else mysql query to take up the first 20 dates - php

Question

I have a sql query which has an if..else function. if the submit button is clicked, it will perform the particular query. else, the query will take in the first 20 dates in my database. however, i am not sure how to do the else query statement to take up only the first 20 dates. do help, thank you :)
<?php
$server = "localhost";
$user = "root";
$password = "";
$database = "sensors_database";
$connection = mysql_connect($server,$user,$password);
$db = mysql_select_db($database,$connection);
if(isset($_POST["usub"])){
$date1 = $_POST["datepicker"];
$date2 = $_POST["datepicker1"];
$query1 = "SELECT time, Ultrasonic FROM pi_sensors_network WHERE date BETWEEN '".$date1."' AND '".$date2."'";
$result1 = mysql_query($query1);
while($row = mysql_fetch_assoc($result1))
{
$dataset1[] = array(strtotime($row['time'])*1000,$row['Ultrasonic']);
//echo strtotime($row['time'])*1000;
}
}
else {
}
?>

Answer 1

Here you go:
SELECT `time`, `Ultrasonic` FROM `pi_sensors_network`
WHERE 1=1 ORDER BY `date` ASC LIMIT 0,20

Answer 2

If you are asking between the dates you mentioned in the query, then use
limit
$query1 = "SELECT time, Ultrasonic FROM pi_sensors_network WHERE date BETWEEN '".$date1."' AND '".$date2."' limit 0,20";

Answer 3

First Create index on table < pi_sensors_network > on column < date >
CREATE INDEX pi_sensors_network01 ON pi_sensors_network (`date`);
Then,
SELECT
`time`,`Ultrasonic`
FROM
pi_sensors_network
WHERE
`date` IN ( SELECT `date` FROM
(SELECT DISTINCT(`date`)
FROM pi_sensors_network
ORDER BY `date` ASC LIMIT 0,20)
AS tmp ) ;
If your table is too large, This query may take long even after indexing,
In this case,
I'ld suggest you to make an array of first 20 dates by separate query and then implode it in your main query !