In this example, lets query to display firstname,lastname,email and salary of all employees in department with id=100.

     SELECT first_name,last_name,email,salary 
     FROM employees
     WHERE department_id=100;
     

You should get a similar result like the one displayed on image below.

Sql Developer and Basic Sql statements

Now lets use an example that makes use of characters, Lets query to display firstname,lastname,salary of all employees who are accountants (job_id='FI_ACCOUNT').

 
SELECT first_name, last_name, salary
FROM employees
WHERE job_id='FI_ACCOUNT';
          

You should get results similar to the one shown in image below:Sql developer and Writing Basic sql statements.
NOTE:when writing strings,it should be in single quotes.

CHARACTER STRINGS AND DATES.

Character strings and Dates in the WHERE clause must be enclosed by single quotation marks (' ') like in the example directly above,while Number constants should not be enclosed by single quotation marks. The default date display is DD/MON/RR.

Comparison Conditions.
Comparison conditon lets you compare one expression to another, the result of such a comparison can be TRUE, FALSE, or NULL.
 =   means equal to
 >  
means greater than
 >=
means greater than or equal to
 <  
means less than
 <=
means less than and equal to
<> , !=
both means not equal to
Between ... and ...  means between two values (inclusive)
IN (set)
means match any of a list of values
LIKE match a character pattern
IS NULL
means is a null value.

Examples Using Comparison Conditions.

Suppose you want to display all the employees with lastname,firstname,job ID that belong to department 100.(using '=' condition). 

SELECT last_name, first_name, job_id
FROM employees
WHERE department_id=100;
          

You should get results similar to the one shown below.SQL developer and basic sql statements

Suppose you want to display all the employees with lastname,firstname,job ID that has salary not greater than 7000. (using '<' '=' condition)

SELECT last_name, first_name,job_id,salary
FROM employees
WHERE salary <=7000;

Image below shows the result of the query.SQL developer and basic sql statements