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How to get RANDOM records from each category in MySQL?

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轮回少年
轮回少年 2020-12-11 18:23

In my MySQL database, I have a table with different questions in different categories.

I would like to write a SQL statement that returns 3 RANDOM questions of EACH

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3条回答
  • 南旧
    2020-12-11 18:27

    Yes, you can do this by enumerating the rows and then fetching the top three:

    select r.id, r.question, r.category
from (select r.*,
             (@rn := if(@c = category, @rn + 1,
                        if(@c := category, 1, 1)
                       )
             ) as seqnum
      from `random` r cross join
           (select @rn := 0, @c := -1) params
      order by category, rand()
     ) r
where seqnum <= 3;
    0 讨论(0)
  • 忘掉有多难
    2020-12-11 18:34

    In addition to the other answer, this is also another way to do it.

    SELECT r.* FROM random r
WHERE (
  SELECT COUNT(*) FROM random r1
  WHERE r.category = r1.category AND r.id < r1.id
) <= 2
ORDER BY r.category ASC, RAND()
    0 讨论(0)
  • 我在风中等你
    2020-12-11 18:39 
    create schema so_gibberish; -- creates database 
use so_gibberish;   -- use it 

-- drop table random;   -- during debug
create table random 
(   id int auto_increment primary key,
    question varchar(50) not null,
    category int not null,
    randomOrder int not null,
    key (category)
);

-- drop table questions_for_user;   -- during debug
create table questions_for_user 
(   physcOrder int auto_increment primary key,
    id int not null,
    question varchar(50) not null,
    category int not null,
    randomOrder int not null,
    key (category)
);

    Create a stored procedure to insert random questions. It creates 300 at a time when you call it.

    DELIMITER $$ 
drop procedure if exists createRandomQuestions$$ 
-- 17 categories of questions randomly created. yes random word questions and categories.

create procedure createRandomQuestions()
BEGIN
set @i=1;
WHILE @i<=300 DO
insert random (question,category) values ('xxx',1);
SELECT @lid:=LAST_INSERT_ID();  -- use id to seed, next 8 guaranteed different i think

UPDATE random SET question=concat(
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(@seed:=round(rand(@lid)*4294967296))*36+1, 1),
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(@seed:=round(rand(@seed)*4294967296))*36+1, 1),
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(@seed:=round(rand(@seed)*4294967296))*36+1, 1),
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(@seed:=round(rand(@seed)*4294967296))*36+1, 1),
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(@seed:=round(rand(@seed)*4294967296))*36+1, 1),
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(@seed:=round(rand(@seed)*4294967296))*36+1, 1),
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(@seed:=round(rand(@seed)*4294967296))*36+1, 1),
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', rand(@seed)*36+1, 1), ' ?'
), category=floor(rand()*17+1),randomOrder=0
WHERE id=@lid;
set @i=@i+1;
END WHILE;
END;
$$
DELIMITER ;

    call it:

    call createRandomQuestions();

    to check:

    select category,count(*) from random group by category order by category;
select count(*) from random;
select * from random limit 10;

    Now get three random questions for each category for a user upon request.

    Come up with a good random SEEDING algorithm. The below is a stub, best to use an external source like http to a service, etc.

    update random set randomOrder=rand()*unix_timestamp();
truncate table questions_for_user;
insert into questions_for_user (id,question,category,randomOrder)
select id,question,category,randomOrder from random order by rand();

select r.* 
FROM questions_for_user r 
WHERE 
( SELECT COUNT(*) FROM random r1 WHERE r.category = r1.category AND r.id < r1.id ) <= 2 
ORDER by r.category ASC, randomOrder;

+------------+-----+------------+----------+-------------+
| physcOrder | id  | question   | category | randomOrder |
+------------+-----+------------+----------+-------------+
|        297 | 266 | JNQH26DD ? |        1 |   841700408 |
|        247 | 286 | ORK15577 ? |        1 |   980764662 |
|         54 | 267 | T2HPRW88 ? |        1 |  1331420535 |
|        190 | 275 | U5BFCUFF ? |        2 |   836160445 |
|        192 | 285 | E3KDJ144 ? |        2 |  1166207975 |
|         55 | 293 | GFWQ0BEE ? |        2 |  1356597807 |
|        266 | 296 | 8MJCWR55 ? |        3 |  1121232849 |
|        173 | 288 | 6GGOI2CC ? |        3 |  1209081435 |
|        205 | 231 | LQMYMV44 ? |        3 |  1330946482 |
|        103 | 289 | FUODJHJJ ? |        4 |   392498036 |
|        274 | 295 | Y85VNBWW ? |        4 |   613800522 |
|        204 | 280 | 441X1YTT ? |        4 |  1141251557 |
|        162 | 273 | CC5FGKFF ? |        5 |    99041401 |
|         36 | 252 | Y7V58ZEE ? |        5 |  1124240820 |
|        143 | 234 | DWZULKBB ? |        5 |  1286225785 |
|         15 | 276 | 3J1KGJZZ ? |        6 |    51285374 |
|        265 | 294 | PTEMZ7GG ? |        6 |   649256602 |
|         91 | 271 | TNG23R11 ? |        6 |  1393790410 |
|         90 | 277 | DXE1W0GG ? |        7 |   622567356 |
|         20 | 274 | LQTFS7RR ? |        7 |  1376505632 |
|        121 | 279 | VQN6TVSS ? |        7 |  1428714367 |
|        268 | 265 | A99UU1YY ? |        8 |   479923553 |
|        242 | 284 | 5PDRR5YY ? |        8 |   658551160 |
|        283 | 282 | NWZSPCZZ ? |        8 |   802197234 |
|        114 | 233 | 3IHU1YKK ? |        9 |   118146688 |
|         85 | 226 | BQTD6A44 ? |        9 |   292147382 |
|        255 | 291 | YNT0YN11 ? |        9 |   559911752 |
|        281 | 258 | IHK46577 ? |       10 |   196799301 |
|        198 | 263 | SHEVXPBB ? |       10 |   545833955 |
|         11 | 298 | QEHHSAEE ? |       10 |  1087006220 |
|         60 | 250 | GEH9PVBB ? |       11 |   225193615 |
|        270 | 272 | 3YY2EMM ?  |       11 |  1143544695 |
|        300 | 290 | O8B4WRCC ? |       11 |  1209867696 |
|         68 | 246 | FTEEVJ00 ? |       12 |   608441021 |
|        177 | 244 | X1JYHUBB ? |       12 |  1192309224 |
|        208 | 240 | W771N588 ? |       12 |  1347800170 |
|        249 | 300 | 97V6UXYY ? |       13 |   500846709 |
|        100 | 292 | 71JZTMQQ ? |       13 |   607266604 |
|         38 | 283 | WBQ0DGLL ? |       13 |  1187952919 |
|         87 | 235 | MBPPXJ44 ? |       14 |   327445349 |
|        178 | 242 | EZ1ET3ZZ ? |       14 |   852480639 |
|         62 | 281 | EIHVH3ZZ ? |       14 |  1420114728 |
|         95 | 251 | PT8E2CII ? |       15 |    72502146 |
|        170 | 253 | 8L35PFYY ? |       15 |  1091765038 |
|         65 | 287 | X5ZR3LHH ? |       15 |  1405199431 |
|        138 | 278 | MBW03OUU ? |       16 |    84358922 |
|          7 | 268 | 2G4G42WW ? |       16 |  1257379717 |
|          1 | 299 | ZT8QRRMM ? |       16 |  1327297420 |
|        221 | 297 | H00HLNRR ? |       17 |   890140945 |
|         40 | 204 | O7VUW2NN ? |       17 |  1238474743 |
|        122 | 229 | 3XEZG0PP ? |       17 |  1359337651 |
+------------+-----+------------+----------+-------------+
51 rows in set (0.01 sec)

    Run it again for next user or same guy:

    update random set randomOrder=rand()*unix_timestamp();
truncate table questions_for_user;
insert into questions_for_user (id,question,category,randomOrder)
select id,question,category,randomOrder from random order by rand();

select r.* 
FROM questions_for_user r 
WHERE 
( SELECT COUNT(*) FROM random r1 WHERE r.category = r1.category AND r.id < r1.id ) <= 2 
ORDER by r.category ASC, randomOrder;

+------------+-----+------------+----------+-------------+
| physcOrder | id  | question   | category | randomOrder |
+------------+-----+------------+----------+-------------+
|        112 | 286 | ORK15577 ? |        1 |   193281314 |
|        176 | 266 | JNQH26DD ? |        1 |   530153269 |
|        296 | 267 | T2HPRW88 ? |        1 |   891041924 |
|         37 | 293 | GFWQ0BEE ? |        2 |   222852606 |
|        239 | 285 | E3KDJ144 ? |        2 |   679635152 |
|        262 | 275 | U5BFCUFF ? |        2 |   846163956 |
|         95 | 288 | 6GGOI2CC ? |        3 |  1244253481 |
|         81 | 231 | LQMYMV44 ? |        3 |  1380298624 |
|         59 | 296 | 8MJCWR55 ? |        3 |  1420850554 |
|         28 | 295 | Y85VNBWW ? |        4 |   806083444 |
|         30 | 289 | FUODJHJJ ? |        4 |   814854070 |
|         34 | 280 | 441X1YTT ? |        4 |  1119188021 |
|        232 | 234 | DWZULKBB ? |        5 |   296692881 |
|        295 | 273 | CC5FGKFF ? |        5 |   337915901 |
|        140 | 252 | Y7V58ZEE ? |        5 |   644987638 |
|        257 | 271 | TNG23R11 ? |        6 |   619359840 |
|         27 | 294 | PTEMZ7GG ? |        6 |   869855570 |
|        218 | 276 | 3J1KGJZZ ? |        6 |  1390090875 |
|         64 | 279 | VQN6TVSS ? |        7 |    33942495 |
|        290 | 277 | DXE1W0GG ? |        7 |   100007602 |
|        173 | 274 | LQTFS7RR ? |        7 |   946909650 |
|        291 | 265 | A99UU1YY ? |        8 |   107468716 |
|         49 | 284 | 5PDRR5YY ? |        8 |  1068298164 |
|        228 | 282 | NWZSPCZZ ? |        8 |  1115906220 |
|        284 | 226 | BQTD6A44 ? |        9 |    45678738 |
|        179 | 291 | YNT0YN11 ? |        9 |   700305900 |
|        164 | 233 | 3IHU1YKK ? |        9 |  1416089612 |
|        193 | 258 | IHK46577 ? |       10 |   460111512 |
|        214 | 298 | QEHHSAEE ? |       10 |   482322673 |
|          7 | 263 | SHEVXPBB ? |       10 |   766681927 |
|        178 | 290 | O8B4WRCC ? |       11 |   341509950 |
|         31 | 272 | 3YY2EMM ?  |       11 |   726662739 |
|        297 | 250 | GEH9PVBB ? |       11 |  1386568968 |
|         32 | 240 | W771N588 ? |       12 |   303493686 |
|        283 | 246 | FTEEVJ00 ? |       12 |   710591266 |
|        177 | 244 | X1JYHUBB ? |       12 |   916685336 |
|        212 | 283 | WBQ0DGLL ? |       13 |   595739692 |
|        159 | 300 | 97V6UXYY ? |       13 |   688431139 |
|         52 | 292 | 71JZTMQQ ? |       13 |  1039681379 |
|         56 | 235 | MBPPXJ44 ? |       14 |   109832248 |
|        207 | 242 | EZ1ET3ZZ ? |       14 |   418951740 |
|          6 | 281 | EIHVH3ZZ ? |       14 |  1182157711 |
|        149 | 287 | X5ZR3LHH ? |       15 |   364819476 |
|         88 | 251 | PT8E2CII ? |       15 |   673475236 |
|        123 | 253 | 8L35PFYY ? |       15 |  1204512525 |
|        190 | 278 | MBW03OUU ? |       16 |   641720378 |
|        160 | 299 | ZT8QRRMM ? |       16 |  1289470813 |
|         75 | 268 | 2G4G42WW ? |       16 |  1427431541 |
|        202 | 297 | H00HLNRR ? |       17 |   374047531 |
|        243 | 204 | O7VUW2NN ? |       17 |   951026810 |
|        300 | 229 | 3XEZG0PP ? |       17 |  1319302198 |
+------------+-----+------------+----------+-------------+
51 rows in set (0.01 sec)

    So the questions are different.

    To get the last results in a shuffled sense:

    select r.* 
FROM questions_for_user r 
WHERE 
( SELECT COUNT(*) FROM random r1 WHERE r.category = r1.category AND r.id < r1.id ) <= 2 
ORDER by randomOrder;

+------------+-----+------------+----------+-------------+
| physcOrder | id  | question   | category | randomOrder |
+------------+-----+------------+----------+-------------+
|         64 | 279 | VQN6TVSS ? |        7 |    33942495 |
|        284 | 226 | BQTD6A44 ? |        9 |    45678738 |
|        290 | 277 | DXE1W0GG ? |        7 |   100007602 |
|        291 | 265 | A99UU1YY ? |        8 |   107468716 |
|         56 | 235 | MBPPXJ44 ? |       14 |   109832248 |
|        112 | 286 | ORK15577 ? |        1 |   193281314 |
|         37 | 293 | GFWQ0BEE ? |        2 |   222852606 |
|        232 | 234 | DWZULKBB ? |        5 |   296692881 |
|         32 | 240 | W771N588 ? |       12 |   303493686 |
|        295 | 273 | CC5FGKFF ? |        5 |   337915901 |
|        178 | 290 | O8B4WRCC ? |       11 |   341509950 |
|        149 | 287 | X5ZR3LHH ? |       15 |   364819476 |
|        202 | 297 | H00HLNRR ? |       17 |   374047531 |
|        207 | 242 | EZ1ET3ZZ ? |       14 |   418951740 |
|        193 | 258 | IHK46577 ? |       10 |   460111512 |
|        214 | 298 | QEHHSAEE ? |       10 |   482322673 |
|        176 | 266 | JNQH26DD ? |        1 |   530153269 |
|        212 | 283 | WBQ0DGLL ? |       13 |   595739692 |
|        257 | 271 | TNG23R11 ? |        6 |   619359840 |
|        190 | 278 | MBW03OUU ? |       16 |   641720378 |
|        140 | 252 | Y7V58ZEE ? |        5 |   644987638 |
|         88 | 251 | PT8E2CII ? |       15 |   673475236 |
|        239 | 285 | E3KDJ144 ? |        2 |   679635152 |
|        159 | 300 | 97V6UXYY ? |       13 |   688431139 |
|        179 | 291 | YNT0YN11 ? |        9 |   700305900 |
|        283 | 246 | FTEEVJ00 ? |       12 |   710591266 |
|         31 | 272 | 3YY2EMM ?  |       11 |   726662739 |
|          7 | 263 | SHEVXPBB ? |       10 |   766681927 |
|         28 | 295 | Y85VNBWW ? |        4 |   806083444 |
|         30 | 289 | FUODJHJJ ? |        4 |   814854070 |
|        262 | 275 | U5BFCUFF ? |        2 |   846163956 |
|         27 | 294 | PTEMZ7GG ? |        6 |   869855570 |
|        296 | 267 | T2HPRW88 ? |        1 |   891041924 |
|        177 | 244 | X1JYHUBB ? |       12 |   916685336 |
|        173 | 274 | LQTFS7RR ? |        7 |   946909650 |
|        243 | 204 | O7VUW2NN ? |       17 |   951026810 |
|         52 | 292 | 71JZTMQQ ? |       13 |  1039681379 |
|         49 | 284 | 5PDRR5YY ? |        8 |  1068298164 |
|        228 | 282 | NWZSPCZZ ? |        8 |  1115906220 |
|         34 | 280 | 441X1YTT ? |        4 |  1119188021 |
|          6 | 281 | EIHVH3ZZ ? |       14 |  1182157711 |
|        123 | 253 | 8L35PFYY ? |       15 |  1204512525 |
|         95 | 288 | 6GGOI2CC ? |        3 |  1244253481 |
|        160 | 299 | ZT8QRRMM ? |       16 |  1289470813 |
|        300 | 229 | 3XEZG0PP ? |       17 |  1319302198 |
|         81 | 231 | LQMYMV44 ? |        3 |  1380298624 |
|        297 | 250 | GEH9PVBB ? |       11 |  1386568968 |
|        218 | 276 | 3J1KGJZZ ? |        6 |  1390090875 |
|        164 | 233 | 3IHU1YKK ? |        9 |  1416089612 |
|         59 | 296 | 8MJCWR55 ? |        3 |  1420850554 |
|         75 | 268 | 2G4G42WW ? |       16 |  1427431541 |
+------------+-----+------------+----------+-------------+
51 rows in set (0.01 sec)

    The reason the other answers seem to be returning the same questions (top 3 or bottom 3) imo is because of the physical ordering of the table, which is why I created the 2nd table. The second table uses a new bogus physical ordering and the insert into it is via order by random (so the physical order problem doesn't exist). Hope that makes sense.

    And of course I poached the answer from pjanaway and couldn't get Gordon's to work on my system though I like his variable approach.

    Good luck.

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