Constraint – Primary Key

/* Create Table classes With Two Fields */
CREATE TABLE classes (
    cid int NOT NULL PRIMARY KEY,
    name varchar(255) UNIQUE
);

/* Create Table teachers With 3 Fields */
CREATE TABLE teachers (
    tid int NOT NULL,
    name varchar(255),
    PRIMARY KEY(tid)
);

/* Make students id field Primary Key */
ALTER TABLE students ADD PRIMARY KEY(id);

/* Drop Primary Key From students Table */
ALTER TABLE students DROP PRIMARY KEY;

/* Get Table Indexs */
SHOW INDEXES FROM students;