PHP encode a partial set of MySQL data into JSON - php

Question

I know how to encode all the data from an SQL query into JSON using PHP, but don't know How to encode a partial set.
In Android (the client), I send an HTTP request which goes something like this:
public interface DataAPI {
#GET("/top500")
public void getResult(#Query("start") int start, #Query("count") int count,
Callback<Top500> response);
}
As you can see, I am using start and count in Android code.
I have around 500 records in my table and I have successfully encoded all the data into JSON, using below PHP Script:
<?php
$con = mysqli_connect(HOST,USER,PASS,DB);
$sql = "select * from Persons";
$res = mysqli_query($con,$sql);
$result = array();
while($row = mysqli_fetch_array($res)){
array_push($result,
array(
'id'=>$row[0],
'name'=>$row[1]
));
}
echo json_encode(array("result"=>$result));
mysqli_close($con);
?>
JSON Result:
{"result": [{ ... }]}
But I need to encode mysql data 20 by 20 like this:
{"count": 20, "start": 0, "total": 500, "result": [{ ...}]}
What's missing is how to use the start and count parameters to get a partial set in order to get slices of the data.

Answer 1

You need to use LIMIT and OFFSET in your query. Applying some nasty logic in PHP is a bad solution. This is an operation that MySQL should do instead because you don't want to fetch all the rows if you don't need all of them.
If you run a query like this:
SELECT *
FROM Persons
ORDER BY Id
LIMIT 10 OFFSET 20
you will get a subset of the matching rows starting from the 20th and long 10 rows. You can then loop over the full results set.
Better to explicitly order by a field to ensure consistency across different pages.
You're final code (using PDO rather than mysqli):
$pdo = new PDO('mysql:dbname=db;host=127.0.0.1', $user, $password);
$count = $_GET['count'];
$start = $_GET['start'];
$stmt = $pdo->prepare('SELECT * FROM Persons ORDER BY Id LIMIT ? OFFSET ?');
$stmt->execute(array($count, $start));
$rows = $stmt->fetchAll();
foreach ($rows as $row) {
// your logic that was inside the while
}

Answer 2

Use below code.
$con = mysqli_connect(HOST,USER,PASS,DB);
$count = "select count(*) as total from Person"; //query to get row count of your table
$count_res = mysqli_query($con,$count);
$count_row = mysqli_fetch_array($count_res);
$total = $count_row['total']; // get total no of records
$start = YOUR_START_VALUE; // from which offset you want to get record
$end = YOUR_VALUE // how many record want to fetch
$sql = "select * from Persons limit $start, $end";
$res = mysqli_query($con,$sql);
$result = array();
while($row = mysqli_fetch_array($res)){
array_push($result,
array(
'id'=>$row[0],
'name'=>$row[1]
));
}
echo json_encode(array("count"=> $end, "start"=> $start, "total"=> $total,"result"=>$result));
mysqli_close($con);
?>

Answer 3

How about something like:
$start = 7;
$size = 3;
$count = 0;
while($row = mysqli_fetch_array($res)){
if($count>=$start && $count <$start+$size) {
array_push($result,array('id'=>$row[0],'name'=>$row[1]));
}
$count++;
}
The code should be self explanatory, but if you have any questions, feel free to comment.
Note: This is more of a PHP pseudo code, as I don't have an environment and haven't coded in PHP in a while.

Answer 4

I can think of 2 ways to solve this problem.
The first is using just MySQL:
SELECT SQL_CALC_FOUND_ROWS * FROM Persons LIMIT 0,20;
SELECT FOUND_ROWS();
With the first query you retrieve the batch of rows you want. With the second you get the total row count in the result set.
The second option is to use array_slice:
$total = count($result);
$batch = array_slice($result, 0, 20);
echo json_encode(array("total" => $total, "start" => 0, "count" => count($batch), "result"=>$batch));