SQL Server/T-SQL/Constraints/Foreign Key

Adding a Foreign Key to an Existing Table

 
3>
4> CREATE TABLE employee(
5>    id          INTEGER NOT NULL PRIMARY KEY,
6>    first_name  VARCHAR(10),
7>    last_name   VARCHAR(10),
8>    salary      DECIMAL(10,2),
9>    start_Date  DATETIME,
10>    region      VARCHAR(10),
11>    city        VARCHAR(20),
12>    managerid   INTEGER
13> );
14> GO
1> INSERT INTO employee VALUES (1, "Jason" ,  "Martin", 5890,"2005-03-22","North","Vancouver",3);
2> GO
(1 rows affected)
1> INSERT INTO employee VALUES (2, "Alison",  "Mathews",4789,"2003-07-21","South","Utown",4);
2> GO
(1 rows affected)
1> INSERT INTO employee VALUES (3, "James" ,  "Smith",  6678,"2001-12-01","North","Paris",5);
2> GO
(1 rows affected)
1> INSERT INTO employee VALUES (4, "Celia" ,  "Rice",   5567,"2006-03-03","South","London",6);
2> GO
(1 rows affected)
1> INSERT INTO employee VALUES (5, "Robert",  "Black",  4467,"2004-07-02","East","Newton",7);
2> GO
(1 rows affected)
1> INSERT INTO employee VALUES (6, "Linda" ,  "Green" , 6456,"2002-05-19","East","Calgary",8);
2> GO
(1 rows affected)
1> INSERT INTO employee VALUES (7, "David" ,  "Larry",  5345,"2008-03-18","West","New York",9);
2> GO
(1 rows affected)
1> INSERT INTO employee VALUES (8, "James" ,  "Cat",    4234,"2007-07-17","West","Regina",9);
2> GO
(1 rows affected)
1> INSERT INTO employee VALUES (9, "Joan"  ,  "Act",    6123,"2001-04-16","North","Toronto",10);
2> GO
(1 rows affected)
1>
2> select * from employee;
3> GO
id          first_name last_name  salary       start_Date              region     city                 managerid
----------- ---------- ---------- ------------ ----------------------- ---------- -------------------- -----------
          1 Jason      Martin          5890.00 2005-03-22 00:00:00.000 North      Vancouver                      3
          2 Alison     Mathews         4789.00 2003-07-21 00:00:00.000 South      Utown                          4
          3 James      Smith           6678.00 2001-12-01 00:00:00.000 North      Paris                          5
          4 Celia      Rice            5567.00 2006-03-03 00:00:00.000 South      London                         6
          5 Robert     Black           4467.00 2004-07-02 00:00:00.000 East       Newton                         7
          6 Linda      Green           6456.00 2002-05-19 00:00:00.000 East       Calgary                        8
          7 David      Larry           5345.00 2008-03-18 00:00:00.000 West       New York                       9
          8 James      Cat             4234.00 2007-07-17 00:00:00.000 West       Regina                         9
          9 Joan       Act             6123.00 2001-04-16 00:00:00.000 North      Toronto                       10
(9 rows affected)
1>
2> create table EmergencyContact(
3> EID INTEGER NOT NULL PRIMARY KEY,
4> Name VARCHAR(20))
5> GO
1>
2> ALTER TABLE EmergencyContact
3> ADD CONSTRAINT FK_EmergencyContact_Employee
4> FOREIGN KEY (EID)
5> REFERENCES Employee (ID)
6> GO
1>
2>
3> drop table EmergencyContact;
4> drop table employee;
5> GO

Define Primary key and foreign key

1> CREATE TABLE Matches (
2>    Society_Group_Id int NOT NULL ,
3>    Match_Id         int IDENTITY (1, 1) NOT NULL PRIMARY KEY CLUSTERED,
4>    Points_Against   smallint NOT NULL
5> )
6> GO
1>
2> CREATE TABLE Match_Scores (
3>    Match_Id     int NOT NULL PRIMARY KEY CLUSTERED,
4>    Player_Id    int NOT NULL ,
5>    Score_Points tinyint NULL
6> )
7> GO
1>
2> ALTER TABLE Match_Scores
3>    ADD CONSTRAINT [FK_Match_Scores_Matches] FOREIGN KEY (Match_Id)
4>    REFERENCES [Matches] ([Match_Id])
5> GO
1>
2> drop table Matches
3> GO
1>
2> drop table Match_scores
3> GO
1>

How to use foreign key constraints

 

The syntax of a column-level foreign key constraint
[FOREIGN KEY] REFERENCES ref_table_name (ref_column_name)
    [ON DELETE {CASCADE|NO ACTION}]
    [ON UPDATE {CASCADE|NO ACTION}]
The syntax of a table-level foreign key constraint
FOREIGN KEY (column_name_1 [, column_name_2]...)
    REFERENCES ref_table_name (ref_column_name_1
[, ref_column_name_2]...)
    [ON DELETE {CASCADE|NO ACTION}]
    [ON UPDATE {CASCADE|NO ACTION}]
A foreign key constraint defined at the column level
A statement that creates the primary key table
28> CREATE TABLE Bankers
29> (BankerID        INT NOT NULL PRIMARY KEY,
30> BankerName       VARCHAR(50) NOT NULL)
31> GO
1>
2> --A statement that creates the foreign key table
3>
4> CREATE TABLE Billings
5> (BillingID       INT NOT NULL PRIMARY KEY,
6> BankerID         INT NOT NULL REFERENCES Bankers (BankerID),
7> BillingTotal     MONEY NULL)
8> GO
1>
2> --An INSERT statement that fails because a related row doesn"t exist
3>
4> INSERT Billings VALUES (1, 99, 100)
5> GO
Msg 547, Level 16, State 1, Server J\SQLEXPRESS, Line 4
The INSERT statement conflicted with the FOREIGN KEY constraint "FK__Billings__Banker__5F9EF494". The conflict occurred in database "master", table "dbo.Bankers", column "BankerID".
The statement has been terminated.
1>
2>
3> drop table Billings;
4> drop table Bankers;
5> GO

Table-level constraints

1> CREATE TABLE employee (emp_no INTEGER NOT NULL CONSTRAINT prim_empl PRIMARY KEY,
2>                    emp_fname CHAR(20) NOT NULL,
3>                    emp_lname CHAR(20) NOT NULL,
4>                    dept_no CHAR(4) NULL)
5>
6> GO
1> CREATE TABLE works_on (emp_no     INTEGER NOT NULL,
2>                        project_no CHAR(4) NOT NULL,
3>                        job        CHAR (15) NULL,
4>                        enter_date DATETIME NULL,
5>                        CONSTRAINT prim_works PRIMARY KEY (emp_no, project_no),
6>                        CONSTRAINT foreign_works FOREIGN KEY (emp_no) REFERENCES employee (emp_no))
7> GO
1> -- Specified with two declarative integrity constraints
2> -- Both constraints are table-level constraints,
3>
4> drop table works_on
5> GO
1>
2> drop table employee
3> GO
1>