fix(parser): parse COMMENT ON ... IS NULL as comment removal (#2562)

src/main/java/net/sf/jsqlparser/statement/comment/Comment.java

@@ -69,7 +69,8 @@ public String toString() {
         } else if (view != null) {
             sql += "VIEW " + view + " ";
         }
-        sql += "IS " + comment;
+        // a null comment stands for PostgreSQL's COMMENT ON ... IS NULL, which removes the comment
+        sql += "IS " + (comment != null ? comment : "NULL");
         return sql;
     }
 

src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt

@@ -15157,9 +15157,12 @@ Comment Comment():
             <K_VIEW> view = Table() { result.setView(view); }
         )
     )
-    <K_IS> comment=<S_CHAR_LITERAL>
+    <K_IS>
+    (
+        comment=<S_CHAR_LITERAL> { result.setComment(new StringValue(comment.image)); }
+        | <K_NULL>
+    )
     {
-        result.setComment(new StringValue(comment.image));
         return result;
     }
 }

src/test/java/net/sf/jsqlparser/statement/comment/CommentTest.java

@@ -78,7 +78,36 @@ public void testCommentColumnDeparse() throws JSQLParserException {
     @Test
     public void testToString() {
         Comment comment = new Comment();
-        assertEquals("COMMENT ON IS null", comment.toString());
+        assertEquals("COMMENT ON IS NULL", comment.toString());
+    }
+
+    @Test
+    public void testCommentTableIsNull() throws JSQLParserException {
+        String statement = "COMMENT ON TABLE schema1.table1 IS NULL";
+        Comment comment = (Comment) CCJSqlParserUtil.parse(statement);
+        assertEquals("table1", comment.getTable().getName());
+        assertThat(comment.getComment()).isNull();
+        assertSqlCanBeParsedAndDeparsed(statement);
+    }
+
+    @Test
+    public void testCommentColumnIsNull() throws JSQLParserException {
+        String statement = "COMMENT ON COLUMN table1.column1 IS NULL";
+        Comment comment = (Comment) CCJSqlParserUtil.parse(statement);
+        assertEquals("column1", comment.getColumn().getColumnName());
+        assertThat(comment.getComment()).isNull();
+        assertSqlCanBeParsedAndDeparsed(statement);
+    }
+
+    @Test
+    public void testCommentViewIsNull() throws JSQLParserException {
+        assertSqlCanBeParsedAndDeparsed("COMMENT ON VIEW myschema.myView IS NULL");
+    }
+
+    @Test
+    public void testCommentIsNullLowercase() throws JSQLParserException {
+        Comment comment = (Comment) CCJSqlParserUtil.parse("comment on table table1 is null");
+        assertThat(comment.getComment()).isNull();
     }
 
     @Test