text: # DATABASE CONSTRAINTS
  # Not Null Constraints
  # a) create a not null column
  
  use pem;
  show tables;
  CREATE TABLE IF NOT EXISTS Books (bookname varchar(50) NOT NULL);
  DESC BOOKS;
  # CREATE TABLE IF NOT EXISTS Books (bookname varchar(50) NOT NULL, author char(20));
  #17:20:07	CREATE TABLE IF NOT EXISTS Books (bookname varchar(50) NOT NULL, author char(20))	0 row(s) affected, 1 warning(s): 1050 Table 'books' already exists	0.000 sec
  
  #ALTER TABLE BOOKS ADD AUTHOR CHAR(20), PRICE INT(10);
  # 17:25:33	ALTER TABLE BOOKS ADD AUTHOR CHAR(20), PRICE INT(20)	Error Code: 1064. You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'PRICE INT(20)' at line 1	0.000 sec
  
  # add a not null constraint to a column:
  SELECT * FROM CUSTOMERS;
  SELECT * FROM PARTICIPANTS;
  INSERT INTO PARTICIPANTS VALUES ('TASHI', 27, 'MALE');
  SELECT * FROM PARTICIPANTS;
  ALTER TABLE PARTICIPANTS MODIFY NAME CHAR(20) NOT NULL;
  DESC PARTICIPANTS;
  # we can see that name column is not null;
  
  # remove a not null constraint from a column; 
  ALTER TABLE PARTICIPANTS MODIFY NAME CHAR(20);
  DESC PARTICIPANTS;
  # we now see that not null contraints is lost;
  
  # UNIQUE CONSTRAINTS;
  # a) create a coulumn with unique data;
  CREATE TABLE IF NOT EXISTS GAMES ( GAMENAME VARCHAR(20), GAMEID INT(20) NOT NULL,
  GAMERID INT(20) UNIQUE );
  SELECT * FROM GAMES;
  DESC GAMES;
  #  here we see that GAMEID is NOT NULL and GAMERID is UNIQUE
  
  ALTER TABLE GAMES MODIFY GAMEID INT(20) UNIQUE;
  DESC GAMES;
  #  we see that GAMEID is now to UNIQUE;
  
  # remove a unique contraint from a column;
  ALTER TABLE GAMES DROP INDEX GAMERID;
  DESC GAMES;
  # WHEN WE REMOVE UNIQUE CONSTRAINTS FROM GAMERID WE USE DROP INDEX
  
  
  
  