Identify change in job code from a mySQL table - php

Question

I am working with a very large table in MySQL containing employee information collected over the last four years. I want to know if someone's job code changed between that time, and if so, push their data into an array for json encoding in php.
An example of the data looks like this:
Year Emp ID Job Code
2015 1234 X908
2014 1234 X908
2013 1234 X908
**2015 5421 Y444**
2014 5421 Z900
2013 5421 Z900
For employee 1234 there has been no job change between 2013-2015; I would however like to catch employee 5421 whose job code changed between 2014 and 2015.
So far, I have written a script in PHP without good results.
$query = mysqli_query("SELECT year, emp_id, job_code from big_table ");
$rows = array();
while ($r = mysql_fetch_assoc($query)) {
if ($r['emp_id'] == $r['emp_id'] and $r['job_code'] != $r['job_code']) {
$rows[] = $r;
}
echo json_encode($rows);
Thank you in advance for your help!

Answer 1

You can get the required data in the SQL query without having to pull all user records into PHP. This will be significantly more efficient than processing the data in PHP.
You can get the job code count for each user:
SELECT emp_id, COUNT(DISTINCT job_code) AS diffjobs FROM big_table GROUP BY emp_id
Then you can conditionally get users with more than 1 distinct job code by wrapping it in a subquery:
SELECT emp_id, diffjobs FROM
(SELECT emp_id, COUNT(DISTINCT job_code) AS diffjobs FROM big_table GROUP BY emp_id) d
WHERE diffjobs > 1

Answer 2

This loop should work:
$rows = array();
$jobChanges = array();
while ($r = mysql_fetch_assoc($query)) {
if (!isset($rows[$r['emp_id']])){
$rows[$r['emp_id']] = $r;
} elseif ($rows[$r['emp_id']]['job_code'] != $r['job_code']) {
// Handle differing job_code...
if (!isset($jobChanges[$r['emp_id'])) {
$jobChanges[$r[emp_id]] = array();
}
// Keep track of all differing job codes per emp_id
// I am not going to go any further than that...
$jobChanges[$r[emp_id]][] = $r;
}
}

Answer 3

You can get the desired employees whose job has changed in between years using GROUP BY and HAVING clause, like this:
$query = mysqli_query($link, "SELECT emp_id from big_table GROUP BY emp_id HAVING COUNT(DISTINCT job_code) > 1");
$rows = array();
while ($r = mysqli_fetch_assoc($query)){
$rows[] = $r;
}
echo json_encode($rows);
Caution: Don't mix mysqli and mysql database extensions in your code.

Answer 4

This can be done with a JOIN query in MySQL, so that you only return the rows that have changed from the previous year -
SELECT
a.year, a.emp_id, a.job_code,
b.year prev_year, b.job_code prev_job_code
FROM big_table a
JOIN big_table b
ON a.emp_id = b.emp_id
AND b.year = a.year-1
WHERE a.job_code != b.job_code
see this SQLFiddle - http://sqlfiddle.com/#!9/95e680/10
so your code could be simplified to -
$query = mysqli_query("SELECT a.year, a.emp_id, a.job_code, b.year prev_year, b.job_code prev_job_code FROM big_table a JOIN big_table b ON a.emp_id = b.emp_id AND b.year = a.year-1 WHERE a.job_code != b.job_code ");
$rows = array();
while ($r = mysqli_fetch_assoc($query)) {
$rows[] = $r;
}
echo json_encode($rows);
note - you had mysqli_query(), but mysql_fetch_assoc(), so I updated to mysqli_fetch_assoc()
If you still wanted to do this in php, you need to save the last row value to a temporary array, and then check if the job_code has changed.
it would look something like this -
$query = mysqli_query("SELECT year, emp_id, job_code from big_table ORDER BY emp_id, year");
$temp = array();
$rows = array();
while ($r = mysqli_fetch_assoc($query)) {
if(!isset($temp[$r['emp_id']]){
$temp[$r['emp_id']] = $r; // add this row to the temp array
}
else {
if ($r['job_code'] != $temp[$r['emp_id']]['job_code']) {
// add the previous values for comparison
$r['prev_year'] = $temp[$r['emp_id']]['year'];
$r['prev_job_code'] = $temp[$r['emp_id']]['job_code'];
// add this row
$rows[] = $r; // add this row
// replace the last temp array with this array
$temp[$r['emp_id']] = $r;
}
}
echo json_encode($rows);
note - I changed the query by adding an ORDER BY -> ORDER BY emp_id, year