Copyright © 2003 - 2012 Curt Hill SQL The Data Definition Language.

Copyright © 2003 - 2012 Curt Hill

SQL

The Data Definition Language

Classification

• SQL statements can be broken into two categories

• Data Definition Language (DDL)– Create databases, tables, views

• Data Manipulation Language (DML)– Change the data– Process queries

• This presentation is about the DDL



Data Definition Language

• Used for creating databases, tables, views

• It also establishes most constraints– Primary keys– Foreign keys– Integrity constraints

• Also deletes databases and tables

Background

• Up to this point we have dealt with existing tables and data– We do not have to worry about what

types of names are legal for table or field names in queries

• Now we have to worry about these things:– What constitutes a legal name?– What data types exist?



Identifiers• A name for an item

– Servers– Databases– Tables– Views– Fields

• There are rules for a legal name– Name must be 1-256 characters


Naming rules• Legal characters

– Letters– Digits– Underscore _ and dollar $

• Start with any type of character including digit– May not be only digits

• Names are case sensitive• Some other names may be used if quoted

– Not usually a good idea

• Reserved words should be avoided


Creation and deletion

• The create reserved word is used to create a database, table, view

• The drop reserved word is used to delete any of these

• At table (or view) creation time the fields, keys and constraints are given

• These may be changed later


Create Database

• Simple form only gives the database name:Create database junkdb

• Some servers allow specification of estimated sizes and disk file names

• In Oracle, databases are linked so this is not done with SQL

SQL Server

• Older versions generally needed to give some disk specifics for first creation, no longer required

• Example:create database college on (name=college, filename='d:\courses\database\sqlserver\college.mdf', size=3MB)

• 3MB is minimum for current versionCopyright © 2003 - 2012 Curt Hill

SQL Server Commentary• The on is used to specify a file where

the database is to be stored– Must be used for the first create– Once file exists other databases may be

placed in it

• It is one file for all the databases and tables

• Every SQL Server database has the following three databases:– Master – the system catalog– Tempdb– Msdb


Tables

• Once a database exists the tables need to be created

• A create table must specify almost everything about it:– Table name– Fields:

• Name • Type, including length

– Constraints• Such as keys



Creating a table• Basic form:

Create table tn (fields…)– Where tn is the table name

• The parenthesized list is the attributes of the file

• These include – Field names and types– Primary and foreign keys– Other constraints


Field Types

• There are several basic types that a field may possess

• Each implementation may augment these with others

• These include char, integer, real etc.

• These may be modified by lengths


Character types• char(n)

– Fixed length character string– Always uses n characters– Padded with blanks– 0 < n < 256

• varchar(n)– Variable length character strings– The n specifies the maximum length– Not all positions need to be stored

depending on the data• MySQL does not like to mix the

two in a table


Numeric types• Int(n) or integer(n)

– N is optional display width– May be prefixed with Unsigned

• Float(p,d)– P is precision – maximum total number of

digits– D is digits to right of decimal point– These are optional and only used for display

• Double or real– A larger float– Also allows p and d

• Decimal or numeric– Usually stored in string format– May also have p and d


Binary Large Objects• AKA BLOB• Stores data that usually cannot be

manipulated in SQL– Word Processor Documents– Graphics– Executables

• Text– Case insensitive blob– Sensitive to character set

SQL Server specific types

• Bigint (64 bit), smallint(16 bit), tinyint(8 bit)– Different sizes

• Bit– Zero or one – a boolean

• Money(64 bit), tinymoney (32 bit)– 4 digits to right of decimal

• Time, date, datetime


Oracle specific types

• Number (p,d)• Varchar2• Date• Raw(len)

– Binary data of given length

• BLOB and CLOB



MySQL specific types

• Tinyint, smallint, bigint• Bit, • bool, boolean• Many others as well


Field definitions• Form of a field definition:

name type col_attr gen_attr• Fields are separated by commas• The name is the field name• Type is one of the above or other types• Column attributes are always optional

– Unsigned is specific to numerics– Lengths have different meanings for

different types

• General attributes are always optional


Primary Key• The most important attribute• A table needs one or more fields

specified as the primary key• The key may be specified as part of the

field:naid int primary key

• It may also be specified after the needed fields are given:Primary key (dept, number)


Example: Students

• Create table students ( s_id int, s_name char(20), s_address char(20), primary key (s_id) )


Attribute: NOT NULL

• NULL or NOT NULL– Whether it is possible for field not to

have a value– Default is NULL– A key can not be null

• Example: name char(20) not null,


Attribute: DEFAULT

• Default constant• Supplies a value when an Insert

does not• Cannot be used on type BLOB or

TEXT• Example:

address char(20) default ‘Unknown’,


Attribute: UNIQUE

• Requires each field to be unique within the table

• Default for the primary key fields• Coded as:

UNIQUE or UNIQUE KEY

• Example:email varchar(50) unique,


Attribute: CHECK• Domain Integrity constraint• Form:

CHECK (relational)• The relational may be almost

anything in a where• Example:crhr int check (crhr < 12 AND crhr>0),


Foreign Key• Enforces referential integrity• Form:

foreign key (local-fields) references field

• Examplesforeign key (dept,course) references course-- Must have same name in course

foreign key (chair) references faculty.naid


Create Order• In the college database that are

tables that have no foreign keys:– Students, courses

• Others that do:– Grades references courses and

students• Tables with foreign keys need to

be created after the tables they depend on

• The same is true for the insertion process


Relationship examples• create table grades (

dept char(5), number int, naid int, score int check (score >= 0 AND score <= 100) , primary key (dept, number, naid), foreign key (naid) references students, foreign key (dept, number) references course);

• create table faculty_teach ( dept char(5), number int, naid int, primary key (dept, number, naid), foreign key (naid) references faculty, foreign key (dept, number) references course);


Drop

• Deletes the entire databaseDrop database junkdb

• Deletes the table:Drop table junktab

Order again

• If there are many foreign keys then creation and insertion may be complicated

• It is also possible to change a table after creation

• This uses the Alter statement• It may add or change fields or

constraints


Alter Example

• Adding a field:alter table faculty add(f_age int, f_years int)

• Removing a field:Alter table studentsdrop column f_balance



Changing constraints• The Alter may also be used to add

or remove constraints• Consider changing a primary key:Alter Table newtabAdd Primary Key (nt_id)

• A Foreign key:Alter Table newtabAdd Foreign Key (fk) references table t(tk)

Cleaning• Since Oracle does not allow either

create or drop on a database removing all the tables to start over is complicated

• Dropping a table that has foreign keys point into it is disallowed

• Typically use Alter to remove all foreign keys from all tables

• The tables may then be dropped in any order


Views• A view is a predefined query that

may be used like a table• It may be as a simple as a

projection upon a single table– Such as removing payroll information

from a personnel table

• It may be complicated rearrangement of data from several tables– Like any query


Syntax• Form:Create View ViewName AsSelect …

• Viewname is the name of the new view and can be any SQL identifier

• View, As and Create are reserved words

• A normal Select/From/Where is the last part of the statement


View Commentary• In a query using a view is just like

using a table• A view does not create a new table• When referenced it is like an

automatic nested query• Not every option of Select is

allowed– Eg. Order By and Into are disallowed


Another View Form• The column names of the view

usually just come from Select– Either original names or their aliases

• They can be specified explicitly• Another form:Create View ViewName (col1, col2,…) AsSelect …

• Column count must match Select


Types of Views• Two classifications of views, simple

and complex• A simple view would be a reduction

of a single table• Example:Create view missing asSelect s_id, S_nameFrom studentsWhere s_address is Null

• A complex view involves multiple tables


Example Complex View

• Multiple table or complex view:create view student_hour (sh_id, sh_name, sh_load) asselect s_id, s_name, sum(crs_crhr)from students, grades, coursewhere s_id = g_naid AND g_dept = crs_dept And g_course = crs_numbergroup by s_id, s_name


Update a View?• A simple view may be updated

– The DBMS translates the update into the single table update

– Not very complicated

• A complex view may not be updated– Figuring out how to update the multiple

tables is difficult to impossible– How could we update a view that

involved computations?– The tables that contribute to the view

should be updated instead


Finally

• We may now create a database• The next presentation will consider

populating it with data


Copyright © 2003 - 2012 Curt Hill SQL The Data Definition Language.

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