Introduction to SQL(Structured Query Language)

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8 years ago

Introduction

SQL(Structured Query Language), pronounced as “sequel” or S.Q.L, is a query language used to access, interact with, and manipulate relational databases. A good understanding of SQL will help us access and manipulate data in databases like Oracle, MySQL, MS SQL Server, MS Access, IBM DB2, etc.

SQL has rules of grammar and syntax like normal English and can be easily understood. SQL is an ANSI standard. (American National Standards Institute) .

Features of SQL

SQL is a standard language to access and manipulate relational databases. Some of the features are as follows:

SQL usage is extremely flexible and uses a free-form syntax. ( The same SQL query can be written in a variety of ways)
SQL allows operations to create, update, and delete database objects. Database objects like tables, views, etc.
SQL allows operations to insert, update, and delete data. SQL is a query language to retrieve and aggregate data.
The SQL statements can be embedded in other high-level programming languages. like C, Java, etc.

SQL Statements

Most of the actions we need to perform on a database are done with SQL statements. Some database systems require a semicolon(;) at the end of each SQL statement. The semicolon (;) is the standard way to separate each SQL statement.

The SELECT statement is used to select data from a database. For example, the following SQL SELECT statement will select all the records in the emp database table:

SELECT * FROM emp;

SQL is not case-sensitive. For example, both the SQL queries are the same.

SELECT * FROM emp;

select * from emp;

Example SQL Query

We can specify the exact columns that we want in the SQL statement. Let’s write a simple query to retrieve employees’ empno, name, salary, and job details 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

SQL Commands

SQL commands can be divided into different types.

https://www.testingdocs.com/types-of-sql-commands/

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Database Tutorials on this website:

https://www.testingdocs.com/oracle-database-tutorials-for-beginners/

More information about Oracle Database:

https://www.oracle.com/database/