CREATE TABLE `stu` (   `sno` char(4) NOT NULL,   `sname` char(8) NOT NULL,   `sex` tinyint(1) DEFAULT NULL,   `mno` char(2) DEFAULT NULL,   `birdate` datetime DEFAULT NULL,   `memo` text,   `totalcredit` smallint(6) DEFAULT NULL,   PRIMARY KEY (`sno`) ); CREATE TABLE `cou` (   `cno` char(4) NOT NULL,   `cname` varchar(30) NOT NULL,   `credit` smallint(6) DEFAULT NULL,   `ptime` char(5) DEFAULT NULL,   `teacher` char(10) DEFAULT NULL,   PRIMARY KEY (`cno`) ); CREATE TABLE `sc` (   `sno` char(4) NOT NULL,   `cno` char(4) NOT NULL,   `grade` decimal(6,1) DEFAULT NULL,   `rank` char(255) DEFAULT NULL,   PRIMARY KEY (`sno`,`cno`),   CONSTRAINT `fk_sc_cno` FOREIGN KEY (`cno`) REFERENCES `cou` (`cno`),   CONSTRAINT `fk_sc_sno` FOREIGN KEY (`sno`) REFERENCES `stu` (`sno`) );