How to Check If Value Already Exists in MySQL Database

check if data already exist inside mysql database

Your forgot to put AND keyword inside SQL query. Update your query like this,

$query = "SELECT * FROM `booked` WHERE 
        `name` = '{$username}' AND 
        `date` = '{$date}' AND 
        `computer_id` = '{$select3}' AND 
        `start_time` = '{$select1}' AND 
        `end_time` = '{$select2}' AND 
        `room` = '{$room}'
        ";

We used AND here to match all the conditions, if all the conditions are matched only then it will return result.

There is another keyword called OR which is also used in WHERE clause. If OR is applied within two conditions (WHERE column1=1 OR column2=2) either of the condition should match to get the results.

You can also use mix of AND and OR.

Reference: http://dev.mysql.com/doc/refman/5.7/en/logical-operators.html

How to check if user already exists in MySQL with PHP

this code works fine for you...

$con=mysqli_connect("localhost","root","","my_db");
$check="SELECT * FROM persons WHERE Email = '$_POST[eMailTxt]'";
$rs = mysqli_query($con,$check);
$data = mysqli_fetch_array($rs, MYSQLI_NUM);
if($data[0] > 1) {
    echo "User Already in Exists<br/>";
}

else
{
    $newUser="INSERT INTO persons(Email,FirstName,LastName,PassWord) values('$_POST[eMailTxt]','$_POST[NameTxt]','$_POST[LnameTxt]','$_POST[passWordTxt]')";
    if (mysqli_query($con,$newUser))
    {
        echo "You are now registered<br/>";
    }
    else
    {
        echo "Error adding user in database<br/>";
    }
}

Check whether the data already exists in MySQL

There were multiple issues in your code. I'll start with answering your question. $check will never be set because your query isn't being executed. The $ is missing from $sql. Additionally, you always need to sanitize/escape user input before using it in a query. If you do not sanitize it, then it is possible that a hacker might inject unwanted code into your query, doing things that you didn't want to be done. See the updated and optimized code below:

<?php 
if($_SERVER['REQUEST_METHOD']=='POST'){
    //Getting values
    if(isset($_POST['project']) && !empty($_POST['project'])){
        //Importing our db connection script
        require_once('dbConnect.php');
        $project = strtoupper($_POST['project']);
        //Security: input must be sanitized to prevent sql injection
        $sanitized_project = mysqli_real_escape_string($con, $project);
        $sql = 'SELECT * FROM Project WHERE project=' . $sanitized_project . ' LIMIT 1';// LIMIT 1 prevents sql from grabbing unneeded records
        $result = mysqli_query($con, $sql);
        if(mysqli_num_rows($result) > 0){
            // a match was found
            // no need insert
        }
        else{
            //Creating an sql query
            $sql = "INSERT INTO Project(project) VALUES ('$sanitized_project')";
            //Executing query to database
            if(mysqli_query($con,$sql)){
            echo('Added Successfully');
        }
        else{
            echo('Could Not Add Project');
        }
    }
    else{
        echo('data is null');
    }
    //Closing the database 
    mysqli_close($con);
}
?>

PHP MySQL: Check if value already exists

As per documentation, mysqli::query for successful SELECT queries returns a mysqli_result object. To extract an object or array from it you should call one of the fetch_* functions listed here. But you just need to check if there any record exists so it's preferrable to use $num_rows property. So your code would look like:

$checkUserID = $db -> query("SELECT shortlink FROM test WHERE `shortlink` = $shortlink");

if ($checkUserID === false) {
    die($db->error());
}

if ($checkUserID->num_rows) {
   header("Location: error.php?title=hovsa");
   exit(0);
}

Check if user already exist in MySql database in Node.js

Don't substitute a variable into the SQL, use a placeholder and a parameter array.

 const selectUsername = conn.query("SELECT username FROM user WHERE username= ?", [username], function (err, row){

How to Check if value exists in a MySQL database

preferred way, using MySQLi extension (supported from PHP 5 onwards):

$mysqli = new mysqli(SERVER, DBUSER, DBPASS, DATABASE);
$result = $mysqli->query("SELECT id FROM mytable WHERE city = 'c7'");
if($result->num_rows == 0) {
     // row not found, do stuff...
} else {
    // do other stuff...
}
$mysqli->close();

deprecated and not supported in PHP 7 or newer:

$result = mysql_query("SELECT id FROM mytable WHERE city = 'c7'");
if(mysql_num_rows($result) == 0) {
     // row not found, do stuff...
} else {
    // do other stuff...
}