Which of the following is used to create and modify the structure of your tables and other objects in the database?

Which of the following is used to create and modify the structure of your tables and other objects in the database?
A .  SQL Data Definition Language (DDL)
B .  SQL Data Manipulation Language (DML)
C .  SQL Data Relational Language (DRL)
D .  SQL Data Identification Language (DIL)

View Answer

Answer: A

Explanation:

The SQL Data Definition Language (DDL) is used to create, modify, and delete views and relations (tables). Data Definition Language

The Data Definition Language (DDL) is used to create and destroy databases and database objects. These commands will primarily be used by database administrators during the setup and removal phases of a database project. Let’s take a look at the structure and usage of four basic DDL commands:

CREATE Installing a database management system (DBMS) on a computer allows you to create and manage many independent databases. For example, you may want to maintain a database of customer contacts for your sales department and a personnel database for your HR department.

The CREATE command can be used to establish each of these databases on your platform. For example, the command:

CREATE DATABASE employees

creates an empty database named "employees" on your DBMS. After creating the database, your next step is to create tables that will contain data. (If this doesn’t make sense, you might want to

read the article Microsoft Access Fundamentals for an overview of tables and databases.) Another variant of the CREATE command can be used for this purpose. The command:

CREATE TABLE personal_info (first_name char(20) not null, last_name char(20) not null, employee_id int not null)

establishes a table titled "personal_info" in the current database. In our example, the table contains three attributes: first_name, last_name and employee_id. Don’t worry about the other information included in the command — we’ll cover that in a future article.

USE The USE command allows you to specify the database you wish to work with within your DBMS. For example, if we’re currently working in the sales database and want to issue some commands that will affect the employees database, we would preface them with the following SQL command:

USE employees

It’s important to always be conscious of the database you are working in before issuing SQL commands that manipulate data.

ALTER Once you’ve created a table within a database, you may wish to modify the definition of it. The ALTER command allows you to make changes to the structure of a table without deleting and recreating it. Take a look at the following command:

ALTER TABLE personal_info ADD salary money null

This example adds a new attribute to the personal_info table — an employee’s salary. The "money" argument specifies that an employee’s salary will be stored using a dollars and cents format. Finally, the "null" keyword tells the database that it’s OK for this field to contain no value for any given employee.

DROP The final command of the Data Definition Language, DROP, allows us to remove entire database objects from our DBMS. For example, if we want to permanently remove the personal_info table that we created, we’d use the following command:

DROP TABLE personal_info Similarly, the command below would be used to remove the entire employees database:

DROP DATABASE employees

Use this command with care! Remember that the DROP command removes entire data structures from your database. If you want to remove individual records, use the DELETE command of the Data Manipulation Language.

That’s the Data Definition Language in a nutshell. Data Manipulation Language

The Data Manipulation Language (DML) is used to retrieve, insert and modify database information. These commands will be used by all database users during the routine operation of the database. Let’s take a brief look at the basic DML commands:

The Data Manipulation Language (DML) is used to retrieve, insert and modify database information. These commands will be used by all database users during the routine operation of the database. Let’s take a brief look at the basic DML commands:

INSERT The INSERT command in SQL is used to add records to an existing table. Returning to the personal_info example from the previous section, let’s imagine that our HR department needs to add a new employee to their database. They could use a command similar to the one shown below:

INSERT INTO personal_info values(‘bart’,’simpson’,12345,$45000)

Note that there are four values specified for the record. These correspond to the table attributes in the order they were defined: first_name, last_name, employee_id, and salary.

SELECT The SELECT command is the most commonly used command in SQL. It allows database users to retrieve the specific information they desire from an operational database. Let’s take a look at a few examples, again using the personal_info table from our employees database.

The command shown below retrieves all of the information contained within the personal_info table. Note that the asterisk is used as a wildcard in SQL. This literally means "Select everything from the personal_info table."

SELECT * FROM personal_info

Alternatively, users may want to limit the attributes that are retrieved from the database. For example, the Human Resources department may require a list of the last names of all employees in the company. The following SQL command would retrieve only that information:

SELECT last_name FROM personal_info

Finally, the WHERE clause can be used to limit the records that are retrieved to those that meet specified criteria. The CEO might be interested in reviewing the personnel records of all highly paid employees. The following command retrieves all of the data contained within personal_info for records that have a salary value greater than $50,000:

SELECT * FROM personal_info WHERE salary > $50000

UPDATE The UPDATE command can be used to modify information contained within a table, either in bulk or individually. Each year, our company gives all employees a 3% cost-of-living increase in their salary. The following SQL command could be used to quickly apply this to all of the employees stored in the database:

UPDATE personal_info SET salary = salary * 103

On the other hand, our new employee Bart Simpson has demonstrated performance above and beyond the call of duty. Management wishes to recognize his stellar accomplishments with a $5,000 raise. The WHERE clause could be used to single out Bart for this raise:

UPDATE personal_info SET salary = salary + $5000 WHERE employee_id = 12345

DELETE

Finally, let’s take a look at the DELETE command. You’ll find that the syntax of this command is similar to that of the other DML commands. Unfortunately, our latest corporate earnings report didn’t quite meet expectations and poor Bart has been laid off. The DELETE command with a WHERE clause can be used to remove his record from the personal_info table:

DELETE FROM personal_info WHERE employee_id = 12345 JOIN Statements

Now that you’ve learned the basics of SQL, it’s time to move on to one of the most powerful concepts the language has to offer C the JOIN statement. Quite simply, these statements allow you to combine data in multiple tables to quickly and efficiently process large quantities of data. These statements are where the true power of a database resides.

We’ll first explore the use of a basic JOIN operation to combine data from two tables. In future installments, we’ll explore the use of outer and inner joins to achieve added power.

We’ll continue with our example using the PERSONAL_INFO table, but first we’ll need to add an additional table to the mix. Let’s assume we have a table called DISCIPLINARY_ACTION that was created with the following statement:

CREATE TABLE disciplinary_action (action_id int not null, employee_id int not null, comments char(500))

This table contains the results of disciplinary actions on company employees. You’ll notice that it doesn’t contain any information about the employee other than the employee number. It’s then easy to imagine many scenarios where we might want to combine information from the DISCIPLINARY_ACTION and PERSONAL_INFO tables.

Assume we’ve been tasked with creating a report that lists the disciplinary actions taken against all employees with a salary greater than $40,000 The use of a JOIN operation in this case is quite straightforward. We can retrieve this information using the following command:

SELECT personal_info.first_name, personal_info.last_name, disciplinary_action.comments FROM personal_info, disciplinary_action WHERE personal_info.employee_id = disciplinary_action.employee_id AND personal_info.salary > 40000

As you can see, we simply specified the two tables that we wished to join in the FROM clause and then included a statement in the WHERE clause to limit the results to records that had matching employee IDs and met our criteria of a salary greater than $40,000

Another term you must be familiar with as a security mechanism in Databases is: VIEW

What is a view?

In database theory, a view is a virtual or logical table composed of the result set of a query. Unlike ordinary tables (base tables) in a relational database, a view is not part of the physical schema: it is a dynamic, virtual table computed or collated from data in the database. Changing the data in a table alters the data shown in the view.

The result of a view is stored in a permanent table whereas the result of a query is displayed in a temporary table.

Views can provide advantages over tables;

They can subset the data contained in a table They can join and simplify multiple tables into a single virtual table Views can act as aggregated tables, where aggregated data (sum, average etc.) are calculated and presented as part of the data Views can hide the complexity of data, for example a view could appear as Sales2000 or Sales2001, transparently partitioning the actual underlying table Views take very little space to store; only the definition is stored, not a copy of all the data they present Depending on the SQL engine used, views can provide extra security. Limit the exposure to which a table or tables are exposed to outer world

Just like functions (in programming) provide abstraction, views can be used to create abstraction. Also, just like functions, views can be nested, thus one view can aggregate data from other views. Without the use of views it would be much harder to normalise databases above second normal form. Views can make it easier to create lossless join decomposition.

Rows available through a view are not sorted. A view is a relational table, and the relational model states that a table is a set of rows. Since sets are not sorted – per definition – the rows in a view are not ordered either. Therefore, an ORDER BY clause in the view definition is meaningless and the SQL standard (SQL:2003) does not allow this for the subselect in a CREATE VIEW statement.

The following reference(s) were used for this question:

The text above is from About.Com at: http://databases.about.com/

The definition of views above is from: http://en.wikipedia.org/wiki/View_%28database%29

KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 47

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