Database Design lecture 3_1 1 Database Design Lecture 3_1 Data definition in SQL

Database Design lecture 3_1 2 SQL Overview  SQL (Structured Query Language) has 3 major components: –A DDL for defining database structure. –A DML for retrieving and updating data. –A DCL for data control, granting of rights etc.  Official pronunciation is ‘S-Q-L'.  Can be used by range of users including DBAs, management, application developers, and other types of end users.  Easy to learn: –Consists of standard English words, case insensitive

Database Design lecture 3_1 3 SQL  An ISO standard now exists for SQL, making it both the formal and de facto standard language for relational databases (Latest version SQL-99).  Supported by many vendors –There are vendor-specific variations  Data types vary from vendor to vendor  Standard SQL data types: see text 8.1  Different conformity to SQL standards  SQL terminology  table, row, column  SQL uses bag-semantics-- a table may contain duplicate rows  We will use Oracle 10g SQL, but omitting most of the complex Oracle-specific details.

Database Design lecture 3_1 4 Oracle Data Types  Basic built-in data types include –Numbers  Number –Character Strings  Char  varchar2 –Date and time  Date –large strings and BLOBs (binary large objects)

Database Design lecture 3_1 5 Number  Syntax for specifying numbers –Number (precision, scale)  Precision is the maximum digits of numbers  Scale specifies the position of decimal point.  Eg  Number(5) 5 digit integer,  Number(6,2) 6 digit (not including decimal point) decimal number with 2 digits after the decimal point,  Can store 1—38 digits precision

Database Design lecture 3_1 6 String  To store strings, you can choose from –Char  stores fixed-length character strings of up to 2000 characters. Eg. char(10)  should use it to store short strings or strings with ‘neat’ lengths –Varchar2  Stores variable-length strings of up to 4000 characters long. Eg. Varchar2(50)  Preferred for most string types –String values are quoted with single quotes  Eg ‘12234’, ‘abcd’, ‘a12’

Database Design lecture 3_1 7 Date and time  Oracle uses the date data type to store both date and time –Always uses 7 bytes for date-time data. –Oracle date has rich formats, you need to specify it SSsecond0-59 MIMinute0-59 HH Hour1-12 HH24Military hour1-24 DDday of month1-31 (depends on month) DAYday of the week Sunday-Saturday Dday of the week1-7 MM month number 1-12 MONmonth abbreviated Jan—Dec Month Month spelled outJanuary-December YYlast 2 digits of yeareg, 98 YYYYfull year valueeg, 1998

Database Design lecture 3_1 8 Date and time  Example of date format: –‘dd-mon-yyyy’ 01-dec-2001 –‘dd/mm/yyyy’ 01/12/2001 –‘mm-dd-yy hh:mi:ss’ :30:59  Default format: ‘dd-mon-yyyy’  Current date and time: –Sysdate  Oracle has built-in functions that converts between data types: eg, date to string, string to date, string to number and number to string.

Database Design lecture 3_1 9 Data Definition  Creating a table create table table-name ( column-name1 datatype [not null], …….. column-nameN datatype [not null]); Table name –can not exceed 30 characters long, –Must begin with an alphabetic character –May contain letters, numbers, $, # and _ –Should not be an oracle reserved word –Should be descriptive

Database Design lecture 3_1 10 Identifying Primary Key create table table-name ( column-name1 datatype [not null], …….. column-nameN datatype [not null], [constraint constraint-name] Primary key (clolumn-nameA,…,column-nameX) ); or create table table-name ( column-name1 datatype [not null] [constraint constraint-name] primary key, …….. column-nameN datatype [not null])

Database Design lecture 3_1 11 Example create table Department ( Dept_nochar(4)PRIMARY KEY, Dept_name varchar2(25)); or create table Department ( dept_nochar(4), Dept_name varchar2(25), CONSTRAINT dept_PK PRIMARY KEY(dept_no));

Database Design lecture 3_1 12 Constraints  Primary key is an example of constraints. Other constraints exist, eg. foreign key, check constraints, null- value constraints.  If you don’t give the constraint name, the system will generate a name automatically, but the name is hard for human understanding.  As seen, primary key constraints (in fact other constraints too) can be specified as row constraints and table constraints. Tables constraints can, for example, identify several columns as the primary key.  Viewing constraints –User constraints are stored in the table user_constraints; –Can use select command to view the constraints.

Database Design lecture 3_1 13 Identifying Foreign Keys create table table-name ( column-name1 datatype [not null] [unique], …….. column-nameN datatype [not null] [unique], [constraint constraint-name1] Primary key (clolumnA,…,columnX), [constraint constraint-name2] Foreign key (Column_list) references referenced_table (column_list_in_referenced_table) ); Create table Staff( staff_no char(3), staff_name varchar2(20), dept_no char(4), Constraint staff_fk foreign key (dept_no) references department (dept_no));

Database Design lecture 3_1 14 More about Foreign Keys  When you delete or update a value of the columns referenced by other tables, the referential integrity constraints may be violated.  You can specify what action to take in such cases –No action or restrict: reject the delete or update (default) –Cascade: cascading the deletion –Set null –Set default

Database Design lecture 3_1 15 Check Constraints  You can use check constraints to limit the values of a column or some columns  Example: Create table staff( Staff_no char(3), Staff_name varchar2(20) not null, Staff_gender char(1) check (staff_gender in (‘M’, ‘F’)), Staff_salary number(8,2) not null, Dept_no char(4), Constraint staff_pk Primary key (staff_no), Constraint staff_fk Foreign key (dept_no) references department (dept_no), Constraint staff_sal check (staff_salary > ));

Database Design lecture 3_1 16 Unique Constraints and Default Values Create table staff( Staff_no char(3), Staff_name varchar2(20) not null, DateofBirth date, Staff_nationality char(10) default ‘Australia’, Staff_salary number(8,2) not null, Dept_no char(4), Constraint staff_pk Primary key (staff_no), Constraint staff_fk Foreign key (dept_no) references department (dept_no) on delete set null, Constraint staff_sal check (staff_salary > ), UNIQUE(staff_name, DateofBirth)); The table can not have two rows having the same values for staff_name and DateofBirth Unlike primary key columns, unique columns are not automatically NOT NULL

Database Design lecture 3_1 17 Modifying Table Definitions Alter table table_name add (column_specification | constraint,..., column_specification| constraint); Alter table table_name modify (column_specification | constraint,..., Column_specification | constraint); Alter table table_name drop column column_name | drop (column_list); Alter table table_name drop primary key; Alter table table_name drop constraint constraint_name;  Compare with standard SQL in text

Database Design lecture 3_1 18 Examples alter table orders add (quantity number (3) not null); alter table orders modify (quantity number(5)); alter table orders drop (quantity);  Be careful if the table contains data.

Database Design lecture 3_1 19 Dropping Tables  Drop table table_name [Cascade Constraints];  Pay attention to referential integrity constraints when dropping tables.  If Cascade Constraints is used, the constraints will be dropped first.  example Drop table Staff; Drop table Department; Drop table Department cascade constraints;

Database Design lecture 3_1 20 Viewing/enabling/disabling/dropping Constraints  To view the constraints defined for table Department, type SELECT constraint_name, constraint_type, search_condition FROM user_constraints WHERE table_name ='DEPARTMENT';  To disable/enable a constraint, use ALTER TABLE Table_name DISABLE CONSTRAINT constraint_name; ALTER TABLE Table_name ENABLE CONSTRAINT constraint_name;  To drop a constraint, use ALTER TABLE Table_name DROP PRIMARY KEY| UNIQUE (column_name) | CONSTRAINT constraint_name;

Database Design lecture 3_1 21 Viewing Tables and Table Structures  To see what tables are there in SQL*plus, type select * from cat;  To see the structure of a table, type describe table_name; or desc table _name;