doc: write about unsupported SQL syntax

src/site/sphinx/index.rst

@@ -15,6 +15,7 @@ Java SQL Parser Library
    migration
    SQL Grammar Stable <syntax_stable>
    SQL Grammar Snapshot <syntax_snapshot>
+   Unsupported Grammar <unsupported>
    Java API Stable <javadoc_stable>
    Java API Snapshot <javadoc_snapshot>
    keywords

src/site/sphinx/unsupported.rst

@@ -0,0 +1,57 @@
+***************************************
+Unsupported Grammar of various RDBMS
+***************************************
+
+*JSQLParser* is a RDBMS agnostic parser with a certain focus on SQL:2016 Standard compliant Queries and the "Big Four" (Oracle, MS SQL Server, Postgres, MySQL/MariaDB).
+We would like to recommend writing portable, standard compliant  SQL in general.
+
+- Postgres Implicit cast is not supported.
+
+    .. code-block:: java
+
+        SELECT date '2022-12-31';
+        SELECT double precision 1;
+
+
+- Oracle PL/SQL blocks are not support.
+
+    .. code-block:: sql
+
+        DECLARE
+            num NUMBER;
+        BEGIN
+            num := 10;
+            dbms_output.put_line('The number is ' || num);
+        END;
+
+
+
+- Oracle `INSERT ALL ...` is not supported
+
+    .. code-block:: sql
+
+        INSERT ALL
+          INTO mytable (column1, column2, column_n) VALUES (expr1, expr2, expr_n)
+          INTO mytable (column1, column2, column_n) VALUES (expr1, expr2, expr_n)
+          INTO mytable (column1, column2, column_n) VALUES (expr1, expr2, expr_n)
+        SELECT * FROM dual;
+
+- DDL statements
+
+    While *JSQLParser* provides a lot of generic support for DDL statements, it is possible that certain RDBMS specific syntax (especially about indices, encodings, compression) won't be supported.
+
+- `JSON` or `XML` specific syntax and functions
+
+    While *JSQLParser* provides a lot of generic support for `JSON` or `XML` processing, it is possible that certain RDBMS specific syntax or functions won't be supported.
+
+- Interval Operators
+
+    Anything like `DAY HOUR MINUTE SECOND [TO HOUR MINUTE SECOND]` is not supported.:
+
+    .. code-block:: sql
+
+        values cast ((time '12:03:34' - time '11:57:23') minute to second as varchar(8));
+
+
+
+