SQL command to create relations between tables

I everybody!

Does anybody know which is the SQL command to create relations between tables?

Thank you very much!

/* create 2 tables named PARENT and CHILD */

CREATE TABLE parent( pfield1 int, pfileld2 char (30) );

CREATE TABLE child ( cfield1 int, cfied2 char(20), cfield3 int );

/* create the primary key for PARENT and CHILD, this field will represent a unique identifier for each of the table */
ALTER TABLE parent add (primary key(pfield1));
ALTER TABLE child add (primary key(cfield1));

/* create the relation between PARENT and CHILD */
ALTER TABLE child ADD ( FOREIGN KEY (cfield3) REFERENCES parent(pfield1));

/* Insert data into parent and child tables */
INSERT INTO parent values(1,'first row');
INSERT INTO parent values(2,'2nd row');
INSERT INTO parent values(3,'tird row');
INSERT INTO child values(1,'first child',2);
INSERT INTO child values(2,'2nd child',2);
INSERT INTO child values(3,'third child',1);
INSERT INTO child values(4,'4th child',1);

/* make a query using relations */
SELECT * FROM parent,child
where parent.pfield1=child.cfield3

Good luck in you lesson !