diff --git a/mysql-test/r/check_constraints.result b/mysql-test/r/check_constraints.result
index 52a4c06b3ee5..372a5b79ddd5 100644
--- a/mysql-test/r/check_constraints.result
+++ b/mysql-test/r/check_constraints.result
@@ -1259,6 +1259,16 @@ ERROR 0A000: ALGORITHM=INPLACE is not supported for this operation. Try ALGORITH
 ALTER TABLE t1 MODIFY COLUMN f1 INT DEFAULT 20, algorithm=copy;
 ERROR HY000: Check constraint 't1_chk_1' is violated.
 DROP TABLE t1;
+#
+# Bug#121124 - An unrelated CHECK constraint on DATETIME should not
+# prevent an online ENUM extension.
+#
+CREATE TABLE t1 (e ENUM('a','b') NOT NULL, d DATETIME(6),
+CONSTRAINT ck CHECK (d IS NULL));
+ALTER TABLE t1 MODIFY e ENUM('a','b','c') NOT NULL, ALGORITHM=INSTANT;
+ALTER TABLE t1 MODIFY e ENUM('a','b','c','d') NOT NULL, ALGORITHM=INPLACE;
+ALTER TABLE t1 MODIFY d DATETIME(6) DEFAULT NULL, ALGORITHM=INPLACE;
+DROP TABLE t1;
 #-----------------------------------------------------------------------
 # Test case to verify check constraint with CHANGE COLUMN syntax.
 #-----------------------------------------------------------------------
diff --git a/mysql-test/t/check_constraints.test b/mysql-test/t/check_constraints.test
index d48890722599..4312104a0d24 100644
--- a/mysql-test/t/check_constraints.test
+++ b/mysql-test/t/check_constraints.test
@@ -789,6 +789,18 @@ ALTER TABLE t1 MODIFY COLUMN f1 INT DEFAULT 20, algorithm=copy;
 DROP TABLE t1;
 
 
+--echo #
+--echo # Bug#121124 - An unrelated CHECK constraint on DATETIME should not
+--echo # prevent an online ENUM extension.
+--echo #
+CREATE TABLE t1 (e ENUM('a','b') NOT NULL, d DATETIME(6),
+                 CONSTRAINT ck CHECK (d IS NULL));
+ALTER TABLE t1 MODIFY e ENUM('a','b','c') NOT NULL, ALGORITHM=INSTANT;
+ALTER TABLE t1 MODIFY e ENUM('a','b','c','d') NOT NULL, ALGORITHM=INPLACE;
+ALTER TABLE t1 MODIFY d DATETIME(6) DEFAULT NULL, ALGORITHM=INPLACE;
+DROP TABLE t1;
+
+
 --echo #-----------------------------------------------------------------------
 --echo # Test case to verify check constraint with CHANGE COLUMN syntax.
 --echo #-----------------------------------------------------------------------
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 5dc08f44e998..d9b8eb990b83 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -7580,8 +7580,7 @@ static bool prepare_key(
     Create_field *sql_field;
     while ((sql_field = it++) &&
            my_strcasecmp(system_charset_info, first_col->get_field_name(),
-                         sql_field->field_name))
-      ;
+                         sql_field->field_name));
     if (!sql_field) {
       my_error(ER_KEY_COLUMN_DOES_NOT_EXITS, MYF(0),
                first_col->get_field_name());
@@ -19204,8 +19203,7 @@ bool mysql_alter_table(THD *thd, const char *new_db, const char *new_name,
              mysql_rename_table(
                  thd, old_db_type, alter_ctx.db, backup_name, alter_ctx.db,
                  backup_name, *schema, alter_ctx.db, alter_ctx.alias,
-                 FN_FROM_IS_TMP | NO_FK_CHECKS | NO_FK_RENAME | NO_CC_RENAME))
-        ;
+                 FN_FROM_IS_TMP | NO_FK_CHECKS | NO_FK_RENAME | NO_CC_RENAME));
     }
     goto err_with_mdl;
   }
@@ -20916,9 +20914,10 @@ static bool is_any_check_constraints_evaluation_required(
             continue;
 
           // Check if data type is changed.
-          if (!my_strcasecmp(system_charset_info, itm_fld->field_name,
+          if (fld.change &&
+              !my_strcasecmp(system_charset_info, itm_fld->field_name,
                              fld.field_name) &&
-              (itm_fld->data_type() != fld.sql_type))
+              (itm_fld->data_type() != real_type_to_type(fld.sql_type)))
             return true;
         }