
             stmt.setOption(OracleMultiInsertStatement.Option.ALL);
-        } else if (lexer.token() == Token.FIRST) {
+        } else if (lexer.token() == Token.FIRST || identifierEquals("FIRST")) {
             lexer.nextToken();
             stmt.setOption(OracleMultiInsertStatement.Option.FIRST);
         }

src/main/java/com/alibaba/druid/sql/dialect/oracle/visitor/OracleOutputVisitor.java

@@ -144,8 +144,11 @@ public class OracleOutputVisitor extends SQLASTOutputVisitor implements OracleAS
     }
 
     public boolean visit(OracleDbLinkExpr x) {
-        x.getExpr().accept(this);
-        print('@');
+        SQLExpr expr = x.getExpr();
+        if (expr != null) {
+            expr.accept(this);
+            print('@');
+        }
         print0(x.getDbLink());
         return false;
     }