在我的MySQL数据库中，我有一个表，在不同的类别中有不同的问题。
我想编写一个返回EACH类别的3个RANDOM问题的SQL语句。
以下是数据库记录的示例：
id question category
1 Question A 1
2 Question B 1
3 Question C 1
4 Question D 1
5 Question D 1
6 Question F 2
7 Question G 2
8 Question H 2
9 Question I 2
10 Question J 2
11 Question K 3
12 Question L 3
13 Question M 3
14 Question N 3
15 Question O 3
16 Question P 3
以下是从上面列表中的每个类别的所有问题中选择并随机排列的3个随机的输出/结果：
2 Question B 1
4 Question D 1
3 Question C 1
10 Question J 2
7 Question G 2
9 Question I 2
11 Question K 3
15 Question P 3
13 Question M 3
到目前为止，我已经使用以下语句进行测试：
SELECT * FROM `random` ORDER BY RAND() LIMIT 0,3;
这只返回所有类别的3个RANDOM问题。
之后我在这个链接上查找了例子：MYSQL select random of each of the categories
并尝试了这个：
(SELECT * FROM `random` WHERE category = 1 ORDER BY RAND() LIMIT 3)
UNION ALL
(SELECT * FROM `random` WHERE category = 2 ORDER BY RAND() LIMIT 3)
UNION ALL
(SELECT * FROM `random` WHERE category = 3 ORDER BY RAND() LIMIT 3)
但在这里我需要手动添加每个类别。
我的问题：我很奇怪是否可以从所有类别的每个类别（自动）中获取3个RANDOM记录/行？
编辑
这不是问题的一部分，而是帮助。
虚拟数据创建者查询代码将名为random，并创建一个名为create_random的存储过程，当您运行存储过程时，它将在随机表中创建随机虚拟数据：
DELIMITER $$
DROP TABLE IF EXISTS `random`;
DROP PROCEDURE IF EXISTS `create_random` $$
CREATE TABLE `random` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`question` VARCHAR(50) NULL DEFAULT NULL,
`category` VARCHAR(50) NULL DEFAULT NULL,
PRIMARY KEY (`id`)
)
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB
AUTO_INCREMENT=401
;
CREATE DEFINER=`root`@`localhost`
PROCEDURE `create_random`()
LANGUAGE SQL
NOT DETERMINISTIC
CONTAINS SQL
SQL SECURITY DEFINER
COMMENT ''
BEGIN
DECLARE v_max int unsigned DEFAULT 100;
DECLARE v_counter int unsigned DEFAULT 0;
DECLARE cat_counter int unsigned DEFAULT 0;
TRUNCATE TABLE `random`;
START TRANSACTION;
WHILE v_counter < v_max DO
IF v_counter %10=0 THEN SET cat_counter=cat_counter+1;
END IF;
INSERT INTO `random` (question, category) VALUES ( CONCAT('Question', FLOOR(0 + (RAND() * 65535))), cat_counter );
SET v_counter=v_counter+1;
END WHILE;
COMMIT;
END
注意：我尝试了所有答案，一切正常。 Gordon Linoff和pjanaway回答选择RANDOM只有前3或下3问题，我已经检查了Gordon的答案，因为他先回答，但这并不意味着其他答案都不好，所有这些都很好，这取决于用户选择正确答案或答案组合。我喜欢所有答案并将其投票。德鲁皮尔斯对这个问题的回答很新，现在更有趣，几乎接近目标。谢谢大家。
是的，您可以通过枚举行然后获取前三个来执行此操作：
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;
除了另一个答案，这也是另一种方法。
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()
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)
);
创建存储过程以插入随机问题。当你调用它时，它会一次创建300个。
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 ;
叫它：
call createRandomQuestions();
去检查：
select category,count(*) from random group by category order by category;
select count(*) from random;
select * from random limit 10;
现在，根据请求为用户提供每个类别的三个随机问题。
想出一个好的随机SEEDING算法。以下是存根，最好使用http等服务等外部源。
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)
为下一个用户或同一个人再次运行它：
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)
所以问题是不同的。
以洗牌的方式获得最后的结果：
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)
其他答案似乎返回相同的问题（前3或后3）imo的原因是因为表的物理排序，这就是我创建第二个表的原因。第二个表使用新的虚假物理排序，并且插入到它中是通过随机顺序（因此物理顺序问题不存在）。希望有道理。
当然，我挖走了pjanaway的答案，虽然我喜欢他的变量方法，但是无法让Gordon在我的系统上工作。
祝好运。