SQL Constraints are rules used to limit the type of data that can go into a table, to maintain the accuracy and integrity of the data inside table.
Constraints provide a standard mechanism to maintain the accuracy and integrity of the data inside a database table.
A Foreign key
is a field which can uniquely identify each row in a another table. And this constraint is used to specify a field as Foreign key
.
A Foreign Key
Constraint is used to establish a relationship between two tables where one column is a Primary Key
of the table and the other column from other table is referenced to the Primary Key
column.
A Foreign Key
column can also have reference to Unique Key
column of another table.
CREATE TABLE table_name( column_name datatype[(size)] [ NULL | NOT NULL ] REFERENCES another_table_name(column_name) [ ON UPDATE | ON DELETE [ NO ACTION | SET NULL | SET DEFAULT | CASCADE ] ], .... );
SQL> CREATE TABLE emp_info( no NUMBER(3,0) PRIMARY KEY, name VARCHAR(30), address VARCHAR(70), contact_no NUMBER(12,0) ); ------------------------ Table created.
SQL> CREATE TABLE emp_salary( no NUMBER(3,0) PRIMARY KEY, users_no NUMBER(3,0) REFERENCES emp_info(no), salary NUMBER(12) ); ---------------------------- Table created