Structured Query Language(SQL)

Document Contents

Introduction

SQL(Structured Query Language) pronounced either as sequel or S.Q.L is a query language used to communicate and interact with a relational database like Oracle, MySQL, etc. SQL has rules of grammar and syntax like normal English and can be easily understood.

Features of SQL

SQL is not merely a query language. It can also be used to create, update and delete database objects.

SQL usage is extremely flexible and uses a free form syntax. ( The same SQL query can be written in a variety of ways)

SQL statements can be embedded in other high-level programming languages. like C, Java, etc.

Types of SQL commands

SQL commands can be classified into different categories based on their functionality.

Data Definition Language(Create, Alter, Drop)

Data Manipulation Language(Insert,Update,Delete)

Data Query Language(Select)

Data Control Language(Grant, Revoke)

Transaction Control Language(Commit, Rollback, etc)

Database administration (Start Audit etc)

Sample SQL query.

Let’s write a simple query to retrieve empno, name, salary and job details for the employees from the emp table.

The table structure of the emp table is shown below:

SQL> DESC emp;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 EMPNO                                     NOT NULL NUMBER(4)
 ENAME                                              VARCHAR2(10)
 JOB                                                VARCHAR2(9)
 MGR                                                NUMBER(4)
 HIREDATE                                           DATE
 SAL                                                NUMBER(7,2)
 COMM                                               NUMBER(7,2)
 DEPTNO                                             NUMBER(2)

SQL> SELECT empno,ename,job,sal FROM emp;

Query result:

SQL> SELECT empno,ename,job,sal FROM emp;

     EMPNO ENAME      JOB              SAL
---------- ---------- --------- ----------
      7369 SMITH      CLERK            800
      7499 ALLEN      SALESMAN        1600
      7521 WARD       SALESMAN        1250
      7566 JONES      MANAGER         2975
      7654 MARTIN     SALESMAN        1250
      7698 BLAKE      MANAGER         2850
      7782 CLARK      MANAGER         2450
      7788 SCOTT      ANALYST         3000
      7839 KING       PRESIDENT       5000
      7844 TURNER     SALESMAN        1500
      7876 ADAMS      CLERK           1100