How to use UNION in mysql properly

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This is the error that I got when I executed it
Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result, bool given in C:\xampp\htdocs\Clinic\Staff\chart.php on line 172

This is my code:

<?PHP
$connect = mysqli_connect("localhost", "root", "", "testingfolder"); 
    $query3 = "SELECT consultationstatus_diagnosis, count(*) as number FROM tbl_consultationstatus WHERE month(consultationstatus_date)=1 GROUP BY consultationstatus_diagnosis ORDER BY count(*) DESC LIMIT 1 UNION ALL SELECT consultationstatus_diagnosis, count(*) as number FROM tbl_consultationstatus WHERE month(consultationstatus_date)=2 GROUP BY consultationstatus_diagnosis ORDER BY count(*) DESC LIMIT 1;";
?>

What I have tried:

PS: I'm doing this because I'm going to graph the data in a line graph.
I need to gather the highest cases per month for forecasting reasons. I'd be grateful if you could assist me with my difficulty.

Posted 11-Nov-21 2:32am

Mark Limuel Fernando

Updated 11-Nov-21 2:55am

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Richard MacCutchan · Answer 1 · 2021-11-11T02:35:00

Solution 1

The error message is telling you that the previous command failed, as explained at PHP: mysql_query - Manual[^].

Posted 11-Nov-21 2:35am

Richard MacCutchan

Comments

Mark Limuel Fernando 11-Nov-21 8:41am

when I didn't add the union it runs perfectly any idea how to combine two MySql Executions?

snorkie · Answer 2 · 2021-11-11T02:55:00

Try taking out the Group By in the top sql Unions don't like that. Group at the end when all the data is collected. If it still breaks, try removing the LIMIT 1 to test.

SQL

SELECT consultationstatus_diagnosis, count(*) as number 
FROM tbl_consultationstatus 
WHERE month(consultationstatus_date)=1 
ORDER BY count(*) DESC 
LIMIT 1 
UNION ALL 
SELECT consultationstatus_diagnosis, count(*) as number 
FROM tbl_consultationstatus 
WHERE month(consultationstatus_date)=2 
GROUP BY consultationstatus_diagnosis 
ORDER BY count(*) DESC 
LIMIT 1;