database programming
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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
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