The WHERE Clause Operators

The first WHERE clause we looked at tests for equalitydetermining if a column contains a specific value. MySQL supports a whole range of conditional operators, some of which are listed in Table 6.1.

Table 6.1. WHERE Clause Operators

Operator	Description
=	Equality
<>	Nonequality
!=	Nonequality
<	Less than
<=	Less than or equal to
>	Greater than
>=	Greater than or equal to
BETWEEN	Between two specified values

Checking Against a Single Value

We have already seen an example of testing for equality. Here's one more:

• Input

SELECT prod_name, prod_price
FROM products
WHERE prod_name = 'fuses';

• Output

+-----------+------------+
| prod_name | prod_price |
+-----------+------------+
| Fuses     |       3.42 |
+-----------+------------+

• Analysis

Checking for WHERE prod_name = 'fuses' returned a single row with a value of Fuses. By default, MySQL is not case sensitive when performing matches, and so fuses and Fuses matched.

Now look at a few examples to demonstrate the use of other operators.

This first example lists all products that cost less than 10:

• Input

SELECT prod_name, prod_price
FROM products
WHERE prod_price < 10;

• Output

+---------------+------------+
| prod_name     | prod_price |
+---------------+------------+
| .5 ton anvil  |       5.99 |
| 1 ton anvil   |       9.99 |
| Carrots       |       2.50 |
| Fuses         |       3.42 |
| Oil can       |       8.99 |
| Sling         |       4.49 |
| TNT (1 stick) |       2.50 |
+---------------+------------+

This next statement retrieves all products costing 10 or less (resulting in two additional matches):

• Input

SELECT prod_name, prod_price
FROM products
WHERE prod_price <= 10;

• Output

+----------------+------------+
| prod_name      | prod_price |
+----------------+------------+
| .5 ton anvil   |       5.99 |
| 1 ton anvil    |       9.99 |
| Bird seed      |      10.00 |
| Carrots        |       2.50 |
| Fuses          |       3.42 |
| Oil can        |       8.99 |
| Sling          |       4.49 |
| TNT (1 stick)  |       2.50 |
| TNT (5 sticks) |      10.00 |
+----------------+------------+

Checking for Nonmatches

This next example lists all products not made by vendor 1003:

• Input

SELECT vend_id, prod_name
FROM products
WHERE vend_id <> 1003;

• Output

+---------+--------------+
| vend_id | prod_name    |
+---------+--------------+
|    1001 | .5 ton anvil |
|    1001 | 1 ton anvil  |
|    1001 | 2 ton anvil  |
|    1002 | Fuses        |
|    1005 | JetPack 1000 |
|    1005 | JetPack 2000 |
|    1002 | Oil can      |
+---------+--------------+

The following is the same example, except this one uses the != operator instead of <>:

• Input

SELECT vend_id, prod_name
FROM products
WHERE vend_id != 1003;

Checking for a Range of Values

To check for a range of values, you can use the BETWEEN operator. Its syntax is a little different from other WHERE clause operators because it requires two values: the beginning and end of the range. The BETWEEN operator can be used, for example, to check for all products that cost between 5 and 10 or for all dates that fall between specified start and end dates.

The following example demonstrates the use of the BETWEEN operator by retrieving all products with a price between 5 and 10:

• Input

SELECT prod_name, prod_price
FROM products
WHERE prod_price BETWEEN 5 AND 10;

• Output

+----------------+------------+
| prod_name      | prod_price |
+----------------+------------+
| .5 ton anvil   |       5.99 |
| 1 ton anvil    |       9.99 |
| Bird seed      |      10.00 |
| Oil can        |       8.99 |
| TNT (5 sticks) |      10.00 |
+----------------+------------+

• Analysis

As seen in this example, when BETWEEN is used, two values must be specifiedthe low end and high end of the desired range. The two values must also be separated by the AND keyword. BETWEEN matches all the values in the range, including the specified range start and end values.

Checking for No Value

When a table is created, the table designer can specify whether individual columns can contain no value. When a column contains no value, it is said to contain a NULL value.

The SELECT statement has a special WHERE clause that can be used to check for columns with NULL valuesthe IS NULL clause. The syntax looks like this:

• Input

SELECT prod_name
FROM products
WHERE prod_price IS NULL;

This statement returns a list of all products that have no price (an empty prod_price field, not a price of 0), and because there are none, no data is returned. The customers table, however, does contain columns with NULL valuesthe cust_email column contains NULL if a customer has no email address on file:

• Input

SELECT cust_id
FROM customers
WHERE cust_email IS NULL;

• Output

+---------+
| cust_id |
+---------+
|   10002 |
|   10005 |
+---------+