MariaDB - clausola Where

WHEREle clausole filtrano varie istruzioni come SELECT, UPDATE, DELETE e INSERT. Presentano i criteri utilizzati per specificare l'azione. In genere vengono visualizzati dopo il nome di una tabella in un'istruzione e la loro condizione segue. La clausola WHERE funziona essenzialmente come un'istruzione if.

Rivedere la sintassi generale della clausola WHERE riportata di seguito:

[COMMAND] field,field2,... FROM table_name,table_name2,... WHERE [CONDITION]

Notare le seguenti qualità della clausola WHERE:

• 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 successfully