#to add one more unique values in the coulmn (likewsie PhoneNumber)
create table contact(
    name varchar(50),
    # named constraint
    mob varchar(50) UNIQUE, constraint  mobno_less_than_10digits check (length(mob)>=10)     #check (length(mob)>=10) --check constraint
    
);
desc contact;
Insert into contact values ('sid','7972384820');
#Insert into contact values ('sid','79384820');
select * from contact;

#alter use
ALTER TABLE contact ADD COLUMN CITY VARCHAR(50);
ALTER TABLE contact ADD COLUMN id int NOT NULL;#sql addnull values when you dont insert any values
desc contact;
select * from contact; 
ALTER table contact drop column id;

desc contact;