SQL’s Data Definition Language (DDL) n DDL statements define, modify and remove objects from data dictionary tables maintained by the DBMS n Whenever you use a DDL statement, the DBMS changes metadata (“data about data”) maintained in the data dictionary n Objects that can be defined include: table, view, sequence, index

Oracle’s Data Dictionary n Data dictionary tables maintain data about the structure of databases n Users cannot directly insert, update or delete information in the data dictionary n DDL statements cause the DBMS to add/update or delete information in the data dictionary tables n Oracle’s data dictionary views such as ALL_OBJECTS, ALL_TABLES, ALL_CONSTRAINTS, ALL_COLUMNS can be queried using SELECT statements and contain information about objects you have access to

SQL’s DDL Statements n Objects are defined using the CREATE statement n Some objects can be modified using the ALTER statement n Objects are removed using the DROP statement n Oracle’s data dictionary views such as USER_OBJECTS, USER_TABLES, USER_CONSTRAINTS, USER_COLUMNS, USER_SEQUENCES, USER_INDEXES can be queried using SELECT statements and contain information about objects you have created

Schema n The DataBase Administrator (DBA) has set up your Oracle account with permission for you to create objects in a schema (schema has same name as your account) n A schema (same as DB2’s collection) is a collection of related tables, views, sequences and indexes, i.e. a database

System Datatypes n System-defined datatypes include: –CHAR(n): Fixed length character string often used for Primary and Foreign Key numeric fields and also for codes such as ProgramCode(CPA/CPD/CNS/…); eg. program_code CHAR(3) –VARCHAR2(n): Variable length character string often used for names; eg. last_name VARCHAR2(30) –NUMBER(n,r): Real numbers with n digits in total and with r digits to the right of the decimal point; eg hrly_pay_rate NUMBER(5,2) can store hourly pay rates from 0.00 to –INTEGER: Integer values up to 2*(10**9) –DATE: dates in the format DD-MON-YYYY or MM/DD/YYYY

Create a Table n You define a table and its columns using the CREATE TABLE statement using the basic format: CREATE TABLE tablename ( column1name datatype(length), column2name datatype(length),… column255name datatype(length)) eg. CREATE TABLE student (id CHAR(9), fname VARCHAR2(30), lname VARCHAR2(30), gpa decimal(2,1) )

Create a Table (ctd) n You can also create a table based on an existing table: CREATE TABLE table1name AS SELECT col1name, …, colnname FROM table2name WHERE … eg. CREATE TABLE honourstudent AS SELECT id, lname FROM student WHERE gpa = 4.0

Change Table Structure n Change structure of a table (i.e. add columns or constraints, remove columns or constraints, change properties of a column) using the ALTER TABLE statement: ALTER TABLE tablename ( ADD col1name datatype(length), DROP col2name, MODIFY col3name datatype(length), MODIFY col4name NOT NULL, … )

ALTER TABLE Examples n ALTER TABLE student ADD (address VARCHAR(40)) n ALTER TABLE student MODIFY (lname VARCHAR2(35) ) n ALTER TABLE student DROP COLUMN gpa

Remove Table from Data Dictionary n Remove table from data dictionary (and also deletes all data in table) DROP TABLE tablename n Deletes all data in table and frees storage but leaves table in data dictionary TRUNCATE TABLE tablename

Constraints n Constraints protect the integrity of data in a database n Constraints are defined on tables and columns in tables n Constraint types: PRIMARY KEY, FOREIGN KEY, NOT NULL, CHECK, UNIQUE n All constraints should be given a name, except for NOT NULLs, so error messages produced will include the name of the constraint violated

Constraints (ctd) n Information about constraints is maintained in the data dictionary and can be queried using the view USER_CONSTRAINTS n Constraints may be defined as part of a CREATE TABLE statement n Constraints may be added, dropped or disabled (but not modified) by using an ALTER TABLE statement

Constraint Example n CREATE TABLE student ( id CHAR(9) PRIMARY KEY, fname VARCHAR(30) NOT NULL, lname VARCHAR2(30) NOT NULL, sem_num SMALLINT, soc_ins CHAR(9) CONSTRAINT soc_ins_uq UNIQUE(soc_ins), prog_code CHAR(3) NOT NULL, CONSTRAINT student_prog_code_fk FOREIGN KEY (prog_code) REFERENCES program(progcode) )

Constraint Examples n ALTER TABLE student ADD CONSTRAINT student_sem_num CHECK (sem_num BETWEEN 1 AND 6) n ALTER TABLE student DROP UNIQUE (soc_ins_uq) n ALTER TABLE student DROP PRIMARY KEY n ALTER TABLE student DROP CONSTRAINT student_sem_num n ALTER TABLE student DISABLE CONSTRAINT student_prog_code_fk