SQL Server/T-SQL/Constraints/Primary key

Combine two columns as the primary key

1>
2> CREATE TABLE ClassGrades(
3>     ClassID int,
4>     StudentID int,
5>     GradeLetter varchar(2),
6>     Constraint PK_ClassGrades PRIMARY KEY(ClassID, StudentID)
7> )
8> GO
1>
2> INSERT ClassGrades VALUES(1,1,"A")
3> GO
(1 rows affected)
1> INSERT ClassGrades VALUES(1,2,"B-")
2> GO
(1 rows affected)
1> INSERT ClassGrades (ClassID, GradeLetter) VALUES(1,"C-")
2> GO
Msg 515, Level 16, State 2, Server sqle\SQLEXPRESS, Line 1
Cannot insert the value NULL into column "StudentID", table "master.dbo.ClassGrades"; column does not allow nulls. INSERT fails.
The statement has been terminated.
1> select * from ClassGrades
2> GO
ClassID     StudentID   GradeLetter
----------- ----------- -----------
          1           1 A
          1           2 B-
(2 rows affected)
1>
2> drop ClassGrades
3> GO
Msg 102, Level 15, State 1, Server sqle\SQLEXPRESS, Line 2
Incorrect syntax near "ClassGrades".
1>
2>

Define Primary key in table creation command

20>
21> CREATE TABLE employee (emp_no     INTEGER  NOT NULL,
22>                        emp_fname  CHAR(20) NOT NULL,
23>                        emp_lname  CHAR(20) NOT NULL,
24>                        dept_no    CHAR(4)  NULL,
25>                        CONSTRAINT prim_empl PRIMARY KEY (emp_no))
26> GO
1>
2> drop table employee
3> GO
1>
2> -- 
3>

Specification of the primary key of the employee table as a column-level constraint

1> -- Specification of the primary key of the employee table as a column-level constraint.
2> CREATE TABLE employee
3>             (emp_no    INTEGER NOT NULL CONSTRAINT prim_empl PRIMARY KEY,
4>              emp_fname CHAR(20) NOT NULL,
5>              emp_lname CHAR(20) NOT NULL,
6>              dept_no   CHAR(4) NULL)
7> GO
1> drop table employee
2> GO
1>