(SQL Tutorials for Citizen Data Scientist)

SQL WHERE Clause

In this tutorial you will learn how to select specific records from a table using SQL.

Selecting Record Based on Condition

In the previous chapter we’ve learnt how to fetch all the records from a table or table columns. But, in real world scenario we generally need to select, update or delete only those records which fulfill certain condition like users who belongs to a certain age group, or country, etc.

The WHERE clause is used with the SELECT, UPDATE, and DELETE. However, you’ll see the use of this clause with other statements in upcoming chapters.

Syntax

The WHERE clause is used with the SELECT statement to extract only those records that fulfill specified conditions. The basic syntax can be given with:

Here, column_list are the names of columns/fields like name, age, country etc. of a database table whose values you want to fetch. However, if you want to fetch the values of all the columns available in a table, you can use the following syntax:

Now, let’s check out some examples that demonstrate how it actually works.

Suppose we’ve a table called employees in our database with the following records:

+--------+--------------+------------+--------+---------+
| emp_id | emp_name     | hire_date  | salary | dept_id |
+--------+--------------+------------+--------+---------+
|      1 | Ethan Hunt   | 2001-05-01 |   5000 |       4 |
|      2 | Tony Montana | 2002-07-15 |   6500 |       1 |
|      3 | Sarah Connor | 2005-10-18 |   8000 |       5 |
|      4 | Rick Deckard | 2007-01-03 |   7200 |       3 |
|      5 | Martin Blank | 2008-06-24 |   5600 |    NULL |
+--------+--------------+------------+--------+---------+

Filter Records with WHERE Clause

The following SQL statement will returns all the employees from the employees table, whose salary is greater than 7000. The WHERE clause simply filtered out the unwanted data.

SELECT * FROM employees
WHERE salary > 7000;

After execution, the output will look something like this:

+--------+--------------+------------+--------+---------+
| emp_id | emp_name     | hire_date  | salary | dept_id |
+--------+--------------+------------+--------+---------+
|      3 | Sarah Connor | 2005-10-18 |   8000 |       5 |
|      4 | Rick Deckard | 2007-01-03 |   7200 |       3 |
+--------+--------------+------------+--------+---------+

As you can see the output contains only those employees whose salary is greater than 7000. Similarly, you can fetch records from specific columns, like this:

SELECT emp_id, emp_name, hire_date, salary
FROM employees
WHERE salary > 7000;

After executing the above statement, you’ll get the output something like this:

+--------+--------------+------------+--------+
| emp_id | emp_name     | hire_date  | salary |
+--------+--------------+------------+--------+
|      3 | Sarah Connor | 2005-10-18 |   8000 |
|      4 | Rick Deckard | 2007-01-03 |   7200 |
+--------+--------------+------------+--------+

The following statement will fetch the records of an employee whose employee id is 2.

SELECT * FROM employees
WHERE emp_id = 2;

This statement will produce the following output:

+--------+--------------+------------+--------+---------+
| emp_id | emp_name     | hire_date  | salary | dept_id |
+--------+--------------+------------+--------+---------+
|      2 | Tony Montana | 2002-07-15 |   6500 |       1 |
+--------+--------------+------------+--------+---------+

This time we got only one row in the output, because emp_id is unique for every employee.

Operators Allowed in WHERE Clause

SQL supports a number of different operators that can be used in WHERE clause, the most important ones are summarized in the following table.

Beginners Guide to SQL – SQL WHERE Clause

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