WHERE clauses filter various statements such as SELECT, UPDATE, DELETE, and INSERT.
They present criteria used to specify action. They typically appear after a table name in a statement, and their condition follows. The WHERE clause essentially functions like an if statement.
Review the general syntax of WHERE clause given below:
[COMMAND] field,field2,... FROM table_name,table_name2,... WHERE [CONDITION]
Note the following qualities of the WHERE clause:
- It is optional.
- It allows any condition to be specified.
- It allows for the specification of multiple conditions through using an AND or OR operator.
- Case sensitivity only applies to statements using LIKE comparisons.
The WHERE clause permits the use of the following operators:
Operator = != > < >= <=
WHERE clauses can be utilized at the command prompt or within a PHP script.
The Command Prompt
At the command prompt, simply use a standard command:
root@host# mysql -u root -p password; Enter password:******* mysql> use PRODUCTS; Database changed mysql> SELECT * from products_tbl WHERE product_manufacturer='XYZ Corp'; +-------------+----------------+----------------------+ | ID_number | Nomenclature | product_manufacturer | +-------------+----------------+----------------------+ | 12345 | Orbitron 4000 | XYZ Corp | +-------------+----------------+----------------------+ | 12346 | Orbitron 3000 | XYZ Corp | +-------------+----------------+----------------------+ | 12347 | Orbitron 1000 | XYZ Corp | +-------------+----------------+----------------------+
Review an example using the AND condition:
SELECT * FROM products_tbl WHERE product_name = 'Bun Janshu 3000'; AND product_id <= 344;
This example combines both AND and OR conditions:
SELECT * FROM products_tbl WHERE (product_name = 'Bun Janshu 3000' AND product_id < 344) OR (product_name = 'Bun Janshu 3000');
PHP Scripts Using Where Clause
Employ the mysql_query() function in operations using a WHERE clause:
<?php
$dbhost = 'localhost:3036';
$dbuser = 'root';
$dbpass = 'rootpassword';
$conn = mysql_connect($dbhost, $dbuser, $dbpass);
if(! $conn )
{
die('Could not connect: ' . mysql_error());
}
$sql = 'SELECT product_id, product_name,
product_manufacturer, ship_date
FROM products_tbl
WHERE product_manufacturer="XYZ Corp"';
mysql_select_db('PRODUCTS');
$retval = mysql_query( $sql, $conn );
if(! $retval )
{
die('Could not get data: ' . mysql_error());
}
while($row = mysql_fetch_array($retval, MYSQL_ASSOC))
{
echo "Product ID :{$row['product_id']} <br> ".
"Name: {$row['product_name']} <br> ".
"Manufacturer: {$row['product_manufacturer']} <br> ".
"Ship Date: {$row['ship_date']} <br> ".
"--------------------------------<br>";
}
echo "Fetched data successfully\n";
mysql_close($conn);
?>
On successful data retrieval, you will see the following output:
Product ID: 12345 Nomenclature: Orbitron 4000 Manufacturer: XYZ Corp Ship Date: 01/01/17 ---------------------------------------------- Product ID: 12346 Nomenclature: Orbitron 3000 Manufacturer: XYZ Corp Ship Date: 01/02/17 ---------------------------------------------- Product ID: 12347 Nomenclature: Orbitron 1000 Manufacturer: XYZ Corp Ship Date: 01/02/17 ---------------------------------------------- mysql> Fetched data successfullyv