Database Programming

Sections 9 – Constraints

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CONSTRAINT TYPES NOT NULL Constraints UNIQUE Constraints PRIMARY KEY Constraints FOREIGN KEY Constraints CHECK Constraints

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Defining CONSTRAINTS

What are constraints? Database rules

Constraints always have a name Given by you/DBA when constraint is

created (preferred method because names are meaningful)

Given by the system when constraint is created (names are not meaningful)

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Defining CONSTRAINTS

Two Ways to Define Constraints during Table Creation Table-Level

NOT NULL cannot be defined at the table-level If the word CONSTRAINT is used in the CREATE

TABLE statement, the constraint must be given a name

Composite-key constraints must be defined at the table-level

Column-Level NOT NULL must be defined at the Column-

Level

Column Level Constraint Refers to a single column CREATE TABLE clients

(client_number NUMBER(4) CONSTRAINT clients_client_num_pk PRIMARY KEY,first_name VARCHAR2(14) NOT NULL,last_name VARCHAR2(13));

Primary key defined at the column level SYS_Cn (where n is a unique integer) NOT NULL

constraint

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Composite key constraint Composite defined at Table level CREATE TABLE clients (

client_number NUMBER(4) NOT NULL,first_name VARCHAR2(20),last_name VARCHAR2(20)phone VARCHAR2(20)email VARCHAR2(10) NOT NULL,CONSTRAINT clients_phone_email_uk UNIQUE (email,phone));

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Defining CONSTRAINTS Table-Level Constraints – at the bottom

EXAMPLE:CREATE TABLE copy_employees(employee_id NUMBER(6),first_name VARCHAR2(20),job_id VARCHAR2(10),CONSTRAINT cemp_emp_id_pk PRIMARY KEY(employee_id), CONSTRAINT cemp_job_id_fk FOREIGN KEY(job_id) REFERENCES jobs(job_id),CONSTRAINT cemp_first_name_uk UNIQUE (first_name),CONSTRAINT cemp_emp_id_ck CHECK (employee_id<=999999));

Note: The words “Foreign Key” are Used at the table

level

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NAMING at TABLE LEVEL

Constraint naming format table_col_type CONSTRAINT constraint_name TYPE OF

CONSTRAINT(column_name) CONSTRAINT cemp_emp_id_pk PRIMARY KEY(employee_id) CONSTRAINT cemp_emp_id_lname_pk PRIMARY KEY(employee_id,last_name)

CONSTRAINT constraint_name TYPE OF CONSTRAINT(column_name) REFERENCES othertablename(column_name) CONSTRAINT cemp_job_id_fk FOREIGN KEY(job_id)

REFERENCES copy_jobs(job_id),

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NAMING at COLUMN LEVEL Column-level assigning a Constraint Name: System Named:

column_name datatype() TYPE OF CONSTRAINT employee_id NUMBER(6) PRIMARY KEY

User Named: column_name datatype() CONSTRAINT constraint

name TYPE OF CONSTRAINT employee_id NUMBER(6) CONSTRAINT

c2emp_emp_id_pk PRIMARY KEY Foreign Key:

column_name datatype() CONSTRAINT constraint_name TYPE OF CONSTRAINT (column it is on) REFERENCES othertablename(column_name)

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Defining CONSTRAINTS Column-Level constraints

Example:CREATE TABLE copy2_employees(employee_id NUMBER(6) CONSTRAINT c2emp_emp_id_pk PRIMARY KEY,CONSTRAINT c2emp_emp_id_ck CHECK(employee_id<=999999),first_name VARCHAR2(20)CONSTRAINT c2emp_first_name_nn NOT NULL,last_name VARCHAR2(20)CONSTRAINT c2emp_last_name_nn NOT NULL,address VARCHAR2(20) CONSTRAINT c2emp_address_ck NOT NULL,job_id VARCHAR2(10)CONSTRAINT c2emp_job_id_fk REFERENCES copy_jobs(job_id));

NOT NULL Constraint

Requires that every row has a value for the NOT NULL column

Named with _nn suffix

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UNIQUE constraint

No two rows have the same value Every value in the column or set of

columns (a composite key) are unique Names with _uk as a suffix If a composite unique key must be

defined at Table-level Column-level unique constraint is

defined a column level

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Primary Key Constraints

Primary key constraint is a column or set of columns that is uniquely identifies each row in a table

Must satisfy both conditions: No column that is part of the primary key

can contain a null value. A table can have only one primary key.

Named with _pk suffix

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Foreign Key(Referential Integrity) constraints CREATE TABLE clients

(client_number NUMBER(4) CONSTRAINT clients_client_num_pk PRIMARY KEY,first_name VARCHAR2(14),last_name VARCHAR2(13),department_id VARCHAR2(4,0),CONSTRAINT clients_dept_id_fk FOREIGH KEY(department_id) REFERENCES departments(department_id));

Names _fk suffix

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Foreign Key syntax

Column-level song_id NUMBER(5) CONSTRAINT

d_track_list_song_id_fk REFERENCES d_songs(id)

Table-level CONSTRAINT d_track_list_song_id_fk

FOREIGN KEY(song_id) REFERENCES d_songs(id)

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DELETE CASCADE option

Used when defining the foreign key enables the dependent rows in the child table to be deleted when a row in the parent table is deleted.

Without this you can not delete the parent record if there are any rows in the child table with the key value.

Use either on definition of FKON DELETE CASCADE or ON DELETE SET NULL

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ON DELETE CASCADE option

Column-level song_id NUMBER(5) CONSTRAINT

d_track_list_song_id_fk REFERENCES d_songs(id) ON DELETE CASCADE

Table-level CONSTRAINT d_track_list_song_id_fk

FOREIGN KEY(song_id) REFERENCES d_songs(id) ON DELETE CASCADE

Could use ON DELETE SET NULL also

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CHECK constraints

Explicitly defines a condition that must be met

Condition must be either TRUE or unknown(due to a null)

May refer to values in any column in the table, but not columns in other tables

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CHECK constraint

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Adding Constraints AFTER Table is created:

First, create a table that does not already have constraints: CREATE TABLE copy3_employees(

employee_id NUMBER(6),first_name VARCHAR2(20),last_name VARCHAR2(20),department_id NUMBER(4));

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Adding Constraints AFTER Table is created:

Secondly, add the constraints:ALTER TABLE copy3_employeesADD CONSTRAINT emp3_emp_id_pkPRIMARY KEY(employee_id);

ALTER TABLE copy3_employeesADD CONSTRAINT emp3_emp_id_fkFOREIGN KEY(department_id)REFERENCES copy_departments(department_id);

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Adding Constraints AFTER Table is created:

NOTE!!! For NOT NULL constraints, use the MODIFY keyword in the ALTER TABLE statement instead of ADD ALTER TABLE copy3_employees

MODIFY (first_name CONSTRAINT emp3_first_name_nn NOT NULL);

NOT NULL constraints can only be added if the column does not already contain null values

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Miscellaneous Constraint Information ...

If the word CONSTRAINT is used in a CREATE TABLE statement, the constraint must be given a name

Constraints that contain more than one column are called composite key constraints and must be specified at the table level by placing a comma between the column names

There is no limit to the number of CHECK CONSTRAINTS that can be specified for a column

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Miscellaneous FK Constraints Information...

Another name for FOREIGN KEY CONSTRAINTS is REFERENCIAL INTEGRITY CONSTRAINTS

When specifying FOREIGN KEY CONSTRAINTS, the table that contains the PRIMARY KEY is called the PARENT TABLE. The table that contains the FOREIGN KEY CONSTRAINT is called the CHILD TABLE.

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DISABLING CONSTRAINTS

Constraints can be disabled Examples:

ALTER TABLE copy3_employeesDISABLE CONSTRAINTemp3_emp_id_pk;

ALTER TABLE copy3_employeesDISABLE CONSTRAINTemp3_emp_id_pk CASCADE; This will cause any FOREIGN KEY that references this primary

key to also be disabled.

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ENABLING CONSTRAINTS

EXAMPLES: ALTER TABLE copy3_employees

ENABLE CONSTRAINT emp3_emp_id_pk;

Note: This does not enable the foreign key in the child tables

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DROPPING CONSTRAINTS

Examples: ALTER TABLE table_name

DROP CONSTRAINT TYPE (column_name)[CASCADE];

ALTER TABLE table_nameDROP CONSTRAINT name[CASCADE];

ALTER TABLE c_clientsDROP PRIMARY KEY CASCADE;

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Viewing Constraint Use the DESCRIBE command to confirm its existence . DESCRIBE can only verify is the NOT NULL constraint.

NOT NULL constraint appears in the data dictionary as a CHECK constraint.

Use a query of the USER_CONSTRAINTS table to view all constraints on your table.

SELECT constraint_name, constraint_typeFROM user_constraintsWHERE TABLE_NAME ='table_name';

SELECT constraint_name, constraint_type FROM user_constraintsWHERE TABLE_NAME ='COPY3_EMPLOYEES‘;

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QUERY THE DATA DICTIONARY

SELECT constraint_name, constraint_type, table_name, statusFROM user_constraints;

Types: P = Primary Key R = Foreign Key (Referential) C = Check (Includes NOT NULL) U = Unique

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Viewing Constraint

SELECT constraint_name, column_nameFROM user_cons_columnsWHERE table_name = 'EMPLOYEES';

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Viewing Constraints