检查数据是否已存在于MySQL中

Question

I'm trying to do a checking data condition. If data already exists, it will not inserted. Otherwise it will inserted. But the problem is data still get inserted although it already exists !

<?php 
    if($_SERVER['REQUEST_METHOD']=='POST'){

        //Getting values
        $project = strtoupper($_POST['project']);
        if($project != null)
        {
            //Importing our db connection script
            require_once('dbConnect.php');
            $sql="SELECT * FROM Project WHERE project='$project'";
            $check=mysqli_fetch_array(mysqli_query($con,sql));
            if(isset($check))
            {
                // no need insert
            }
            else{
                //Creating an sql query
        $sql = "INSERT INTO Project(project) VALUES ('$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);
    }
?>

Answer 1

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);
}
?>

Answer 2

Correct this line $check=mysqli_fetch_array(mysqli_query($con,sql)); , you missed $ before sql . That's why condition is evaluating to false.

Answer 3

执行sql应该放入“需要插入”其他{}。