MySQL combined primary key

DROP DATABASE ks;
CREATE DATABASE db1;
USE db1;
/***
 Primary key: primary key
 uniquely determines a set of data in a table, so a table has only one primary key.
 The field constrained by the primary key has a unique constraint by default, not unll
 ; It can be one field or multiple fields
***/

CREATE TABLE kecbiao (
 
 kcname VARCHAR(20) NOT NULL,
 keteach VARCHAR(20) ,
 kcstart DATE NOT NULL,
 kcend DATE NOT NULL COMMENT'end time',
 kctype ENUM ('compulsory','optional') NOT NULL,
 kcks TINYINT UNSIGNED NOT NULL COMMENT'total hours',
 kcxf TINYINT UNSIGNED NOT NULL COMMENT'credits'
)

/*
 1.
 After adding the direct field of the primary key, add the constraint primar key
 
*/

ALTER TABLE kecbiao ADD kc INT PRIMARY KEY;
/*
 Add a primary key to an existing table
*/
ALTER TABLE kecbiao DROP PRIMARY KEY;
ALTER TABLE kecbiao MODIFY kc INT PRIMARY KEY;
ALTER TABLE kecbiao ADD PRIMARY KEY('kcid');
ALTER TABLE kecbiao DROP

DROP TABLE kecbiao;

CREATE TABLE xubiao(
 xu_name VARCHAR(10),
 xu_xh ENUM('Male','Female'),
 xu_xh VARCHAR(10) PRIMARY KEY,
 xu_bj VARCHAR(10),
 xu_zy VARCHAR(15),
 xu_lx TINYINT UNSIGNED
);
USE DATABASE db1 ;
-- Combined primary key: Composite primary key. The primary key is composed of two fields
CREATE TABLE xkb(
 kc_id INT UNSIGNED NOT NULL,
 stuid INT UNSIGNED NOT NULL,
 score DOUBLE UNSIGNED,
 PRIMARY KEY(kc_id,stuid)
);