Transcript SQL Data Definition Language

SQL
The Data Definition Language
Copyright © 2003 - 2012 Curt Hill
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
Copyright © 2003 - 2012 Curt Hill
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
Copyright © 2003 - 2012 Curt Hill
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?
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Identifiers
• A name for an item
–
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Servers
Databases
Tables
Views
Fields
• There are rules for a legal name
– Name must be 1-256 characters
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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
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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
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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
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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 version
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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
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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
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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
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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
Copyright © 2003 - 2012 Curt Hill
Character types
• char(n)
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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
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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
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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
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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
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Oracle specific types
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•
•
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Number (p,d)
Varchar2
Date
Raw(len)
– Binary data of given length
• BLOB and CLOB
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MySQL specific types
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•
•
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Tinyint, smallint, bigint
Bit,
bool, boolean
Many others as well
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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
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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)
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Example: Students
• Create table students (
s_id int,
s_name char(20),
s_address char(20),
primary key (s_id) )
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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,
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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’,
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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,
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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),
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Foreign Key
• Enforces referential integrity
• Form:
foreign key (local-fields) references
field
• Examples
foreign key (dept,course)
references course
-- Must have same name in course
foreign key (chair)
references faculty.naid
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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
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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);
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Drop
• Deletes the entire database
Drop database junkdb
• Deletes the table:
Drop table junktab
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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
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Alter Example
• Adding a field:
alter table faculty
add(f_age int, f_years int)
• Removing a field:
Alter table students
drop column f_balance
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Changing constraints
• The Alter may also be used to add or
remove constraints
• Consider changing a primary key:
Alter Table newtab
Add Primary Key (nt_id)
• A Foreign key:
Alter Table newtab
Add Foreign Key (fk)
references table t(tk)
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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
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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
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Syntax
• Form:
Create View ViewName As
Select …
• 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
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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
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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,…) As
Select …
• Column count must match Select
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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 as
Select s_id, S_name
From students
Where s_address is Null
• A complex view involves multiple
tables
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Example Complex View
• Multiple table or complex view:
create view student_hour
(sh_id, sh_name, sh_load) as
select s_id, s_name,
sum(crs_crhr)
from students, grades, course
where s_id = g_naid AND
g_dept = crs_dept And
g_course = crs_number
group by s_id, s_name
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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
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Finally
• We may now create a database
• The next presentation will consider
populating it with data
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