77

SQL Data DefinitionSQL Data Definition

Spread throughout chapter 7

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A Data Dictionary for the CH06 Database

Table 6.3

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Data TypesData Types

Each attribute will be defined to have a particular data type

Data type selection is usually dictated by the nature of the data and by the intended use

Pay close attention to the expected use of attributes for sorting and data retrieval purposes

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Some Common SQL Data Types

Numeric NUMBER(L,D)INTEGERSMALLINTDECIMAL(L,D)

Character CHAR(L)VARCHAR(L)

Date DATE

Data Type Format

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Data Definition CommandsData Definition Commands Creating Table Structures

CREATE TABLE <table name>(<attribute1 name and attribute1 characteristics,attribute2 name and attribute2 characteristics,attribute3 name and attribute3 characteristics,primary key designation,foreign key designation and foreign key requirements>);

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DDL - Create TableDDL - Create TableCREATE TABLE STUDENT

( SSN CHAR(9) NOT NULL,

FNAME VARCHAR(15) NOT NULL,

MIDINIT CHAR,

LNAME VARCHAR(15) NOT NULL,

YEAR CHAR(2) NOT NULL DEFAULT ‘Fr’,

MAJOR CHAR(4) DEFAULT ‘Und’,

CREDIT INT DEFAULT 0,

GPA DECIMAL(4,2),

HOMETOWN VARCHAR(15),

BALANCE DECIMAL(7,2) DEFAULT 0,

CONSTRAINT STDPK

PRIMARY KEY (SSN) )

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DDLDDL Foreign KeysCREATE TABLE enrollments

(classindex INT NOT NULL,

stdssn CHAR(9) NOT NULL,

CONSTRAINT ENRLPK

PRIMARY KEY(CLASSINDEX,STDSSN),

CONSTRAINT ENRLSECT

FOREIGN KEY (CLASSINDEX) REFERENCES SECTION(INDEX)

ON DELETE CASCADE ON UPDATE CASCADE,

CONSTRAINT ENRLSTUD

FOREIGN KEY (STDSSN) REFERENCES STUDENT(SSN)

ON DELETE CASCADE ON UPDATE CASCADE )

DomainsCREATE DOMAIN ZIP_CODE_TYP AS CHAR(9);

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Other SQL ConstraintsOther SQL Constraints

NOT NULL constraint

Ensures that a column does not accept nulls

UNIQUE constraint

Ensures that all values in a column are unique

DEFAULT constraint

Assigns a value to an attribute when a new row is added to a table

CHECK constraint

Validates data when an attribute value is entered

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Data Definition CommandsData Definition Commands

SQL Integrity Constraints Entity Integrity

PRIMARY KEY NOT NULL and UNIQUE

Referential Integrity FOREIGN KEY ON DELETE ON UPDATE

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DomainsDomains

Domain is a set of permissible values for an attribute. Defining a domain requires:

Name Data type Default value Domain constraint or condition

CREATE DOMAIN depttyp AS CHAR(4);

Then can use in defining any attributes of that type – one benefit, we can ensure that we define the PK and FK the same way

Syntax:

CREATE DOMAIN <domain name> AS <data type>

DEFAULT <value> CHECK <condition>

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SQL IndexesSQL Indexes

When a primary key is declared, DBMS automatically creates a unique index

Often need additional indexes

Using the CREATE INDEX command, SQL indexes can be created on the basis of any selected attribute

77CREATE INDEX P_CODEXON PRODUCT(P_CODE);

CREATE UNIQUE INDEX P_CODEXON PRODUCT(P_CODE);

Composite index Index based on two or more attributesCREATE INDEX meetingX ON SECTION ( Sem, Year, Time, Room);

Often used to prevent data duplication

SQL IndexesSQL Indexes

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DDLDDL

DROP SCHEMA DROP TABLE ALTER TABLE

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Advanced Data Definition CommandsAdvanced Data Definition Commands

All changes in the table structure are made by using the ALTER command

Followed by a keyword that produces specific change

Three options are available

ADD

MODIFY

DROP

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Advanced Data Management Commands

Advanced Data Management Commands

Changing Table Structures

ALTER TABLE <table name>MODIFY (<column name> <new column characteristics>);

ALTER TABLE <table name>ADD (<column name> <new column characteristics>);

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Changing a Column’s Data TypeChanging a Column’s Data Type

ALTER can be used to change data type

Some RDBMSs (such as Oracle) do not permit changes to data types unless the column to be changed is empty

ALTER TABLE PRODUCTMODIFY (V_CODE CHAR(5));

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Changing a Column’s Data Characteristics

Changing a Column’s Data Characteristics

Use ALTER to change data characteristics

If the column to be changed already contains data, changes in the column’s characteristics are permitted if those changes do not alter the data type

ALTER TABLE PRODUCTMODIFY (P_PRICE DECIMAL(9,2));

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Adding or Dropping a ColumnAdding or Dropping a Column

Use ALTER to add a column

Do not include the NOT NULL clause for new column

ALTER TABLE PRODUCTADD (P_SALECODE CHAR(1));

Use ALTER to drop a column

Some RDBMSs impose restrictions on the deletion of an attribute

ALTER TABLE StudentsDROP COLUMN Year;

77 Primary and Foreign Key Designation

ALTER TABLE PRODUCTADD PRIMARY KEY (P_CODE);

ALTER TABLE PRODUCTADD FOREIGN KEY (V_CODE) REFERENCES VENDOR;

ALTER TABLE PRODUCT ADD PRIMARY KEY (P_CODE) ADD FOREIGN KEY (V_CODE) REFERENCES VENDOR;

Advanced Data Management Commands

Advanced Data Management Commands

77 Deleting a Table from the Database

DROP TABLE <table name>;

DROP TABLE PART;

Advanced Data Management Commands

Advanced Data Management Commands

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ViewsViews A view is a virtual table - looks like it exists but doesn’t View has a name (like a table) and you can do many of

the same things to views as tables Content of a view is derived from the contents of some

real table(s). (“base tables”) View is defined via a SELECT statement … see upcoming

A user may see the view and think it is the actual DB

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Example ViewsExample Views

CREATE VIEW CS_STUD

AS SELECT *

FROM STUDENT

WHERE MAJOR = ‘CS’; CREATE VIEW PRIV_STUD

AS SELECT SSN,FNAME,LNAME,YEAR,MAJOR

FROM STUDENT CREATE VIEW ENROLL_STATS (DEPT,LARGE,SMALL,AVE,TOTAL)

AS SELECT DEPT, MAX(ENROLL),MIN(ENROLL),AVG(ENROLL), SUM(ENROLL)

FROM SECTIONS

GROUP BY DEPT

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Example ViewsExample Views

CREATE VIEW STUD_ENROLL_INFO

AS SELECT ENROLLMENTS.INDEX,STUDENT.SSN, FNAME,LNAME, YEAR, MAJOR, GPA

FROM ENROLLMENTS,STUDENT

WHERE ENROLLMENTS.STUDENT = STUDENT.SSN;

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Updates involving ViewsUpdates involving Views NOT ALL views are updatable (e.g. ENROLL_STATS,

STUD_ENROLL_INFO) If a given update is ambiguous as to what the user would

want, it is hard for the DBMS to decide for them Updatable:

a view based on a single table is updatable if the view includes the PK

Not Updateable: views involving joins view involving grouping and aggregate functions

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Advantages of ViewsAdvantages of Views

(Some) Logical Independence Show user only the info they need to see Views can simplify the user’s interaction with DB

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End SQL DDLEnd SQL DDL