Adding & deleting constraint using Alter ||SQL||MasterTech

create table Customer_tbl(
C_id int ,
C_Name varchar(50),
C_Address varchar(max),
City varchar(50) );

delete Customer_tbl where C_Name='GSingh';
select * from Customer_tbl;
insert into Customer_tbl (C_id,C_Name,C_Address,City) values(1,'GSingh','home no :8','Mohali')

--not null
-- primary
-- foreign key
--unique
--check
--default
alter table Customer_tbl add C_Age int ;

alter table Customer_tbl alter column C_id int not null;

alter table Customer_tbl alter column C_id int null;
alter table Customer_tbl drop constraint ;
alter table Customer_tbl add primary key(C_id);
alter table Customer_tbl add unique(C_id);
alter table Customer_tbl add check(C_Age==18) ;
alter table Customer_tbl add default 18 for C_Age ;

alter table [order] add foreign key(C_id) references Customer_tbl(C_id)

create table [order](
C_id int ,
P_Name varchar(50),

Price varchar(50) );
select * from [order];