问题描述
当您执行IN子句为空的SQL查询时会发生什么?
What happens when you do a SQL query where the IN clause is empty?
例如:
SELECT user WHERE id IN ();
MySQL是否会按预期方式处理此问题(即始终为false)，否则，我的应用程序在动态构建IN子句时如何处理这种情况?
Will MySQL handle this as expected (that is, always false), and if not, how can my application handle this case when building the IN clause dynamically?
推荐答案
如果我有一个正在动态构建IN列表的应用程序，但它可能最终为空，我有时要做的就是使用一个不可能的值，并添加到它.例如.如果是用户名列表，我将以空字符串开头，因为这不是可能的用户名.如果它是一个auto_increment ID，我将使用-1，因为实际值始终为正.
If I have an application where I'm building the IN list dynamically, and it might end up empty, what I sometimes do is initialize the list with an impossible value and add to that. E.g. if it's a list of usernames, I'll start with an empty string, since that's not a possible username. If it's an auto_increment ID, I'll use -1 because the actual values are always positive.
如果由于没有不可能的值而导致此操作不可行，则必须使用条件语句来确定是否在WHERE子句中包含AND column IN ($values)表达式.
If this isn't feasible because there are no impossible values, you have to use a conditional to decide whether to include AND column IN ($values) expression in the WHERE clause.
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