Multiple row retrieval with their latest inserted value using MySQL - mysql

Question

I have two tables, first one is 'file_details':
+---------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+-------------+------+-----+---------+-------+
| file_name | varchar(40) | YES | | NULL | |
| creation_date | date | YES | | NULL | |
+---------------+-------------+------+-----+---------+-------+
and second one is 'logs':
+-----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| sl_no | varchar(20) | YES | | NULL | |
| file_name | varchar(40) | YES | | NULL | |
| status | varchar(100) | YES | | NULL | |
+-----------+--------------+------+-----+---------+-------+
values in the tables are:
file_details:
+-----------+---------------+
| file_name | creation_date |
+-----------+---------------+
| a1 | 2020-01-09 |
| a2 | 2020-01-08 |
+-----------+---------------+
logs:
+-------+-----------+---------+
| sl_no | file_name | status |
+-------+-----------+---------+
| 1 | a1 | created |
| 2 | a1 | step1 |
| 1 | a2 | created |
| 2 | a2 | step1 |
| 3 | a2 | step2 |
+-------+-----------+---------+
now I want to retrieve the following data:
+-----------+---------------+--------+
| file_name | creation_date | status |
+-----------+---------------+--------+
| a1 | 2020-01-09 | step1 |
| a2 | 2020-01-08 | step2 |
+-----------+---------------+--------+
using the below query:
select f.file_name,f.creation_date,
l.status
from file_details f
inner join logs l on f.file_name=l.file_name
and l.status=(select status
from logs
where sl_no=(
select max(convert(sl_no,unsigned))
from logs));
But the above query gives me the below output:
+-----------+---------------+--------+
| file_name | creation_date | status |
+-----------+---------------+--------+
| a2 | 2020-01-08 | step2 |
+-----------+---------------+--------+
which is not a required solution. So, please help me out.

Answer 1

So there is a couple things to discuss here, you mentioned in the comments that you are new to SQL, so I will provide some links to look at, first off being normalization, this is used to reduce data redundancy (which you have with your status descriptions).
Also what you are trying to do is essentially make the engine "guess" what status is the most up to date one, using the MAX like you have will only deal with alphabetical orders and as such is not scale-able for if you say want to add a status such as "completed", so what you would have to do is hard code the order in something like a case statement which gets really messy with multiple conditions.
And lastly here is a tutorial site on SELECT query basics with links to other data manipulation commands.
So the answer I came up with, I made a status table to store the description and then in the log table I store the status_id, doing this addresses the normalization issue I mentioned earlier. Creating this table also allows me to assign the statuses a rank to order with, which is another issue I discussed earlier.
SELECT t.file_name,
t.creation_date,
s.description
FROM status_details s
JOIN (SELECT f.file_name,
f.creation_date,
MAX(s2.rank_no) rank_no
FROM file_details f
JOIN logs l
ON l.file_name = f.file_name
JOIN status_details s2
ON s2.status_id = l.status_id
GROUP BY f.file_name,
f.creation_date) t
ON t.rank_no = s.rank_no
Now I don't want you so blindly copy this query without understanding what it is doing, so the general gist is that the inner select gets the file names and creation dates with the rank number of the status, note this only gets the status with the highest rank number, then the outer select takes the data already retrieved and joins back onto the status table to grab the status description from the rank number. Giving the output
file_name creation_date description
a1 2020-01-09 step1
a2 2020-01-08 step2
If you would like to see the query working I have created a fiddle for you to try.
These are the data scripts I used to create the environment:
create table file_details( file_name varchar(40), creation_date date)
create table logs (sl_no varchar(20), file_name varchar(40), status_id int)
create table status_details (status_id int, description varchar(100), rank_no int)
insert into file_details values ('a1', '2020-01-09')
insert into file_details values ('a2', '2020-01-08')
insert into status_details values (1, 'created', 1)
insert into status_details values (2, 'step1', 2)
insert into status_details values (3, 'step2', 3)
insert into logs values ('1', 'a1' , 1)
insert into logs values ('2', 'a1' , 2)
insert into logs values ('1', 'a2' , 1)
insert into logs values ('1', 'a2' , 2)
insert into logs values ('3', 'a2' , 3)

Answer 2

max(convert(sl_no,unsigned)) from logs) will return 3 in your example and therefore it only matches
+-------+-----------+---------+
| sl_no | file_name | status |
+-------+-----------+---------+
| 3 | a2 | step2 |
+-------+-----------+---------+