Limit Rows Selected
Objectives
This tutorial covers the following objectives:
- Apply SQL syntax to restrict the rows returned from a query
- Demonstrate application of the WHERE clause syntax
- Explain why it is important, from a business perspective, to be able to easily limit data retrieved from a table
- Construct and produce output using a SQL query containing character strings and date values
Purpose
- Have you ever had “information overload”?
- The television is on, your Mom is asking you how school went today, the phone rings, and the dog is barking.
- Wouldn’t it be nice to be able to restrict the amount of information you have to process at one time?
- In SQL, this is the job of the WHERE clause.
- It is important to be able to choose the information you need to see from a table.
- Tables can have millions of rows of data, and it is a waste of resources to search and return data you don’t need or want.
SELECT Statement
- You use SELECT to retrieve information from the database.
- A SELECT statement must include at a minimum a SELECT clause and a FROM clause.
- The WHERE clause is optional.
SELECT*|{[DISTINCT] column | expression alias]..}
FROM table
[WHERE condition(s)];
SELECT clause general syntax: The [ ] mean this is optional. The | means “or.”
WHERE Clause
- When retrieving data from the database, you may need to limit the rows of data that are displayed.
- You can accomplish this using the WHERE clause.
- A WHERE clause contains a condition that must be met, and it directly follows the FROM clause in a SQL statement.
- The syntax for the WHERE clause is:
WHERE column_name comparison_condition comparison_value
Note: An alias cannot be used in the WHERE clause!
- Examine the following SQL statement from the Employees database:
SELECT employee_id, first_name, last_name
FROM employees;

- By adding a WHERE clause, the rows are limited to those rows where the value of employee_id is 101.

Comparison Operators in the WHERE Clause
- As you saw above, the = sign can be used in the WHERE clause.
- In addition to the “equal to” operator (=), other comparison operators can be used to compare one expression to another:
- = equal to
- > greater than
- >= greater than or equal to
- < less than
- <= less than or equal to
- <> not equal to (or != or ^=)
- In the example below, the department_id column is used in the WHERE clause, with the comparison operator = .
- All employees with a department_id of 90 are returned.
SELECT employee_id, last_name, department_id
FROM employees
WHERE department_id = 90;

Character and Date Strings in the WHERE Clause
- Character strings and dates in the WHERE clause must be enclosed in single quotation marks ‘ ‘.
- Numbers, however, should not be enclosed in single quotation marks.
- Look at the following example from the Employees database.
- The WHERE clause contains a string and is enclosed in single quotation marks.
SELECT first_name, last_name
FROM employees
WHERE last_name = 'Taylor';
- What do you think will happen if the WHERE clause is written as:
WHERE last_name = 'jones';
- All character searches are case-sensitive.
- Because the employees table stores all the last names in the proper case, no rows are returned in this example.
- This is an important point to remember.
- In another lesson, you will learn to use other SQL keywords UPPER, LOWER, and INITCAP that will make it easier to avoid a case-sensitive mistake.
Comparison Operators in the WHERE Clause
- Comparison operators can be used in all of the following ways in the WHERE clause:
WHERE hire_date < '01-Jan-2000'
WHERE salary >= 6000
WHERE job_id = 'IT_PROG'
Like character strings, Date values must also be enclosed in single quotes.
- In the following example from the Employees database, which rows will be selected?
- Will salaries of 3000 be included in the result set?
SELECT last_name, salary
FROM employees
WHERE salary <= 3000;
Terminology
Key terms used in this tutorial included:
- WHERE Clause
- Comparison Operators
Summary
In this tutorial, you should have learned how to:
- Apply SQL syntax to restrict the rows returned from a query
- Demonstrate application of the WHERE clause syntax
- Explain why it is important, from a business perspective, to be able to easily limit data retrieved from a table
- Construct and produce output using a SQL query containing character strings and date values
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