PHP & MySQL - Using Joins

The MySQL JOIN clause is used to combine rows of two or more tables based on common column between them. There are four types of JOINs in MySQL:

• INNER JOIN - It is sometimes called simple JOIN. It returns records based on matching rows in both tables.
• LEFT JOIN - It is sometimes called LEFT OUTER JOIN. It returns records which contains all rows from left table and matching rows from right tables.
• RIGHT JOIN - It is sometimes called RIGHT OUTER JOIN. It returns records which contains all rows from right table and matching rows from left tables.
• CROSS JOIN - Returns records which contains all rows from both tables. It is sometimes called CARTESIAN JOIN because in the absence of a WHERE condition it behaves like a CARTESIAN PRODUCT i.e., the number of rows in the result-set is the product of the number of rows of the two tables.

Consider a database containing tables called Employee and Contact_Info with the following records:

Table 1: Employee table

Table 2: Contact_Info table

In the query below, the INNER JOIN clause is used with Employee and Contact_Info tables based on common column EmpID. It returns Name, Age and Address columns based on match found in both tables.

SELECT Employee.Name, Employee.Age, Contact_Info.Address 
FROM Employee
INNER JOIN Contact_Info
ON Employee.EmpID = Contact_Info.EmpID;

This will produce the result as shown below:

Please note that, to connect to the MySQL server, mysqli_connect() function can be used. After establishing the connection, mysqli_query() function can be used to perform a query on the database.

The num_rows() function can be used to check if there are more than zero rows returned. Then, the fetch_assoc() function can be used to fetch the result set as an associative array. Later on the free_result() function can be used to free the memory associated with the result.

MySQL JOIN - Object-oriented style

The example below demonstrates how to perform INNER JOIN discussed above using object-oriented style.

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDatabase";

//establishing connection 
$mysqli = new mysqli($servername, $username, $password, $dbname);

//checking connection
if ($mysqli->connect_errno) {
  echo "Failed to connect to MySQL: ". $mysqli->connect_error;
  exit();
}

//getting query result from the database
$sql = 'SELECT Employee.Name, Employee.Age, Contact_Info.Address 
FROM Employee
INNER JOIN Contact_Info
ON Employee.EmpID = Contact_Info.EmpID;';

$result = $mysqli->query($sql);

//fetching associative array
while ($row = $result->fetch_assoc()) {
  printf("Name: %s, Age: %d, Address: %s\n", 
          $row["Name"], $row["Age"], $row["Address"]);
}

//free result set
$result->free_result();

//closing the connection
$mysqli->close();
?>

The output of the above code will be:

Name: Marry, Age: 24, Address: Brooklyn, New York, USA
Name: Jo, Age: 27, Address: Grenelle, Paris, France
Name: Kim, Age: 30, Address: Geuzenveld, Amsterdam, Netherlands
Name: Huang, Age: 28, Address: Yizhuangzhen, Beijing, China 

MySQL JOIN - Procedural style

To obtain the same result using procedural style, the following script can be used.

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDatabase";

//establishing connection 
$mysqli = mysqli_connect($servername, $username, $password, $dbname);

//checking connection
if (mysqli_connect_errno()) {
  echo "Failed to connect to MySQL: ". mysqli_connect_error();
  exit();
}

//getting query result from the database
$sql = 'SELECT Employee.Name, Employee.Age, Contact_Info.Address 
FROM Employee
INNER JOIN Contact_Info
ON Employee.EmpID = Contact_Info.EmpID;';
$result = mysqli_query($mysqli, $sql);

//fetching associative array
while ($row = mysqli_fetch_assoc($result)) {
  printf("Name: %s, Age: %d, Address: %s\n", 
          $row["Name"], $row["Age"], $row["Address"]);
}

//free result set
mysqli_free_result($result);

//closing the connection
mysqli_close($mysqli);
?>

The output of the above code will be:

Name: Marry, Age: 24, Address: Brooklyn, New York, USA
Name: Jo, Age: 27, Address: Grenelle, Paris, France
Name: Kim, Age: 30, Address: Geuzenveld, Amsterdam, Netherlands
Name: Huang, Age: 28, Address: Yizhuangzhen, Beijing, China 

Complete PHP MySQLi Reference

For a complete reference of all properties, methods and functions of PHP MySQLi extension, see PHP MySQLi Reference.