Download Data Definition Language and Data Manipulation Language

DDL 1
Data Definition Language and Data Manipulation Language
SQL supports functions such as building and manipulating database objects, populating database tables with
data, updating existing data in tables, deleting data, performing database queries, controlling database access and
overall database administration. Such functions can be classified into a number of categories and the most well
known two categories are Data Definition Language (DLL) and Data Manipulation Language (DML).
DDL allows user to create and restructure database objects, such as creating and deleting database tables.
Besides, DDL can be used to define table indexes as well as foreign keys between tables. Some of the
commonly used DDL commands are:
 CREATE TABLE
 ALTER TABLE
 DROP TABLE
 CREATE INDEX
 ALTER INDEX
 DROP INDEX
 CREATE VIEW
 DROP VIEW
DML allows users to manipulate data within the objects of a database. Some of the commonly used DML
commands are:
 SELECT
 INSERT INTO
 UPDATE
 DELETE
DDL allows database users to define database objects whereas DML allows database users to retrieve, insert,
delete and update data in a database.
1.
Create table with primary key
For each table, it is necessary to have a field or a combination of selected fields such that their values can
be used to identify each table row uniquely. Such an identifier is known as a candidate key. The concept
of candidate key is essential to good database design. The most commonly used candidate key of a table
is typically selected to be the primary key of the table.
The PRIMARY KEY keyword is used to specify the fields in a table that compose the table’s primary key.
Syntax
CREATE TABLE TableName
(
Column1 DataType, NOT NULL
Column2 DataType, NOT NULL
.......
PRIMARY KEY (Column1, Column2, …)
)

Technically, all fields in a primary key should be defined to be UNIQUE and NOT NULL.
Although some databases like Microsoft Access 2003 may take all primary key fields as UNIQUE
and NOT NULL even though they are not specified, it is a good practice to specify them explicitly.
Example
To create a table called “Student” with the primary key “StdID”, we can use the following statement:
CREATE TABLE Student
(
StdID
char(7) NOT NULL UNIQUE,
Name
varchar(30),
Class
char(10),
Age
smallint,
OverduePay
decimal(5,2),
PRIMARY KEY (StdID)
)
2.
Create table with foreign key
A foreign key (which may be composite) to another table ensures that the value of the foreign key
field(s) can be found in the primary key of the foreign table. The following example shows how
to create a table in a database with foreign key.
Syntax
CREATE TABLE TableName1
(
Column1 DataType1,
Column2 DataType2,
.......
FOREIGN KEY (ColumnX, ColumnY) REFERENCES TableName2
)
DDL 2
Example
In this example, we would like to create a table “LoanRecord” with a primary key “LoanRecID” and
two foreign keys “StdID” and “BookID” that references tables “Student” and “Book” respectively by
using the following statement.
CREATE TABLE LoanRecord
(
LoanRecID
char(8) NOT NULL,
StdID
char(7) NOT NULL,
BookID
char(8) NOT NULL,
Dateofborrow date,
Status
char(1),
PRIMARY KEY (LoanRecID),
FOREIGN KEY (StdID) REFERENCES Student,
FOREIGN KEY (BOOKID) REFERENCES Book
)
3.
Creating and Deleting Data View
Create a data view
With the use of the CREATE VIEW statement, users may create a special view on one or more
tables (or views) in the database in form of a new “virtual” table. The data view is created with
the use of an associated SELECT statement. Most SQL statements that apply to a database table
can also be applied to a data view.
Syntax
CREATE VIEW ViewName (Column1, Column2…)
AS Select-Statement;
Example
CREATE VIEW BookOnLoan_n_Borrower_View (StdID, Name, BookID)
AS SELECT Student.StdID, Name, BookID
FROM Student, LoanRecord WHERE Student.StdID = LoanRecord.StdID;
Delete a data view
A data view can be deleted with the use of the DROP VIEW statement.
Syntax
DROP VIEW ViewName;
4.
Create/Drop Table Index
Indexing is commonly used to enhance the performance of a database system. With the CREATE
INDEX statement, we can create indexing structures (which is a pre-processed list) for database
tables so as to provide an efficient access path to various table rows. When running a query,
database will examine any relevant index for a more efficient data access instead of traversing the
entire table. To delete an index, the DROP INDEX statement can be used.
Syntax (CREATE INDEX)
CREATE INDEX IndexName ON TableName(Column1,Column2,...)
Syntax (DROP INDEX)
DROP INDEX IndexName ON TableName
Example (CREATE INDEX)
In the following, we create an index for the class and student number combination in the Student table
as the two fields are often accessed by various queries.
CREATE INDEX ind_class_stdID
ON Student (class, StdID)
Example (DROP INDEX)
The following delete the index that was created in the previous example.
DROP INDEX ind_class_stdID ON Student