MySQL: Select penult values - sql

Question

There are 2 tables:
table1:
id |phone| order|
---|-----|------|
1 | 122 | 6 |
2 | 122 | 4 |
3 | 122 | 3 |
4 | 123 | 6 |
5 | 123 | 5 |
6 | 123 | 3 |
7 | 124 | 6 |
8 | 124 | 5 |
9 | 125 | 6 |
10| 125 | 5 |
table2:
|phone |
|------|
|122 |
|123 |
|124 |
I have to select id and last order according next conditions:
If order not equals 3 take row with max id value for this phone
If order equals 3 take pre-max id for this phone
Id is in table2.
So result should be:
|phone | order|
|------ |------|
|122 | 4 |
|123 | 5 |
|124 | 5 |
MySQL version: Ver 15.1 Distrib 5.5.64-MariaDB

Answer 1

Basically you want to look at the last two records; if the last record has order 3, then use the previous one.
That would have been a simple query with window functions and/or lateral joins be your old MySQL version does not support these features. User variables are an option, as demonstrated by nbk, but they are tricky to use - and MySQL 8.0 annonced that this feature will be deprecated in a future version.
I am going to recommend correlated subqueries and a little logic:
select t2.id,
coalesce(
nullif((select ord from table1 t1 where t1.id = t2.id order by odering_id desc limit 1), 3),
(select ord from table1 t1 where t1.id = t2.id order by odering_id desc limit 1, 1)
) as ord
from table2 t2
The first subquery gets the latest value; nullif() checks the returned value and returns null if it has order 3; this indicate coalesce() that it should return the result of the second subquery, that gets the previous value.
order is a language keyword, so I used ord instead.
Demo in MySQL 5.5:
id | ord
--: | --:
122 | 4
123 | 5
124 | 5

Answer 2

Your mariadb version is a little old
Thta will use the row number sorted by the order column and it will select onl ythe second one.
the LIMIT in the subquery is needed,, because mariadb follows the standard and would not sort the subselect.
CREATE TABLE Table1
(`id` int, `order` int)
;
INSERT INTO Table1
(`id`, `order`)
VALUES
(122, 6),
(122, 4),
(122, 3),
(123, 6),
(123, 5),
(123, 3),
(124, 6),
(124, 5),
(125, 6),
(125, 5)
;
CREATE TABLE Table2
(`id` int)
;
INSERT INTO Table2
(`id`)
VALUES
(122),
(123),
(124)
;
SELECT id,`order`
FROM (SELECT
t1.`order`
, IF ( #id = t1.id ,#rn := #rn +1, #rn:= 1) AS rownum
, #id := t1.`id` as id
FROM Table1 t1 INNER JOIN Table2 t2 ON t1.id = t2.id,(SELECT #id := 0,#rn := 0) t3
ORDEr BY t1.id,t1.`order` DESC LIMIT 18446744073709551615) t4
WHERE rownum = 2
id | order
--: | ----:
122 | 4
123 | 5
124 | 5
db<>fiddle here