Sql query with three conditions - mysql

Question

I have a database with a table having content as below :
message_number message_type message_chat
0 IN Hi
1 OB Hello
2 IN Help
3 IN Want to find this thing
4 OB Sure
5 OB Please let me know
I have written 5 rows since i want to incorporate all possible cases that i want in my query in the example table that i showed.
Now in my query output, i want something like :
message_in message_out
Hi Hello
Help NULL
Want to find this string Sure
NULL Please let me know
So the cases that i want to consider are :
suppose if message_number=0 and message_number=1 both have message_type value as IN then put message_chat_in as message_chat(at message_number=0) and message_chat out as NULL and the iterate over message_number=1
if message_number =0 have message_type=IN and message_number =1 have message_type=OB, then show message_chat(at message_number=0) as message_chat_in and message_chat(at message_number=1) as message_out and dont iterate over message_number=1;
hope i have clarified the condition though i have included all three condition in the expected output.How should my sqlquery look like?
Edit : I am using mysql version 5.5.8

Answer 1

Try the following query
SELECT
q1.message_number in_num,
q1.message_chat in_chat,
q2.message_number out_num,
q2.message_chat out_chat
FROM
(
SELECT *,#i1:=IFNULL(#i1,0)+1 num
FROM Chat
ORDER BY message_number
) q1
LEFT JOIN
(
SELECT *,#i2:=IFNULL(#i2,0)+1 num
FROM Chat
ORDER BY message_number
) q2
ON q2.num=q1.num+1 AND q2.message_type<>q1.message_type
WHERE q1.message_type='IN'
UNION ALL
SELECT
q1.message_number in_num,
q1.message_chat in_chat,
q2.message_number out_num,
q2.message_chat out_chat
FROM
(
SELECT *,#i3:=IFNULL(#i3,0)+1 num
FROM Chat
ORDER BY message_number
) q1
RIGHT JOIN
(
SELECT *,#i4:=IFNULL(#i4,0)+1 num
FROM Chat
ORDER BY message_number
) q2
ON q2.num=q1.num+1 AND q2.message_type<>q1.message_type
WHERE q2.message_type='OB'
AND q1.message_type IS NULL
ORDER BY IFNULL(in_num,out_num)
SQL Fiddle - http://sqlfiddle.com/#!9/95a515/1
The second variant
SET #i1 = 0;
SET #i2 = 0;
SET #i3 = 0;
SET #i4 = 0;
-- the same query
SQL Fiddle - http://sqlfiddle.com/#!9/95a515/2
Or
SELECT 0,0,0,0 INTO #i1,#i2,#i3,#i4;
-- the same query
SQL Fiddle - http://sqlfiddle.com/#!9/95a515/5

Answer 2

why not using a analytic function here? I would do it with Lead() like this:
with inc as (
--Do the incorporation in this block. could be subquery too
--but its easier to read this way.
select
case when message_type = 'IN'
then message_chat
end as message_in
,case when LEAD(message_type) OVER (Order by message_number) = 'OB' --get the next message by number if it is type OB
then LEAD(message_chat) OVER (order by message_number)
end as message_out
from input
)
select *
from inc
where coalesce(message_in, message_out) is not null --filter out rows where with in & out is null

Answer 3

Ok, since there is no analytical functions in MySQL less than 8 the code may not be easy to follow:
with data_rn as
(
-- this isolate consecutive rows with the same message_type
select d1.*, count(d2.message_number) rn
from data d1
left join data d2 on d1.message_number > d2.message_number and d1.message_type != d2.message_type
group by d1.message_number
),
data_rn2 as
(
-- this marks the rows where new rows has to be added (i.e. when rn2 != 0)
select d1.*, count(d2.message_number) rn2
from data_rn d1
left join data_rn d2 on d1.rn = d2.rn and d1.message_type = d2.message_type and d1.message_number > d2.message_number
group by d1.message_number
),
data_added as
(
-- this add new rows
select message_number, message_type, message_chat
from data_rn2
union all
select message_number - 0.5, 'OB', NULL from data_rn2 where message_type = 'IN' and rn2 != 0
union all
select message_number - 0.5, 'IN', NULL from data_rn2 where message_type = 'OB' and rn2 != 0
order by message_number
), data_added_rn as
(
-- this compute new row numbering
select d1.*, ceil((count(d2.message_number)+1)/2) rn
from data_added d1
left join data_added d2 on d1.message_number > d2.message_number
group by d1.message_number
)
-- this will do the final formating
select max(case when message_type = 'IN' then message_chat end) message_in,
max(case when message_type = 'OB' then message_chat end) message_out
from data_added_rn
group by rn
demo
I have tried to comment each section appropriately.