Render correct DDL for unsetting table comments

Fixed issue where the DDL emitted for :class:`.DropTableComment`, which
will be used by an upcoming version of Alembic, was incorrect for the MySQL
and Oracle databases.

Fixes: #4436
Change-Id: I196de09495a37adface4caa9dcbc29a6d0ad159a

3 files changed, 15 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index 4a0d82fe1..6e1b4e49a 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -1724,6 +1724,11 @@ class MySQLDDLCompiler(compiler.DDLCompiler):
             ),
         )
 
+    def visit_drop_table_comment(self, create):
+        return "ALTER TABLE %s COMMENT ''" % (
+            self.preparer.format_table(create.element)
+        )
+
     def visit_set_column_comment(self, create):
         return "ALTER TABLE %s CHANGE %s %s" % (
             self.preparer.format_table(create.element.table),
diff --git a/lib/sqlalchemy/dialects/oracle/base.py b/lib/sqlalchemy/dialects/oracle/base.py
index b58d038cf..7e9a79909 100644
--- a/lib/sqlalchemy/dialects/oracle/base.py
+++ b/lib/sqlalchemy/dialects/oracle/base.py
@@ -994,6 +994,11 @@ class OracleDDLCompiler(compiler.DDLCompiler):
 
         return text
 
+    def visit_drop_table_comment(self, drop):
+        return "COMMENT ON TABLE %s IS ''" % self.preparer.format_table(
+            drop.element
+        )
+
     def visit_create_index(self, create):
         index = create.element
         self._verify_index_table(index)
diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index e77b2880f..898ca75b2 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -1868,6 +1868,11 @@ class PGDDLCompiler(compiler.DDLCompiler):
             colspec += " NOT NULL"
         return colspec
 
+    def visit_drop_table_comment(self, drop):
+        return "COMMENT ON TABLE %s IS NULL" % self.preparer.format_table(
+            drop.element
+        )
+
     def visit_create_enum_type(self, create):
         type_ = create.element