Find missing values in an alphanumeric sequence - mysql

Question

I want to identify missing values in an alphanumeric sequence.
The table is defined as such:
CREATE TABLE `seqtest` (
`ID` int(11) NOT NULL,
`PoleNo` text,
`Pre` char(1) DEFAULT NULL,
`Num` int(3) DEFAULT NULL,
PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
The data is as shown below and will always be one letter (A-Z) followed by three numbers from 000 to 999.
| PoleNo | Pre | Num |
|------------------------|
| A000 | A | 000 |
| A001 | A | 001 |
| A002 | A | 002 |
| A004 | A | 003 |
| **** | * | *** |
| A998 | A | 998 |
| A999 | A | 999 |
| B000 | B | 000 |
| B001 | B | 001 |
| B002 | B | 002 |
| **** | * | *** |
| B998 | B | 998 |
| B999 | B | 999 |
| C000 | C | 000 |
| C001 | C | 001 |
| C005 | C | 005 |
| C006 | C | 006 |
|------------------------|
I want the query to find that, for example, C002, C003 AND C004 are missing as shown below.
| Pre | start | stop |
| C | 2 | 4 |
|----------------------|
Im using the following:
SELECT l.Pre, l.Num + 1 as start, min(fr.Num) - 1 as stop
FROM seqtest as l
LEFT OUTER JOIN seqtest as r ON l.Num = r.Num - 1 AND l.Pre = r.Pre
LEFT OUTER JOIN seqtest as fr ON l.Num < fr.Num AND l.Pre = fr.Pre
WHERE r.Num is null AND l.Num < 999
GROUP BY l.Pre, l.Num, r.Num
which is based on this.
It gives me the range that is missing and works well except for one case...when 'Pre' changes from one letter to the next.
IE With the following data:
| PoleNo | Pre | Num |
|------------------------|
| B995 | B | 995 |
| B996 | B | 996 |
| B997 | B | 997 |
| C003 | C | 003 |
| C004 | C | 004 |
| C005 | C | 005 |
| C006 | C | 006 |
|------------------------|
Id like to be able to return this:
| Pre | start | stop |
| B | 998 | 999 |
| C | 0 | 2 |
|----------------------|
Is this possible? Im using the Pre and Num fields which are simply the PoleNo field broken up...but if anyone sees a way to do it just using the PoleNo field, that would work as well.

Answer 1

This is much easier in MySQL 8+, because you have lead(). But, you can do what you want as:
select st.pre,
(st.num + 1) as start,
(st.next_num - 1) as stop
from (select st.*,
(select st2.num
from seqtest st2
where st2.pre = st.pre and
st2.num > st.num
order by st2.num asc
limit 1
) as next_num
from seqtest st
) st
where next_num <> num + 1;
EDIT:
This gets the ranges at the beginning and end as well:
select st.pre,
(st.num + 1) as start,
(st.next_num - 1) as stop
from (select st.pre, num,
coalesce( (select st2.num
from seqtest st2
where st2.pre = st.pre and
st2.num > st.num
order by st2.num asc
limit 1), 1000
) as next_num
from seqtest st
union
select st.pre, 0 as num, min(st.num) as next_num
from seqtest st
group by st.pre
) st
where next_num <> num + 1
order by pre, start;
Here is a db<>fiddle.