mysql，一次select能不能获取两条数据？

Posted 2023-04-22

比如
$row=select * from a order by b asc
和
$row=select * from a order by b desc

相差无几，重复查询影响速度，能不能一次查询既返回order by b asc的数据，又返回order by b desc的数据？

两个语句之间加一个“union all”就可以了，虽然实际上还是两次查询，但是是在一个statement里面实现的，返回一个resultSet 参考技术A 你这样union上其实也是查了两次
你可以查一次结果 然后在php里做反序使用 参考技术B 既升序又降序？？

mysql怎样高效率随机获取n条数据

ID连续的情况下（注意不能带where，否则结果不好）：

SELECT *
FROM `mm_account` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `mm_account`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id ASC LIMIT 4;
ID不连续的情况下：

SELECT * FROM `mm_account`
WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `mm_account`))) and city="city_91" and showSex=1
ORDER BY id LIMIT 4;

假设你的表叫做mm_account，并且有一个字段叫id。最快的方法如下（随机获取5条）：

SELECT * FROM mm_account WHERE id >= ((SELECT MAX(id) FROM mm_account)-(SELECT MIN(id) FROM mm_account)) * RAND() + (SELECT MIN(id) FROM mm_account) limit 5;
如果带where语句，上面就不适合了。带where语句请看下面：

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