SQL Server constraints

--PRIMARY KEY constraint (Primary key constraint): The data of the primary key column is required to be unique and is not allowed to be empty.

--Unique Constraint: Requires the column to be unique, allowed to be empty, but only one null value can occur.

--Check Constraint: A column value range limit, formatting restrictions, and so on, such as age-related constraints.

--Default Constraint: The default value of a column, such as our male classmates more, gender default is male.

--FOREIGN KEY constraint (Foreign key): The column that references the primary table is required to establish a relationship between the two tables.

1, using SQL statement to implement ADD, remove constraints back to top

--PRIMARY KEY constraint (Primary key constraint): The data of the primary key column is required to be unique and is not allowed to be empty. --Unique Constraint: Requires the column to be unique, allowed to be empty, but only one null value can occur. --Check Constraint: A column value range limit, formatting restrictions, and so on, such as age-related constraints. --Default Constraint: The default value of a column, such as our male classmates more, gender default is male. --FOREIGN KEY constraint (Foreign key): The column that references the primary table is required to establish a relationship between the two tables. --syntax as follows--alter table name--ADD constraint constraint name constraint type specific constraint description--Example:--add PRIMARY KEY constraint ALTER TABLE STUINFOADD constraint Pk_stuno primary key (Stuno)--add unique key constraint ALTER TABLE STUINFOADD constraint Uq_stuid unique (stuid)--Add DEFAULT constraint ALTER TABLE STUINFOADD constraint df_ Stuaddress default (' Address Unknown ') for stuaddress--add CHECK constraint ALTER TABLE STUINFOADD constraint ck_stuage check (Stuage between 4 0)--add FOREIGN KEY constraint ALTER TABLE STUINFOADD constraint Fk_stuno foreign key (Stuno) references Stuinfo (Stuno)--delete Constraint--alter table table name D ROP constraint constraint name

SQL Server constraints