In previous discussions and examples, we examined retrieving from a single table, or retrieving multiple values from multiple sources. Most real-world data operations are much more complex, requiring aggregation, comparison, and retrieval from multiple tables.

JOINs allow merging of two or more tables into a single object. They are employed through SELECT, UPDATE, and DELETE statements.

Review the general syntax of a statement employing a JOIN as shown below −

SELECT column
FROM table_name1
INNER JOIN table_name2
ON table_name1.column = table_name2.column;

Note the old syntax for JOINS used implicit joins and no keywords. It is possible to use a WHERE clause to achieve a join, but keywords work best for readability, maintenance, and best practices.

JOINs come in many forms such as a left join, right join, or inner join. Various join types offer different types of aggregation based on shared values or characteristics.

Employ a JOIN either at the command prompt or with a PHP script.

The Command Prompt

At the command prompt, simply use a standard statement −

root@host# mysql -u root -p password;
Enter password:*******
mysql> use PRODUCTS;
Database changed

mysql> SELECT products.ID_number, products.Nomenclature, inventory.inventory_ct
   FROM products
   INNER JOIN inventory
   ON products.ID_numbeer = inventory.ID_number;
+-------------+----------------+-----------------+
| ID_number   | Nomenclature   | Inventory Count |
+-------------+----------------+-----------------+
| 12345       | Orbitron 4000  | 150             |
+-------------+----------------+-----------------+
| 12346       | Orbitron 3000  | 200             |
+-------------+----------------+-----------------+
| 12347       | Orbitron 1000  | 0               |
+-------------+----------------+-----------------+

PHP Script Using JOIN

Use the mysql_query() function to perform a join operation −

<?php
   $dbhost = ''localhost:3036'';
   $dbuser = ''root'';
   $dbpass = ''rootpassword'';
   $conn = mysql_connect($dbhost, $dbuser, $dbpass);

   if(! $conn ) {
      die(''Could not connect: '' . mysql_error());
   }

   $sql = ''SELECT a.product_id, a.product_manufacturer, b.product_count   
      FROM products_tbl a, pcount_tbl b 
      WHERE a.product_manufacturer = b.product_manufacturer'';

   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 "Manufacturer:{$row[''product_manufacturer'']} <br&gt ".
         "Count: {$row[''product_count'']} <br&gt ".
         "Product ID: {$row[''product_id'']} <br&gt ".
         "--------------------------------<br&gt";
   }

   echo "Fetched data successfullyn";
   mysql_close($conn);
?>

On successful data retrieval, you will see the following output −

ID Number: 12345
Nomenclature: Orbitron 4000
Inventory Count: 150
--------------------------------------
ID Number: 12346
Nomenclature: Orbitron 3000
Inventory Count: 200
--------------------------------------
ID Number: 12347
Nomenclature: Orbitron 1000
Inventory Count: 0
--------------------------------------
mysql> Fetched data successfully