mysql高效率隨機獲取n條數據寫法

今天做項目遇到這個問題，本來想用mysql自帶的隨機函數來實現，但是想到這樣做功能是實現了，但是效率真的好差！一下子想不到好的方法，就去網上找了一下，記錄下來，好好研究學習一下。

ID連續的情況下（注意不能帶where，否則結果不好）：

SELECT *
FROM `talbe` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `table`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id ASC LIMIT 4;

ID不連續的情況下：

SELECT * FROM `table` 
WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `table`)))  and a = "a" and b = b
ORDER BY id LIMIT 4;

假設你的表叫做mm_account，並且有一個字段叫id。最快的方法如下（隨機獲取5條）：

SELECT * FROM talbe WHERE id >= ((SELECT MAX(id) FROM table)-(SELECT MIN(id) FROM table)) * RAND() + (SELECT MIN(id) FROM table) limit 5;

如果帶where語句，上面就不適合了。帶where語句請看下面：

SELECT *
FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * (
(SELECT MAX(id) FROM `table` where id < 1000 )-(SELECT MIN(id) FROM `table` where id < 1000 ))+(SELECT MIN(id) FROM `table` where id < 1000 )) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id LIMIT 5;