MCQs Test!! VB Basic Test !! Essentials of VB !! Creating User Services !! function !! Anatomy !! GK March-2019 !! DATA TYPES IN JAVA !! Classes ,Constructor ,Methods,Inheritance !! Ecology-I !! Biology 2019 !! Gk april 2019 !! Biology (living world) !! Plant anatomy !! CELL STRUCTURES AND FUNCTION !! General biology !! Human physiology !! Data structure !! Ecology-II !! GK TEST !! Reproduction in plants !! Human reproduction !! The ABC of C !! Operating system set1 !! Operating system set2 !! Gk and current affairs !! Animal kingdom !! Number system !! Animal kingdom-II !! Gk TEST-III !! Number series !! Microorganisms !! Interview question in c !! Time and Work !! Percentage !! General history !! Average !! Cells !! Locomotion and Movement !! paging !! Bacteriology !! Page replacement policy !! Secondary memory and Disk scheduling !! Introduction of Nervous System and neurons !! Plant harmones !! Transport and Mineral Nutrition in Plants !! Neurotransmitters !! Modern Indian History !! Photosynthesis in Plants !! Modern Indian History-II !! Plant respiration !! Environment & Biodiversity !! Environment & Biodiversity - II !! Algae, Bryophytes, Pteridophytes !! Area !! Cricket !! Days of the Year (India and the World) !! Plant Pathology - I !! Indian Transport System !! Plant Pathology - II !! Aptitude !! Biology !! Botany !! C language !! Computer Science !! Core Java !! Gate !! GK !! Mysql !! Operating system !! Physics !! VB-Visual Basic !! Zoology


The FOREIGN KEY Constraint
Previous Home Next

FOREIGN KEY constraint to remove the inconsistency in two tables when data in one table depends on data in another table.

A FOREIGN KEY constraint associates one or more columns of a table (the foreign key) with an identical set of columns on which a PRIMARY KEY constraint has been defined (a primary key column in another table).

CREATE TABLE table_name
(column_name data_type   REFERENCES  table_name (emp_id))


CONSTRAINT  constraint_name  FOREIGN KEY  (column_name,..)
REFERENCE  table_name  (column_name,..)



(emp_name  char(4), emp_id  varchar (16)  REFERENCES  engineer (emp_id))

The above command creates a FOREIGN KEY on the attribute emp_id of the employee table that reference the primary key emp_id of the engineer table. This will ensure that the employeeID code that is inserted into the employee table is checked against the engineer table for the validity.


If the employee table is exist and does not have foreign key defined, then table also modified using ALTER TABLE command.

ALTER TABLE  employee
ADD CONSTRAINT  fkemp_id  FOREIGN KEY  (emp_id)  REFERENCES  engineer (emp_id)


Previous Home Next