Introduction to SQL
CIS 205, Spring 2005
page 1 of 4
An Introduction to SQL
by C. Herbert
for CIS 205 – Database Management Systems
Community College of Philadelphia
March, 2005
What is SQL?
SQL stands for Structured Query Language. According to Oracle Corporation, SQL is “The
standardized relational database language for querying, manipulating, and updating
information in a relational database.“ The first standardized version of SQL was approved by
ANSI in 1986, and an international standard was adopted by ISO in 1992. Many commercially
available database management systems, such as Oracle 9i, MySQL, and Microsoft SQL Server
are based on the SQL-92 standard. Each of these software packages also contain extension to
SQL 92 that provide increased capability, but only work within their own systems. Generally,
SQL databases are compatible with one another.
SQL is not the same as SEQUEL, which is an older database management product from IBM.
SQL is a Data Definition Language (DDL), a Data Manipulation Language (DML), and a Data
Control Language (DCL) all in one.
A DDL allows users to define a database, including creating, altering, and dropping tables and
establishing database constraints. A constraint is a rule for the database, such as defining a
particular attribute as a primary key, or establishing that a particular attribute cannot be NULL.
A DML allows the user to manipulate a database. Most importantly, a DML provides the ability
to query a database.
A DCL is used to control the database administratively, such as scheduling events, committing
data, and managing user rights and privileges within a database. In CIS 205, we will look at SQL
as a DDL and a DML, but not as a DCL. I will need to use the DCL features of SQL to set up
Oracle accounts for you on our systems, and you may see some DCL commands, but we will
focus on implementing and querying a database using SQL’s DDL and DML features.
Using a standardized database language, such as SQL:
 increases productivity among both database managers and users,
 makes database application more portable and more useful over time,
 reduces dependence on a single vendor, a single programmer, or programming team, such
as a particular consulting firm,
 reduced training costs for those who work with databases,
 allows multiple database management systems to communicate with one another.
Introduction to SQL
CIS 205, Spring 2005
page 2 of 4
In SQL, a schema is the description of a database and the tables it contains. We might say that
the schema describes the structure of a database, including its metadata.
SQL as a DDL
In SQL, CREATE statements are used to define database objects, such as tables.
Note: SQL is not case-sensitive, therefore, it is important for people to use uppercase
and lowercase language to make SQL statements easier to read. Many people use
uppercase lettering for SQL statements and key words, and lowercase lettering for the
names of tables, columns, etc., as in this example:
SELECT last, first, hours, rate, (hours * rate) as gross
FROM payroll
WHERE hours <= 40;
Also note that SQL statements end with a semicolon (;).
The most important SQL DDL statement is “CREATE TABLE”, which is used to define a table
and its columns.
Two other commonly used SQL DDL statements are:
ALTER TABLE, which can be used to change the structure of a table, and
DROP TABLE, which is used to remove tables from a database.
The INSERT statement, which is used to add data to a database, can be considered either a DDL
or a DML statement. So can the UPDATE statement, which can be used to change data in a
database.
I will not explain each how statement works here, as that information is contained in the tutorials
and in the reference information that I will provide next week.
Here are some examples of each of these statements:
1. CREATE TABLE student
(stnumber
VARCHAR(10) NOT NULL,
lname
VARCHAR(15),
fname
VARCHAR(15),
address
VARCHAR(15),
city
VARCHAR(15),
state
VARCHAR(15),
zip
VARCHAR(15),
major
VARCHAR(25) NOT NULL,
GPA
NUMBER(5,3)),
CONSTRAINT student_pk PRIMARY KEY (stnumber));
2. ALTER TABLE inventory ADD (stockno VARCHAR(10));
3. DROP TABLE courses;
Introduction to SQL
CIS 205, Spring 2005
page 3 of 4
4. INSERT INTO customer VALUES (001, ‘Contemporary Casuals’, 1355 S. Himes Blvd.’,
‘Gainesville’, ‘FL’, 32601);
5. INSERT INTO product (product_id, product_description,product_finish, standard_price,
product_on_hand) VALUES (1, ‘End Table’, ‘Cherry’, 175, 8);
6. INSERT INTO customerB SELECT * FROM CUSTOMERA WHERE state = ‘CA’;
7. DELETE FROM customer WHERE state = ‘HI’;
8. UPDATE product SET unit_price = 775 WHERE product_id = “SK37”;
SQL as a DML – The SELECT Statement
Perhaps the most important statement in SQL is the SELECT statement. The SELECT Statement
can be used for queries on a single table or on multiple tables.
The SELECT … FROM … WHERE statement combines both projection and selection from
relational algebra in a single statement. Remember that projection determines which attributes
we see in our result set (which columns) and selection contains a condition that determines
which instances of the entity (which rows) we will see. The SELECT … FROM … WHERE …
statement does both, and more.
Here is an example of the statement:
SELECT last, first, hours
FROM employees
WHERE hours > 40;
This statement tells us which columns we want (last, first, hours) just like a projection operation
in relational algebra, and from rows that meet the condition (hours > 40), just like a selection
operation in relational algebra. The result will be a table, called a “result set” that contains rows
and columns with the data we requested. Of course, it is possible for the result set table to be a
single cell, for the result to be a NULL value if no data is returned, or for the result to be an error
message if our SQL statement cannot be understood by the database management system
processing the SQL statement.
In its simplest form, the statement has three parts, or clauses, which are easy to see — the
SELECT clause, the FROM clause, and the WHERE clause, as follows:

the SELECT clause contains the word SELECT, followed by a list of the columns from
which we wish to retrieve data,

the FROM clause contains the word FROM, followed by the names of one or more tables
from which we wish to retrieve data, and
Introduction to SQL

CIS 205, Spring 2005
page 4 of 4
the WHERE clause includes the word WHERE followed by the condition that will
determine which rows are included in the result set.
In this form of the statement, the list of columns in the SELECT clause is just like the list of
attributes we need for relational algebra’s projection operation, and the condition in the FROM
clause mirrors the condition in a relation algebra selection operation.
More sophisticated uses of the SELECT statement
The SELECT statement has many additional features beyond the basic SELECT … FROM …
WHERE statement. We can also use aliases aggregate functions, and additional modifying
clauses in the SELECT statement.
An alias is as alternative name for a column that will appear in the result set, such as in the
following:
SELECT enumber as employee, last, first, hours as hrs
FROM employees
WHERE hours > 40;
In the example above, employee and hrs are aliases that will be used in the result set as the
names of two of the columns in place of enumber and hours.
An aggregate function returns a result set with summary information about a table, such as:
SELECT COUNT(*)
FROM employees
WHERE hours > 40;
This would return a result set with a single cell containing the number of rows n the employees
table with hours greater than 40.
In addition to the SELECT, FROM, and WHERE, clauses, several extension clauses can be
added to the statement to modify how it works, including the following:
ORDER BY - Sorts the rows in the result according to specified criteria
GROUP BY – allows categorization of the results by a particular attribute
HAVING - indicate the conditions under which a category (group) will be included in the
result set
We will be learning SQL over the next few weeks. Next week I will give you a reference
manual for MySQL, which contains extended descriptions of many SQL statements. For now,
more information on the statements of SQL that we will start with is contained in the tutorials.