Advanced Database
Lecture 1
SQL- Data Definition Language(DDL)
Darbandikhan Technical Institute
Department of Computer Science
Ardalan
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Content
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What is SQL?
What is DDL?
Creating a Database
Creating a Table
Creating a Table with Constraints
Alter table
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What is SQL?
• SQL structure query language
• SQL is a standard language for accessing and manipulating
databases.
 What Can SQL do?
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SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert records in a database
SQL can update records in a database
SQL can delete records from a database
SQL can create new databases
SQL can create new tables in a database
SQL can create stored procedures in a database
SQL can create views in a database
SQL can set permissions on tables, procedures, and views
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What is DDL?
• Data Definition Language (DDL) is the part
of SQL that allows one to set up a database’s
schema, that is, to define attributes, domains
for those attributes, defines tables, define
relationships among those tables and so on.
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SQL Data Types
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BOOLEAN
• A Boolean variable is one that is either true
or false.
– In the Employee table in the Department
database, regular field is a Boolean. Either one is
a regular employee of the Department or one is
not.
– In some cases, there may be a third choice for a
Boolean variable: NULL.
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CHAR and VARCHAR
• CHAR and VARCHAR are character strings
• When defining an attribute of the character type,
one includes a length
– characterID CHAR(6)
– fName VARCHAR(30)
the CHAR variable must have the specified length,
whereas the VARCHAR variable can be up to the
specified length.
• Nchar and nvarchar both are Unicode variable
length can store both non-Unicode and Unicode
characters
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Difference between char, nchar, varchar and nvarchar
Char
Nchar
Varchar
Nvarchar
Character Data
Type
Non-Unicode
fixed-length
Unicode fixedNon-Unicode
length can store
variable length
both nonUnicode and
Unicode
characters (i.e.
Japanese, Korean
etc.)
Unicode variable
length can store
both nonUnicode and
Unicode
characters (i.e.
Japanese, Korean
etc.)
Maximum
Length
up to 8,000
characters
up to 4,000
characters
up to 8,000
characters
up to 4,000
characters
Character Size
takes up 1 byte
per character
takes up 2 bytes
per
Unicode/NonUnicode
character
takes up 1 byte
per character
takes up 2 bytes
per
Unicode/NonUnicode
character
Usage
use when data
length is
constant or fixed
length columns
use only if you
need Unicode
support such as
the Japanese
used when data
length is variable
or variable length
columns and if
actual data is
always way less
than capacity
use only if you
need Unicode
support such
as the
Japanese
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INTEGER and SMALLINT
• Used for whole numbers, such as: 56, 0 or –77
• There is not always a distinction between
INTEGER and SMALLINT, but if there is
SMALLINT is used if the numbers are fairly
small and one wants to save space.
– rooms SMALLINT
– population INTEGER
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NUMERIC and DECIMAL
• Used for numbers that have an exact fractional part.
• They have a PRECISION (total number of digits)
and a SCALE (number of digits after the decimal
place).
– The number 1234.56789 has a PRECISION of 9 and a
SCALE of 5.
• The implementation will have a default
PRECISION and SCALE to use if they are not
specified.
– Salary DECIMAL(7,2)
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NUMERICAL vs DECIMAL
• NUMERICAL is stricter on the limits of
possible numbers.
• For example, with PRESICION 9 and
SCALE 5,
– the largest NUMERICAL value is strictly
9999.99999
– The largest DECIMAL value may be larger if the
implementation can handle it
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FLOATS, REALS, etc.
• In floating point numbers (like scientific
notation 6.022  1023), the decimal place
moves and digits on the right-hand side (least
significant) might be dropped during various
calculations.
• This allows for a larger range of numbers, but
the numbers are handled somewhat more
approximately.
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Decimal VS Floating number
• The main difference is Floats and Doubles
are binary floating point types and a Decimal
will store the value as a floating decimal point
type.
• Decimals have much higher precision and
are usually used within monetary (financial)
applications that require a high degree of
accuracy.
• In performance wise Decimals are slower
than double and float types.
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DATE and TIME
• Used for variables that track when something
happened.
• DATE keeps track of year, month and day.
• TIME keeps track of hours, minutes and seconds.
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Establishing Domains for Attributes
• The type tells the computer how to interpret
data.
• The type also puts restrictions on the values
the data is allowed to have.
• Further restrictions can be placed on the data
by introducing a domain constraint.
• A domain is an allowed set of values.
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NULL or NOT NULL
• A simple but important domain constraint is
whether or not a value is allowed to be NULL.
• NULL is used if the data is unknown or not
applicable.
• NULL is distinct from 0 for numbers and “” (the
empty string) for characters.
• The SQL key words are NULL and NOT NULL
– NULL means the data can have the NULL value
– NOT NULL means it cannot
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NULL is the default
• If nothing is said, NULL is implied.
• For example, in the Character table in the
Department database
– lName VARCHAR(30)
– lName VARCHAR(30) NULL
would be the same and say that a character may not
have a last name
– characterID CHAR(6) NOT NULL
says that a character must have an ID.
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Constraints- Value Constraints
Value Constraints:
–
Allows value inserted in the column to be checked condition in the column constraint.
•
Check clause is used to create a constraint in SQL
•
Example:
CREATE TABLE Movies
(movie_title
varchar(40)
PRIMARY KEY,
studio_id
Number,
budget
Number check (budget > 50000)
)
•
Table level constraints can also be defined using the Constraint keyword
•
Example:
CREATE TABLE Movies
(movie_title
varchar(40)
PRIMARY KEY,
studio_id
Number,
budget
Number check (budget > 50000),
release_date
Date,
CONSTRAINT release_date_constraint Check (release_date between ’01-Jan-1980’ and ’31-dec1989))
•
Such constraints can be activated and deactivated as required.
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Constraints- Default Value
Default Value:
–
•
A default value can be inserted in any column by using the Default
keyword.
Example:
CREATE TABLE Movies (
movie_title
varchar(40)
NOT NULL,
release_date
date
DEFAULT sysdate
genre
varchar(20)
DEFAULT ‘Comedy’
In (‘Comedy’, ‘Horror’, ‘Drama’)
)
NULL,
Check genere
•
Table level constraints can also be defined using the
Constraint keyword
•
release_date defaults to the current date, however Null value is enabled in
the column which will need to be added explicitly when data is added.
Note: Any valid expression can be used while specifying constraints
•
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PRIMARY KEY
•
•
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A primary key, also called a primary keyword, is
a key in a relational database that is unique for
each record.
A primary key can not be NULL and that the
primary key must uniquely identify a record
(known as Entity Integrity).
The keyword PRIMARY KEY specifies that
attribute(s) will serve this purpose.
PRIMARY KEY(characterID)
PRIMARY KEY(episodeID, characterID)
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Candidate Keys
• A candidate key is a combination of attributes that can be uniquely
used to identify a database record without referring to any
other data. Each table may have one or more candidate. One of
these candidate keys is selected as the table primary key.
• A table contains only one primary key, but it can contain several
candidate keys. If a candidate key is composed of two or more
columns, then it's called a composite key
• A candidate key is an attribute or set of attributes having the same
features as the primary key (uniqueness).
• To specify that a set of attributes must have this uniqueness
property, use the keyword UNIQUE
PRIMARY KEY(date, time, room)
UNIQUE(date, time, tutee)
UNIQUE(date, time, tutor)
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Foreign Keys
• A FOREIGN KEY is a key used to link two tables together.
• A FOREIGN KEY is a field (or collection of fields) in one table that
refers to the PRIMARY KEY in another table.
• The table containing the foreign key is called the child table, and the
table containing the candidate key is called the referenced or parent
table.
• A foreign key is an attribute in a table that is either NULL or must
match a value from another table (called Referential Integrity).
• It realizes a relationship between two tables.
• The SQL is
FOREIGN KEY (personID) REFERENCES
RealPerson
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Referential integrity (RI)
• Referential integrity (RI) is a relational database concept,
which states that table relationships must always be
consistent.
• primary key field changes must be applied to all foreign
keys, or not at all. The same restriction also applies to
foreign keys in that any updates (but not necessarily
deletions) must be propagated to the primary parent key.
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What happens to old relationships?
• A foreign key takes it values from another
table (its parent in the relationship). What
happens if the parent value is updated or
deleted?
• How will referential integrity be maintained?
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Four choices for maintaining referential integrity
• CASCADE: make same update to child as made
to parent
• SET NULL: set the child value to NULL when
the parent is updated
• SET DEFAULT: set the child value to its
DEFAULT when the parent is updated
• NO ACTION: prevent parent from being
updated if children are affected
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ON DELETE / ON UPDATE
• Different choices can be specified depending
on what is happening to parent
FOREIGN KEY (stateSymbol) REFERENCES
State ON DELETE NO ACTION
FOREIGN KEY (stateSymbol) REFERENCES
State ON UPDATE CASCADE
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Main DDL statements
• SQL allows one to create, alter and destroy domains,
tables, views and schemas.
• The main SQL DDL statements are:
CREATE SCHEMA, DROP SCHEMA
CREATE DOMAIN, ALTER DOMAIN, DROP
DOMAIN
CREATE TABLE, ALTER TABLE, DROP TABLE
CREATE VIEW, DROP VIEW
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CREATE TABLE TableName
{(colName dataType [NOT NULL] [UNIQUE]
[DEFAULT defaultOption]
Define fields
[CHECK searchCondition] [,...]}
Identify primary key [PRIMARY KEY (listOfColumns),]
{[UNIQUE (listOfColumns),] […,]}
Identify candidate
keys
{[FOREIGN KEY (listOfFKColumns)
Identify foreign
REFERENCES ParentTableName
keys, relationships
[(listOfCKColumns)],
and actions to
[ON UPDATE referentialAction]
maintain
[ON DELETE referentialAction ]] [,…]}
referential
integrity.
{[CHECK (searchCondition)] [,…] })
Business rules
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CREATE TABLE PropertyForRent (
propertyNo Pnumber NOT NULL, ….
rooms Prooms NOT NULL DEFAULT 4,
rent Prent NOT NULL, DEFAULT 600,
ownerNo OwnerNumber NOT NULL,
staffNo StaffNumber
Constraint StaffNotHandlingTooMuch ….
branchNo BranchNumber NOT NULL,
PRIMARY KEY (propertyNo),
FOREIGN KEY (staffNo) REFERENCES Staff
ON DELETE SET NULL ON UPDATE CASCADE ….);
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Modifying Records
Alter Statement
•
Alter Statement:
–
used to make changes to the schema of the table.
Columns can be added and the data type of the columns
changed as long as the data in those columns conforms
to the data type specified.
• ALTER TABLE can
–
–
–
–
Add a new column
Remove an existing column
Add a new constraint
Remove an existing constraint
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ALTERing Columns
To add or remove
columns use
ALTER TABLE <table>
ADD COLUMN <col>
ALTER TABLE <table>
DROP COLUMN <name>
Examples
ALTER TABLE Student
ADD COLUMN
Degree VARCHAR(50)
ALTER TABLE Student
DROP COLUMN Degree
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ALTERing Constraints
To add or remove
columns use
ALTER TABLE <table>
ADD CONSTRAINT
<definition>
ALTER TABLE <table>
DROP CONSTRAINT
<name>
Examples
ALTER TABLE Module
ADD CONSTRAINT
ck UNIQUE (title)
ALTER TABLE Module
DROP CONSTRAINT ck
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Deleting Tables
• To delete a table use
• DROP TABLE
•
[IF EXISTS]
•
<name>
• Example:
• DROP TABLE Module
• BE CAREFUL with any
SQL statement with DROP
in it
– You will delete any
information in the table as
well
– You won’t normally be
asked to confirm
– There is no easy way to undo
the changes
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