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Text lesson

MySQL WHERE

Select and Filter Data From a MySQL Database

The WHERE clause filters records by extracting only those that meet a specified condition.

SELECT column_name(s) FROM table_name WHERE column_name operator value

Select and Filter Data With MySQLi

The following example retrieves the id, firstname, and lastname columns from the MyGuests table where the lastname is “Doe” and displays the results on the page:

Example (MySQLi Object-oriented)

<?php
$servername = “localhost”;
$username = “username”;
$password = “password”;
$dbname = “myDB”;

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
  die(“Connection failed: “ . $conn->connect_error);
}

$sql = “SELECT id, firstname, lastname FROM MyGuests WHERE lastname=’Doe'”;
$result = $conn->query($sql);

if ($result->num_rows > 0) {
  // output data of each row
  while($row = $result->fetch_assoc()) {
    echo “id: “ . $row[“id”]. ” – Name: “ . $row[“firstname”]. ” “ . $row[“lastname”]. “<br>”;
  }
} else {
  echo “0 results”;
}
$conn->close();
?>

Code lines explained from the example above:

First, we create an SQL query to select the id, firstname, and lastname columns from the MyGuests table where the lastname is “Doe”. The subsequent line executes the query and stores the resulting data in a variable named $result.

Next, the num_rows() function checks if any rows were returned.

If rows are present, the fetch_assoc() function retrieves all the results into an associative array, which we then loop through using a while() loop to display the data from the id, firstname, and lastname columns.

The following example demonstrates the same process using the MySQLi procedural approach:

Example (MySQLi Procedural)

<?php
$servername = “localhost”;
$username = “username”;
$password = “password”;
$dbname = “myDB”;

// Create connection
$conn = mysqli_connect($servername, $username, $password, $dbname);
// Check connection
if (!$conn) {
  die(“Connection failed: “ . mysqli_connect_error());
}

$sql = “SELECT id, firstname, lastname FROM MyGuests WHERE lastname=’Doe'”;
$result = mysqli_query($conn, $sql);

if (mysqli_num_rows($result) > 0) {
  // output data of each row
  while($row = mysqli_fetch_assoc($result)) {
    echo “id: “ . $row[“id”]. ” – Name: “ . $row[“firstname”]. ” “ . $row[“lastname”]. “<br>”;
}
} else {
  echo “0 results”;
}

mysqli_close($conn);
?>

You can also present the results in an HTML table:

Example (MySQLi Object-oriented)

<?php
$servername = “localhost”;
$username = “username”;
$password = “password”;
$dbname = “myDB”;

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
  die(“Connection failed: “ . $conn->connect_error);
}

$sql = “SELECT id, firstname, lastname FROM MyGuests WHERE lastname=’Doe'”;
$result = $conn->query($sql);

if ($result->num_rows > 0) {
  echo “<table><tr><th>ID</th><th>Name</th></tr>”;
  // output data of each row
  while($row = $result->fetch_assoc()) {
    echo “<tr><td>”.$row[“id”].“</td><td>”.$row[“firstname”].” “.$row[“lastname”].“</td></tr>”;
  }
  echo “</table>”;
} else {
  echo “0 results”;
}
$conn->close();
?>

Select Data With PDO (+ Prepared Statements)

The following example uses prepared statements to select the id, firstname, and lastname columns from the MyGuests table where the lastname is “Doe” and displays the results in an HTML table:

Example (PDO)

<?php
echo “<table style=’border: solid 1px black;’>”;
echo “<tr><th>Id</th><th>Firstname</th><th>Lastname</th></tr>”;

class TableRows extends RecursiveIteratorIterator {
  function __construct($it) {
    parent::__construct($it, self::LEAVES_ONLY);
}

  function current() {
    return “<td style=’width:150px;border:1px solid black;’>” . parent::current(). “</td>”;
  }

  function beginChildren() {
    echo “<tr>”;
  }

  function endChildren() {
    echo “</tr>” . “\n”;
  }
}

$servername = “localhost”;
$username = “username”;
$password = “password”;
$dbname = “myDBPDO”;

try {
  $conn = new PDO(“mysql:host=$servername;dbname=$dbname”, $username, $password);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  $stmt = $conn->prepare(“SELECT id, firstname, lastname FROM MyGuests WHERE lastname=’Doe'”);
  $stmt->execute();

  // set the resulting array to associative
$result = $stmt->setFetchMode(PDO::FETCH_ASSOC);
  foreach(new TableRows(new RecursiveArrayIterator($stmt->fetchAll())) as $k=>$v) {
    echo $v;
  }
}
catch(PDOException $e) {
  echo “Error: “ . $e->getMessage();
}
$conn = null;
echo “</table>”;
?>