php - 当总行数超过一个数字时，从表中删除所有行

Question

我正在尝试优化我的查询，并且我有以下代码:

<?php
$counter = mysql_query("SELECT COUNT(*) AS c FROM table");
$row = mysql_fetch_array($counter);
$count = $row["c"];
if($count>500){
mysql_query("DELETE FROM table");
}
?>

我尝试了以下语法作为测试，它有效:

Select case when (Select count(*) as t from `table`) > 500 then 1 else 2 end

但这会引发错误:

Select case when (Select count(*) as t from `table`) > 500 then (DELETE FROM table) else null end

1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DELETE FROM table else null end LIMIT 0, 25' at line 1

我知道存在语法错误，但我的主要问题是为什么当我在条件中放置整数时 case 语句有效，但 delete 语句抛出错误。我尝试了一个 select 语句，它运行良好:

Select case when (Select count(*) as t from `table`) < 500 then (select count(*) from table) else null end

Answer 1

如果您真的想用一个delete来做到这一点:

delete t
     from table t cross join
          (select count(*) as cnt from table t) c
     where c.cnt > 500;

请注意(正如 BWS 所做的那样)truncate table 更有效，尽管您不能使用 where 子句或 join with truncate table