MYSQL 取随机数
引自：http://hi.baidu.com/dbaeyes/blog/item/0ab73aed7063f7d8b31cb145.html
mysql 取随机数
--对一个表取任意随机数
SELECT *
FROM TMP_XF_TEST
WHERE ID >= (SELECT FLOOR(RAND() * (SELECT MAX(ID) FROM TMP_XF_TEST)))
order by id LIMIT 1;
--有条件性的取随机数
SELECT *
FROM TMP_XF_TEST
WHERE ID >= (SELECT FLOOR(RAND() * ((SELECT MAX(ID) FROM TMP_XF_TEST WHERE GID = 9) -
(SELECT MIN(ID) FROM TMP_XF_TEST WHERE GID = 9))) +
(SELECT MAX(ID) FROM TMP_XF_TEST WHERE GID = 9))
AND GID = 9
order by id LIMIT 1;
--gid上存在索引
或者
SELECT *
FROM TMP_XF_TEST AS t1 JOIN
(SELECT ROUND(RAND() * ((SELECT MAX(id) FROM TMP_XF_TEST WHERE GID = 9)-(SELECT MIN(id) FROM TMP_XF_TEST WHERE GID = 9))
+(SELECT MIN(id) FROM TMP_XF_TEST WHERE GID = 9)) AS id) AS t2
WHERE t1.id >= t2.id AND t1.GID = 9
ORDER BY t1.id LIMIT 1;
#########
不要用下面的杯具写法
mysql> insert into tmp_xf_test(user_nick,gid,item_id,gmt_create,gmt_modified,memo)
-> select user_nick,gid,item_id,gmt_create,gmt_modified,memo from tmp_xf_test;
Query OK, 165888 rows affected (9.65 sec)
Records: 165888 Duplicates: 0 Warnings: 0
mysql> SELECT *
-> FROM `tmp_xf_test`
-> WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `tmp_xf_test` )
-> ORDER BY id LIMIT 1;
+-----+-----------+-----+---------+---------------------+---------------------+--------------------+
| id | user_nick | gid | item_id | gmt_create | gmt_modified | memo |
+-----+-----------+-----+---------+---------------------+---------------------+--------------------+
| 467 | 玄风 | 9 | 123 | 2010-04-26 14:56:39 | 2010-04-26 14:56:39 | 玄风测试使用的数据 |
+-----+-----------+-----+---------+---------------------+---------------------+--------------------+
1 row in set (51.12 sec)
mysql> explain SELECT *
-> FROM `tmp_xf_test`
-> WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `tmp_xf_test` )
-> ORDER BY id LIMIT 1\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: tmp_xf_test
type: index
possible_keys: NULL
key: PRIMARY
key_len: 8
ref: NULL
rows: 1
Extra: Using where
*************************** 2. row ***************************
id: 2
select_type: UNCACHEABLE SUBQUERY
table: tmp_xf_test
type: index
possible_keys: NULL
key: idx_tmp_xf_test_gid
key_len: 4
ref: NULL
rows: 331954
Extra: Using index
2 rows in set (0.01 sec)
---
mysql> SELECT * FROM `tmp_xf_test` t1 join
-> (SELECT FLOOR( MAX(id) * RAND()) as id FROM `tmp_xf_test` ) as t2
-> where t1.id >=t2.id
-> ORDER BY t1.id LIMIT 1;
+-------+-----------+-----+---------+---------------------+---------------------+--------------------+-------+
| id | user_nick | gid | item_id | gmt_create | gmt_modified | memo | id |
+-------+-----------+-----+---------+---------------------+---------------------+--------------------+-------+
| 40311 | 玄风 | 9 | 123 | 2010-04-28 15:47:19 | 2010-04-28 15:47:19 | 玄风测试使用的数据 | 40311 |
+-------+-----------+-----+---------+---------------------+---------------------+--------------------+-------+
1 row in set (0.14 sec)
##############
mysql> SELECT * FROM `tmp_xf_test`
-> WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `tmp_xf_test`)))
-> ORDER BY id LIMIT 1;
+------+-----------+-----+---------+---------------------+---------------------+--------------------+
| id | user_nick | gid | item_id | gmt_create | gmt_modified | memo |
+------+-----------+-----+---------+---------------------+---------------------+--------------------+
| 1352 | 玄风 | 9 | 123 | 2010-04-28 15:47:19 | 2010-04-28 15:47:19 | 玄风测试使用的数据 |
+------+-----------+-----+---------+---------------------+---------------------+--------------------+
1 row in set (0.00 sec)
mysql> explain SELECT * FROM `tmp_xf_test`
-> WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `tmp_xf_test`)))
-> ORDER BY id LIMIT 1\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: tmp_xf_test
type: index
possible_keys: NULL
key: PRIMARY
key_len: 8
ref: NULL
rows: 1
Extra: Using where
*************************** 2. row ***************************
id: 3
select_type: SUBQUERY
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: Select tables optimized away
2 rows in set, 1 warning (0.00 sec)
对应的另外一种杯具写法是：
SELECT *
FROM TMP_XF_TEST
WHERE ID >= (SELECT FLOOR(RAND() * (MAX(ID) - MIN(ID))) + MIN(ID) MID
FROM TMP_XF_TEST
WHERE GID = 9)
AND GID = 9 LIMIT 1;