首页 \ 问答 \ 如何选择子项不存在的记录(How to select records where a child does not exist)

如何选择子项不存在的记录(How to select records where a child does not exist)

在rails我有2个表:

bans(ban_id, admin_id)
ban_reasons(ban_reason_id, ban_id, reason_id)

我想找到某个管理员的所有bans ,其中ban_reasons表中没有记录。 如何在Rails中执行此操作而不循环遍历所有禁止记录并使用ban.ban_reasons.nil?过滤掉所有禁止记录ban.ban_reasons.nil? 我希望使用单个SQL语句(希望)这样做。

我只需要这样做:(但我想以“rails”的方式做到这一点)

SELECT bans.* FROM bans WHERE admin_id=1234 AND 
                              ban_id NOT IN (SELECT ban_id FROM ban_reasons)

In rails I have 2 tables:

bans(ban_id, admin_id)
ban_reasons(ban_reason_id, ban_id, reason_id)

I want to find all the bans for a certain admin where there is no record in the ban_reasons table. How can I do this in Rails without looping through all the ban records and filtering out all the ones with ban.ban_reasons.nil? I want to do this (hopefully) using a single SQL statement.

I just need to do: (But I want to do it the "rails" way)

SELECT bans.* FROM bans WHERE admin_id=1234 AND 
                              ban_id NOT IN (SELECT ban_id FROM ban_reasons)

原文:https://stackoverflow.com/questions/3757285
更新时间:2022-03-10 06:03

最满意答案

C字符串文字"12"需要3个字符, 1和空终止符。

如果要初始化2 char数组,请执行以下操作:

static const char _basex2 [2] = {'1','2'};


The C-string literal "12" takes 3 characters, 1, 2, and the null terminator.

If you want to initialize the 2 char array, do this:

static const char _basex2[2] = { '1', '2' };

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