mysql如果行不存在，则获取默认值

本文介绍了mysql如果行不存在，则获取默认值的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我正在为系统创建首选项模块，以便用户可以为某些部分(诸如布局，颜色方案，主屏幕等之类的东西)定义自己的首选项.表中的每个首选项都有一个默认值(以防用户尚未定义)，一旦用户更改了首选项，系统应使用该用户定义的值.

I'm making a preferences module for a system so users can define their own preferences for certain parts (stuff like lay-out, color-scheme, homescreen etc.). Every preference in the table has got a default value (in case the user hasn't defined one yet) and once the user has changed their preferences the system should use that user-defined value.

我在选择所需的值时遇到了麻烦，我的查询使用的是用户定义的值，但是遗憾的是，当用户尚未定义首选项时，它会返回null.

I'm having trouble with selecting the desired value, my query works with the user-defined value, but sadly it returns null when the user hasn't defined a preference yet.

我有2张桌子

table **preferences**: id name default_value table **preferences_defined**: id preference_id user_id defined_value

我想创建一个函数，通过提供应用了首选项的人员的user_id和首选项的名称来轻松选择所需的值.目前，我的查询如下:

I want to create a function to easily select the desired value by giving the user_id of the person to whom the preference applies and the name of the preference. At the moment my query looks like this:

SELECT IF(ISNULL(defined_value),default_value,defined_value) AS result FROM preferences LEFT JOIN preferences_defined ON preferences_defined.id = preferences.id WHERE user_id = 17 AND name = "menu_items"

我想WHERE user_id = 17部分出了问题，因为当没有可用的定义值时，这意味着也没有名为user_id的列. 我需要找到一种方法来完成这项工作. 因此，我需要选择定义的值(如果存在)(考虑到首选项的user_id和name)，如果不存在，则必须返回默认值.

I guess the WHERE user_id = 17 part is where things go wrong because when there is no defined value available it means there also isn't a column named user_id. I need to find a way to make this work. So, I need to select the defined value if it exist (given the user_id and the name of the preference), if not, it will have to return the default value.

推荐答案

WHERE (user_id = 17 OR user_id IS NULL) AND name = "menu_items"

只是为了清理其余查询:

And just to clean up the rest of the query:

不需要在首选项中定义id，而是使用组合键preference_id和user_id.

The id in preferences defined doesn't need to be there, use a combined key of preference_id and user_id instead.

如果preferences的id被命名为preference_id，则长ON语句可以替换为USING(preference_id)

If the id of preferences was named preference_id, the long ON statement could be replaced with USING(preference_id)

IF函数可以替换为COALESCE(defined_value, default_value)