Mysql GROUP BY and Union based on date condition - php

Question

Here is my query - it mostly works, but I can see it failing on one condition - explained after the query:
$firstDay = '2020-03-01' ;
$lastDay = '2020-03-31' ;
SELECT * FROM clubEventsCal
WHERE ceFreq!=1
AND (ceDate>='$firstDay' AND ceDate<='$lastDay')
UNION SELECT * FROM clubEventsCal
WHERE ceFreq=1
AND (ceDate>='$firstDay' AND ceDate<='$lastDay')
GROUP BY ceStopDate ORDER BY ceID,ceDate ;
The first select gives me all Event records between the two dates. The second select gives me grouped/summarized Event records between the two dates. The problem though is if the value ceDate spans days across two months: IE: 2020-03-30 thru 2020-04-02. When I pull the records for March, all is good - the above query pulls the 2020-03-30 record (grouped) as the first instance of the 4 days/records - allowing us to charge for a single 4 day event. But when I pull the records for April its also going to pull 2020-04-01 as a new grouped Event record for the last two days of the 4 day event and try to charge the customer for a new Event - when in fact those two days were already a part of March's bill.
How can I write the query so that when ceDate starts in Month X but ends in Month Y that when records are pulled for Month Y its not trying to pull records that actually belong to an Event that started in Month X?
Examples of an Event record would look like this:
rid | ceID | ceActive | ceFreq | ceDate | ceStopDate
------------------------------------------------
1 1108 1 3 2020-03-09 | 2020-03-09
2 1111 1 2 2020-03-15 | 2020-03-15
3 1112 1 2 2020-03-17 | 2020-03-17
4 1117 1 1 2020-03-30 | 2020-04-02
5 1117 1 1 2020-03-31 | 2020-04-02
6 1106 1 3 2020-03-21 | 2020-03-21
7 1110 1 2 2020-03-05 | 2020-03-05
8 1113 1 2 2020-03-24 | 2020-03-24
9 1117 1 1 2020-04-01 | 2020-04-02
10 1117 1 1 2020-04-02 | 2020-04-02
The above query pulls all records where ceFreq != 1, and it pulls a single record for the ceFreq = 1 records (rids: 4 & 5). For March, we don't necessarily care that ceID 1117 spills into April. But when we pull records for April - we need to exclude rid 9 & 10, because the Event (ceID=1117), was already accounted for in March.

Answer 1

SELECT * FROM clubEventsCal
...
GROUP BY ceStopDate
This is gibberish.
MySQL (depending on configuration) allows it without choking - but it's semantically wrong and stands out as an anti-pattern.
There are some edge cases where the values returned might contain significant data, but they very unusual. Trying to explain a problem with code which does not work is perhaps not a good strategy.
Looking at your code, its possible that you don't need a union - but there's not enough information in your example records to say if this would actually give the result you expect (it will be significantly faster depending on your indexes):
SELECT IF(cefreq=1, rid, null) AS consolidator
, ceid
, cefreq
, MIN(cedate), MAX(cedate)
, ceStopDate
FROM clubEventsCal
WHERE cID=1001
AND ceActive!=2
AND (ceDate>='$firstDay' AND ceDate<='$lastDay')
GROUP BY IF(cefreq=1, rid, null)
, ceid
, cefreq
, ceStopDate
;
I would have added the ORDER BY - but I don't know where clId came from. Also This will give different resuts to what I think you were trying to achieve for any record where cefreq is null (if you really do want to exclude them, add a predicate in the WHERE clause).