Database Programming

Sections 9 & 10 – DDL Data Definition Language,

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Data Definition Language DDL

Data Definition Language DDL ALTER TABLE DROP TABLE RENAME TRUNCATE COMMENT

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ALTER TABLE Use ALTER TABLE to:

ADD a new column (Always goes at the end of the table)

MODIFY an existing column (Change data type, size, or default value – see restrictions)

Define a DEFAULT value for a column DROP COLUMN (Cannot drop all columns) SET UNUSED/DROP UNUSED

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ALTER TABLE Examples ALTER TABLE

copy_items ADD(price number(8));

ALTER TABLE copy_items MODIFY(price number(7));

ALTER TABLE copy_items DROP COLUMN price;

adding a new column

modifying an existing column width, datatype, default value only affects subsequent insertions

dropping a column – one at a time must have at least one column left

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ALTER TABLE Examples cont’d ALTER TABLE copy_items

SET UNUSED (qty_on_hand);

ALTER TABLE copy_itemsDROP UNUSED COLUMNS;

set unused so it can be dropped later – no access once set unused

reclaim disk space

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TRUNCATE Removes all rows in a table Releases storage space where as DROP

does not release storage space Example:

TRUNCATE TABLE copy_items;

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COMMENT ON TABLE Table Example:

COMMENT ON TABLE itemsIS ‘Inventory items begin with a default qty_on_hand of zero’;

View this comment using:SELECT * FROM USER_TAB_COMMENTS;

Column Example:COMMENT ON COLUMN copy_items.qty_on_handIS ‘begin with a default value of one’;

View this comment usingDESC copy_items;

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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

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

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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));

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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