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What is SQL?
Oracle Tips by Burleson
 

Structured Query Language (SQL) is a relatively young language compared to languages like COBOL and Fortran, and it’s quite different from those early ancestors.

SQL is a fourth-generation programming language (4GL), which means that it allows the user to describe the data he or she wants without giving precise instructions to the computer about how to retrieve the data. This was a remarkable idea when SQL was first introduced, and SQL remains the only 4GL in widespread use today. All of the major relational database systems use SQL for accessing data (although all of them, like Oracle’s SQL*Plus, include proprietary modifications to the language).

SQL statements come in two varieties: data definition language and data manipulation language.

Data Definition Language

Data definition language (DDL) is language that defines how data is stored within the relational database. A simple example of a DDL statement is shown in Listing 2.5.

Listing 2.5 A simple DDL statement.

CREATE TABLE STUDENTS AS
(ssn                   NOT NULL   number(9),
 first_name            NOT NULL   varchar2 (10),
 last_name             NOT NULL   varchar2 (12),
 middle_name                      varchar2 (10),
 street_address        NOT NULL   varchar2 (30),
 apartment_number                 varchar2 (4),
 city                  NOT NULL   varchar2 (30),
 state                 NOT NULL   varchar2 (2),
 zip_code              NOT NULL   number (5),
 home_phone            NOT NULL   number (10));

This statement creates a table within the Oracle database to hold information about students. Obviously, this is a very simple example. As you become a more advanced developer, you’ll learn more about DDL.

Data Manipulation Language

Data manipulation language (DML) is the most common type of SQL that you’ll encounter. In case you haven’t already figured it out by reading the name, this is the SQL that creates, deletes, reads, and modifies data. Listing 2.6 displays a simple DML statement.

Listing 2.6 A simple DML statement.

SELECT last_name || ', ' || first_name || ' ' || middle_name
FROM   STUDENTS
WHERE  ssn = 999999999;

This statement returns the name of the student whose social security number is 999-99-9999 from the STUDENTS table, in the format “Doe, John Adam”. In addition to querying data from tables using the SELECT statement, you can add data to tables with the INSERT statement, modify the data in tables with the UPDATE statement, and remove data from tables with the DELETE statement.

  

This is an excerpt from the book "High Performance Oracle Database Automation" by Jonathan Ingram and Donald K. Burleson, Series Editor.

  
 

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