how to group two specific items together - mysql

Question

I have a database with questions with columns Question, Answer, Type.
Currently, this is the sql statement I am running:
SELECT Question, Answer, Type FROM goodquestions ORDER BY RAND() LIMIT 0,20
As you can see, I select random values from the table and I would like it to be that way. However, when type is 12 I would like to access the table row prior to that entry and print them out in conjunction
Like this
RANDOM
RANDOM
RANDOM
Question before 12 type
12 type question
RANDOM
RANDOM
RANDOM
It can also be like this:
Question before 12 type
12 type question
RANDOM
RANDOM
RANDOM
RANDOM
RANDOM
RANDOM
I just need them to be together and I am unable to do this right now.

Answer 1

I guess I see what you want.
Please try this query, I changed limit 1,20 to limit 1, 10:
select #next_line_id:=0, #next_line_type:=0;
select g.*, tt.is_property
from
(select * from
(select *,
rand() as rand_val,
case
when #next_line_type= 12 or Type = 12 then 1
else 0
end is_property,
#next_line_id as next_line_id,
#next_line_id:=id as current_id,
#next_line_type:=Type as current_type
from goodquestions order by id desc
) t
where t.Type <> 12
order by rand_val limit 0,10) tt
join goodquestions g on g.id = tt.id or (g.id = tt.next_line_id and tt.is_property = 1 and tt.Type <> 12)
group by g.id, g.Question, g.Answer, g.Type, tt.is_property
order by is_property desc, id
limit 0, 10;
The following are the query of creating test table:
create table goodquestions (
id int unsigned auto_increment primary key,
Question varchar(255) not null,
Answer varchar(255) not null,
Type int unsigned,
index idx_type (Type)
) engine=innodb DEFAULT CHARSET=latin1;
insert into goodquestions (Question, Answer, Type)
values ('q1', 'a1', 1),
('q2', 'a2', 2),
('q3', 'a3', 3),
('q4', 'a4', 4),
('q5', 'a5', 5),
('q6', 'a6', 6),
('q7', 'a7', 7),
('q8', 'a8', 8),
('q9', 'a9', 9),
('q10', 'a10', 10),
('q11', 'a11', 11),
('q12', 'a12', 12),
('q13', 'a13', 13),
('q14', 'a14', 14),
('q15', 'a15', 15),
('q16', 'a16', 16),
('q17', 'a17', 17),
('q18', 'a18', 18);
Please note, using rand() function may have a bad performance for a large
table. If there are performance issue, I could provide another solution for better performance.
The following query which result list must have and only have one record of type 12:
select #total_type_12:=(select count(*) from goodquestions where Type=12);
select #random_type_12:=(floor(rand()*#total_type_12) + 1) * 2;
select #next_line_id:=0, #next_line_type:=0, #is_property:=0;
select g.*, tt.is_property
from
(select * from
(select *,
case
when (#next_line_type= 12 or Type = 12) and #random_type_12 > 0 and #random_type_12 <= 2 then #is_property:=1
else #is_property:=0
end is_property,
rand() as rand_val,
#random_type_12 as cur_random_type_counter,
case
when (#next_line_type= 12 or Type = 12) and #random_type_12 > 0 then #random_type_12:=#random_type_12-1
else #random_type_12
end as next_rand_type_counter,
#next_line_id as next_line_id,
#next_line_id:=id as current_id,
#next_line_type:=Type as current_type
from goodquestions order by id desc
) t
where t.Type <> 12
order by is_property desc, rand_val limit 0,10) tt
join goodquestions g on g.id = tt.id or (g.id = tt.next_line_id and tt.is_property = 1 and tt.Type <> 12)
group by g.id, g.Question, g.Answer, g.Type, tt.is_property
order by is_property desc, id
limit 0, 10;
Test data set is as following:
mysql> select * from goodquestions;
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 84
Current database: test
+----+----------+--------+------+
| id | Question | Answer | Type |
+----+----------+--------+------+
| 1 | q1 | a1 | 1 |
| 2 | q2 | a2 | 2 |
| 3 | q3 | a3 | 3 |
| 4 | q4 | a4 | 4 |
| 5 | q5 | a5 | 5 |
| 6 | q6 | a6 | 6 |
| 7 | q7 | a7 | 7 |
| 8 | q8 | a8 | 8 |
| 9 | q9 | a9 | 9 |
| 10 | q10 | a10 | 10 |
| 11 | q11 | a11 | 11 |
| 12 | q12 | a12 | 12 |
| 13 | q13 | a13 | 13 |
| 14 | q14 | a14 | 14 |
| 15 | q15 | a15 | 15 |
| 16 | q16 | a16 | 16 |
| 17 | q17 | a17 | 17 |
| 18 | q18 | a18 | 18 |
| 19 | q21 | a21 | 12 |
| 20 | q22 | a22 | 22 |
| 21 | q23 | a23 | 12 |
+----+----------+--------+------+
21 rows in set (0.34 sec)

Answer 2

For MySQL 8+ it can be something similar to
WITH
-- SELECT 20 random rows
cte AS ( SELECT Question, Answer, Type
FROM goodquestions
ORDER BY RAND() LIMIT 0,20 )
( SELECT Question, Answer, Type
FROM cte )
-- add pre-row if Type=12 row is selected and pre-row is not selected
UNION DISTINCT
( SELECT Question, Answer, Type
FROM goodquestions
WHERE Type = 'pre-type for type 12'
AND EXISTS ( SELECT NULL
FROM cte
WHERE Type = 12 ) )
-- sort placing pre-row and type=12 row at the top
ORDER BY Type = 'pre-type for type 12' DESC,
Type = 12 DESC,
RAND()
-- remove excess row if Type=12 row was selected in CTE
-- and pre-row was not selected in CTE but added in UNION
LIMIT 0, 20
The query assumes that goodquestions.Type is unique.