MariaDB [bugreport]> select version();
+----------------+
| version()      |
+----------------+
| 5.5.38-MariaDB |
+----------------+
1 row in set (0.00 sec)
MariaDB [bugreport]> # Show that comment execution is honored:
MariaDB [bugreport]> /*!60101 select 1 */;
Query OK, 0 rows affected (0.00 sec)

MariaDB [bugreport]> /*!50101 select 1 */;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

MariaDB [bugreport]> # Prepare for bug demo:
MariaDB [bugreport]> # (1) unconditionally:
MariaDB [bugreport]>          create table t1 (id int not null primary key auto_increment) engine = innodb   ;
Query OK, 0 rows affected (0.00 sec)

MariaDB [bugreport]> # (2) conditionally via executable comment:
MariaDB [bugreport]> /*!50101 create table t2 (id int not null primary key auto_increment) engine = innodb */ ;
Query OK, 0 rows affected (0.00 sec)

MariaDB [bugreport]> show create table t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bugreport]> show create table t2\G
*************************** 1. row ***************************
       Table: t2
Create Table: CREATE TABLE `t2` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bugreport]> # All fine for now.
MariaDB [bugreport]> # Now for tables that refer to t1.ID:
MariaDB [bugreport]> # (1) unconditionally:
MariaDB [bugreport]>          create table t3 (id int not null primary key auto_increment, rid int not null, CONSTRAINT a FOREIGN KEY (rid) REFERENCES t1(id)) engine = innodb   ;
Query OK, 0 rows affected (0.00 sec)

MariaDB [bugreport]> # (2) conditionally via executable comment:
MariaDB [bugreport]> /*!50101 create table t4 (id int not null primary key auto_increment, rid int not null, CONSTRAINT b FOREIGN KEY (rid) REFERENCES t1(id)) engine = innodb */;
Query OK, 0 rows affected (0.00 sec)

MariaDB [bugreport]> show create table t3\G
*************************** 1. row ***************************
       Table: t3
Create Table: CREATE TABLE `t3` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `rid` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `a` (`rid`),
  CONSTRAINT `a` FOREIGN KEY (`rid`) REFERENCES `t1` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bugreport]> show create table t4\G
*************************** 1. row ***************************
       Table: t4
Create Table: CREATE TABLE `t4` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `rid` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `b` (`rid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bugreport]> # BUG: Missing CONSTRAINT `a` FOREIGN KEY (`rid`) REFERENCES `t1` (`id`) in table t4!
MariaDB [bugreport]> # Trying to add it:
MariaDB [bugreport]> # (1) conditionally via executable comment:
MariaDB [bugreport]> /*!50101 alter table t4 add FOREIGN KEY (rid) REFERENCES t1(id) */ ;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [bugreport]> show create table t4\G
*************************** 1. row ***************************
       Table: t4
Create Table: CREATE TABLE `t4` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `rid` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `rid` (`rid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bugreport]> # BUG: Missing CONSTRAINT `a` FOREIGN KEY (`rid`) REFERENCES `t1` (`id`) in table t4!
MariaDB [bugreport]> # Trying to add it (2) unconditionally:
MariaDB [bugreport]>          alter table t4 add FOREIGN KEY (rid) REFERENCES t1(id)  ;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [bugreport]> show create table t4\G
*************************** 1. row ***************************
       Table: t4
Create Table: CREATE TABLE `t4` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `rid` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `rid` (`rid`),
  CONSTRAINT `t4_ibfk_1` FOREIGN KEY (`rid`) REFERENCES `t1` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

# CONSTRAINT are only created if statement is not executed within /*!#### */