SQL is a computer/programming language which enables a user or any application to interact with an SQL capable database management systems or DBMS. SQL is a great tool to deal with databases. It is used to access and manipulate databases & It is very easy to implement.

In this tutorial, we will explain about the basics of SQL language, its history and its implementations.

SQL stands for Structured Query Language.

It is an ANSI (American National Standards Institute) standard language used for accessing and manipulating databases.

There are different databases, such as, MySQL, SQL Server, Access, Oracle, Sybase, DB2, and other database systems. We will basically deal with Oracle.

Uses of SQL:

SQL is a very useful language. It can be used for:

• It is used to insert records in a database.
• It is used to update records in a database.
• It is used to execute queries against a database.
• It is used to retrieve data from a database.
• It is used to delete records from a database.
• It can create new database.
• It can create new tables in a database.
• It can be used to create views in a database.

History of SQL:

Before moving on its implementations, let us see the history of SQL.

IBM developed the original version of SQL at its San Jose Research Laboratory (presently known as Almaden Research Center).

1. In 1970s, IBM implemented the language, originally called Sequel, as a part of the system R project.
2. Later on, in 1974, the name of Sequel changed to SQL (Structured Query Language).
3. In 1986, the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO) published in SQL standard, called SQL-86.
4. In 1987, IBM published its own corporate SQL standard, the System Application Architecture Database Interface (SAA-SQL).
5. ANSI published an extended standard for SQL, called SQL-89, in 1989.

Components / Parts of SQL Language:

• Data Definition Language (DDL): DDL is used for defining relational schemas, deleting relations, and modifying relational schemas.
• Data Manipulation Language (DML): It includes query language. It also includes commands to insert tuples , delete tuples and modify tuples in the database.
• Transaction Control: SQL includes commands for specifying the beginning and ending of transactions.

Elements of an SQL Language:

The SQL language is sub-divided into several language elements, including:

• Clauses
• Expressions
• Predicates
• Queries
• Statements

Chart to illustrate SQL Elements:

The use of sequence is very important in Oracle. A sequence is a database item that generates a sequence of integers. It is a set of integers that are used to create a unique number to act as a primary key.

Sytax of an Oracle Sequence

The syntax for a sequence is:

CREATE SEQUENCE sequence_name
MINVALUE value
MAXVALUE value
START WITH value
INCREMENT BY value
CACHE value;

SQL Sequence Example:

Let us consider a simple example. Suppose we want to create a sequence object called “s”, it starts with 1, the minimum and maximum values are 1 and 100 respectively, it is incremented by 1 and will cache up to 20 values. Then the following command will be used:

CREATE SEQUENCE s_seq
MINVALUE 1
MAXVALUE 100
START WITH 1
INCREMENT BY 1
CACHE 20;

The default value of MAXVALUE option is 999999999999999999999999999. It is the maximum value up to which it is used.

NEXTVAL Operator:

Nextval Operator is used to retrieve the next value in the sequence order.

The general way to use this command is as follows:

s.NEXTVAL

where “s” is the sequence object.

This would retrieve the next value from “s”.

CURRVAL Operator:

Currval Operator is used to retrieve the current value in the sequence order.

The general way to use this command is as follows:

s.CURRVAL

where “s” is the sequence object.

This would retrieve the current value from “s”.

INSERT Statement & Sequences:

Insert statement is used to insert values in the table.

Let us understand this with a simple example:

INSERT INTO s
(s1_id, s1_name)
VALUES
(s.NEXTVAL, ‘hello student’);

The insert statement would insert a new record into the “s” table. The s1_id field would be assigned the next number from the “s” sequence. The s1_name field would be set to ‘hello student’.

Let us take few more examples:

• Creating a sequence “ t ”

CREATE SEQUENCE t
START WITH 10
INCREMENT BY 5
MINVALUE 10
MAXVALUE 20
CYCLE CACHE 2 ORDER ;

Sequence created.

• Creating a sequence “ d ” start with 50 increment by 10

CREATE SEQUENCE d
START WITH 50
INCREMENT BY 10 ;

Sequence created.

SELECT statement & Sequences:

Select command is used to select a data from a database and then display it.

Syntax:

SELECT column name
FROM table name;

Let us take an example:

SELECT d.nextval
FROM dhr;

Output will be as follows:

NEXTVAL
———-
50

DROP a Sequence:

A drop statement is used for deleting a sequence. If the sequence is deleted, it will not be restored again.

Syntax:

DROP SEQUENCE name;

For example:

DROP SEQUENCE t;

Sequence dropped.

Note: The sequence “ t ” is dropped and will not be restored again.