How to select from relational database? - php

Question

I am starting to learn how databases work and I have always had all my data in one table. I am looking over an open-source project which I would like to create an interface for and the database structure is too complicated for me to understand.
I am sure it has a relational database structure (if I have learned properly). This is what the database structure looks like:
-- Table structure for table `databasechangelog`
--
CREATE TABLE IF NOT EXISTS `databasechangelog` (
`ID` varchar(255) NOT NULL,
`AUTHOR` varchar(255) NOT NULL,
`FILENAME` varchar(255) NOT NULL,
`DATEEXECUTED` datetime NOT NULL,
`ORDEREXECUTED` int(11) NOT NULL,
`EXECTYPE` varchar(10) NOT NULL,
`MD5SUM` varchar(35) DEFAULT NULL,
`DESCRIPTION` varchar(255) DEFAULT NULL,
`COMMENTS` varchar(255) DEFAULT NULL,
`TAG` varchar(255) DEFAULT NULL,
`LIQUIBASE` varchar(20) DEFAULT NULL,
`CONTEXTS` varchar(255) DEFAULT NULL,
`LABELS` varchar(255) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
-- --------------------------------------------------------
-- Table structure for table `databasechangeloglock`
--
CREATE TABLE IF NOT EXISTS `databasechangeloglock` (
`ID` int(11) NOT NULL,
`LOCKED` bit(1) NOT NULL,
`LOCKGRANTED` datetime DEFAULT NULL,
`LOCKEDBY` varchar(255) DEFAULT NULL,
PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
-- --------------------------------------------------------
-- Table structure for table `devices`
--
CREATE TABLE IF NOT EXISTS `devices` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(128) NOT NULL,
`uniqueid` varchar(128) NOT NULL,
`status` varchar(128) DEFAULT NULL,
`lastupdate` timestamp NULL DEFAULT NULL,
`positionid` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `uk_device_uniqueid` (`uniqueid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=2 ;
-- --------------------------------------------------------
-- Table structure for table `positions`
--
CREATE TABLE IF NOT EXISTS `positions` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`protocol` varchar(128) DEFAULT NULL,
`deviceid` int(11) NOT NULL,
`servertime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`devicetime` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`fixtime` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`valid` bit(1) NOT NULL,
`latitude` double NOT NULL,
`longitude` double NOT NULL,
`altitude` float NOT NULL,
`speed` float NOT NULL,
`course` float NOT NULL,
`address` varchar(512) DEFAULT NULL,
`attributes` varchar(4096) NOT NULL,
PRIMARY KEY (`id`),
KEY `position_deviceid_fixtime` (`deviceid`,`fixtime`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=145 ;
-- --------------------------------------------------------
-- Table structure for table `server`
--
CREATE TABLE IF NOT EXISTS `server` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`registration` bit(1) NOT NULL DEFAULT b'1',
`latitude` double NOT NULL DEFAULT '0',
`longitude` double NOT NULL DEFAULT '0',
`zoom` int(11) NOT NULL DEFAULT '0',
`map` varchar(128) DEFAULT NULL,
`language` varchar(128) DEFAULT NULL,
`distanceunit` varchar(128) DEFAULT NULL,
`speedunit` varchar(128) DEFAULT NULL,
`bingkey` varchar(128) DEFAULT NULL,
`mapurl` varchar(128) DEFAULT NULL,
`readonly` bit(1) NOT NULL DEFAULT b'0',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=2 ;
-- --------------------------------------------------------
-- Table structure for table `users`
--
CREATE TABLE IF NOT EXISTS `users` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(128) NOT NULL,
`email` varchar(128) NOT NULL,
`hashedpassword` varchar(128) NOT NULL,
`salt` varchar(128) NOT NULL,
`readonly` bit(1) NOT NULL DEFAULT b'0',
`admin` bit(1) NOT NULL DEFAULT b'0',
`map` varchar(128) NOT NULL DEFAULT 'osm',
`language` varchar(128) NOT NULL DEFAULT 'en',
`distanceunit` varchar(128) NOT NULL DEFAULT 'km',
`speedunit` varchar(128) NOT NULL DEFAULT 'kmh',
`latitude` double NOT NULL DEFAULT '0',
`longitude` double NOT NULL DEFAULT '0',
`zoom` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`),
UNIQUE KEY `uk_user_email` (`email`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=3 ;
-- --------------------------------------------------------
-- Table structure for table `user_device`
--
CREATE TABLE IF NOT EXISTS `user_device` (
`userid` int(11) NOT NULL,
`deviceid` int(11) NOT NULL,
KEY `fk_user_device_deviceid` (`deviceid`),
KEY `user_device_user_id` (`userid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
--
-- Constraints for dumped tables
--
--
-- Constraints for table `positions`
--
ALTER TABLE `positions`
ADD CONSTRAINT `fk_position_deviceid` FOREIGN KEY (`deviceid`) REFERENCES `devices` (`id`) ON DELETE CASCADE;
--
-- Constraints for table `user_device`
--
ALTER TABLE `user_device`
ADD CONSTRAINT `fk_user_device_deviceid` FOREIGN KEY (`deviceid`) REFERENCES `devices` (`id`) ON DELETE CASCADE,
ADD CONSTRAINT `fk_user_device_userid` FOREIGN KEY (`userid`) REFERENCES `users` (`id`) ON DELETE CASCADE;
When a user registers, it adds an entry to the "users" table and the user gets an id.
Then when the user adds a device, the user_device table looks like this:
And the devices table looks like this:
This is a GPS tracking software, so each device then posts it's position in the "positions" table which looks like this:
So, if a user logs in, how do I query the positions table and get all info from all devices of that user? I am assuming I would need to join the tables but I don't know how to do that. Any help would be greatly appreciated.

Answer 1

Let's build it incrementally. First, what is the actual data you want?
query the positions table
Ok, start with that:
SELECT
*
FROM
positions
Now how do you want to further filter that data?
from all devices of that user
Ok, so we need to relate this table (positions) through devices and to the user. So ultimately we'll need a WHERE clause like this:
WHERE
userid = ?
But positions doesn't have a userid, so let's walk through joining some tables until we have one. positions does have a deviceid, so we can join that:
SELECT
*
FROM
positions
INNER JOIN devices
ON positions.deviceid = devices.id
We still don't have a userid, though. And devices itself doesn't reference anything to further help us. But there's a table which does reference devices, user_device. Let's join that:
SELECT
*
FROM
positions
INNER JOIN devices
ON positions.deviceid = devices.id
INNER JOIN user_device
ON devices.id = user_device.deviceid
You'll see that we're getting a lot of data now. But you'll also see that we're now getting a userid column, so we can finally add our WHERE clause:
SELECT
*
FROM
positions
INNER JOIN devices
ON positions.deviceid = devices.id
INNER JOIN user_device
ON devices.id = user_device.deviceid
WHERE
userid = ?
At this point we should be getting all of the unique positions from any device related to that user. You can get just the position data by replacing SELECT * with SELECT positions.*, or you can explicitly define which columns from which tables you want to select.

Answer 2

it would appear from your notes that you know when the user logs in which device he has. if this is correct, you should not need a join. Just do a select on the positions table where the positions.deviceid = the login device.