MySQL Multiple Table Update Statement - php

Question

I am working on a study planner. The SQL query below is supposed to run when a logged in user tries to save a study plan from the welcome page. However, the code does not do anythin. The input values just disappear upon submission with no initialisation of the $result variable, no flag changes and no any error print-outs when I run the application via NetBeans...
// Set up a flag for monitoring the individual queries
$flag = true;
$query = "UPDATE
`modulecodes`
LEFT JOIN `moduletitles` ON `moduletitles`.`modulecodeid` = `modulecodes`.`modulecodeid`
LEFT JOIN `studyplans` ON `studyplans`.`modulecodeid` = `modulecodes`.`modulecodeid`
LEFT JOIN `comments` ON `comments`.`modulecodeid` = `modulecodes`.`modulecodeid`
SET
modulecode = '$moduleCode', moduletitle = '$moduleTitle', studydate = '$moduleStudyDate', numberofstudyhours = '$moduleAllocatedHours', comment = '$studyPlanComments'
WHERE userid = '$userid';";
// Execute the query and put the result in the variable $result
$result = mysqli_query($mysqli, $query);
if (!$result) {
$flag = false;
echo $flag;
echo "Error details for Result: " . mysqli_error($mysqli) . ".";
}
...And with hard-coded values (as per below) and direct test in phpMyAdmin, it runs but comes back with 0 rows affected. (Query took 0.0069 seconds.) and no changes in the database, whatsoever. So, I feel like I haven’t structured it well. Can someone kindly guide me on how to convert the query into one with a subquery or something more efficient and reliable, please or at the least, help point out what I am doing wrong?
I'd suspect other parts of the code but the fact that a direct test in phpMyAdmin tells me, the issue is with the SQL statement, besides I have already used other parts of the code elsewhere and with another query and it runs fine.
UPDATE
`modulecodes`
LEFT JOIN `moduletitles` ON `moduletitles`.`modulecodeid` = `modulecodes`.`modulecodeid`
LEFT JOIN `studyplans` ON `studyplans`.`modulecodeid` = `modulecodes`.`modulecodeid`
LEFT JOIN `comments` ON `comments`.`modulecodeid` = `modulecodes`.`modulecodeid`
SET
modulecode = 'P00100', moduletitle = 'Java', studydate = '2017/04/10', numberofstudyhours = '1', comment = 'Java'
WHERE userid = 20;
Database Code:
CREATE TABLE `comments` (
`commentid` int(10) NOT NULL,
`modulecodeid` int(10) NOT NULL,
`comment` text,
`created` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
-- --------------------------------------------------------
--
-- Table structure for table `modulecodes`
--
CREATE TABLE `modulecodes` (
`modulecodeid` int(10) NOT NULL,
`userid` int(10) NOT NULL,
`modulecode` varchar(10) NOT NULL,
`created` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
-- --------------------------------------------------------
--
-- Table structure for table `moduletitles`
--
CREATE TABLE `moduletitles` (
`moduletitleid` int(10) NOT NULL,
`modulecodeid` int(10) NOT NULL,
`moduletitle` varchar(100) NOT NULL,
`created` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
-- --------------------------------------------------------
--
-- Table structure for table `studyplans`
--
CREATE TABLE `studyplans` (
`studyplan` int(10) NOT NULL,
`modulecodeid` int(10) NOT NULL,
`studydate` date NOT NULL,
`numberofstudyhours` int(10) NOT NULL,
`created` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
-- --------------------------------------------------------
--
-- Table structure for table `users`
--
CREATE TABLE `users` (
`userid` int(10) NOT NULL,
`firstname` varchar(100) NOT NULL,
`lastname` varchar(100) NOT NULL,
`created` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Answer 1

To test the relationships, see if this returns the rows you expect to be updated:
SELECT
a.`modulecode`,
b.`moduletitle`,
c.`studydate`,
c.`numberofstudyhours`,
d.`comment`
FROM `modulecodes` a
LEFT JOIN `moduletitles` b ON b.`modulecodeid` = a.`modulecodeid`
LEFT JOIN `studyplans` c ON c.`modulecodeid` = a.`modulecodeid`
LEFT JOIN `comments` d ON d.`modulecodeid` = a.`modulecodeid`
WHERE `userid` = '$userid';
If any of the columns are NULL (joined table row not found), then the update will fail.