Question

我有以下查询

SELECT s.id AS id, s.grade_id as grade_id, g.name as grade_name, s.name AS name, s.scheduled_days as scheduled_days, s.period_id as period_id, p.name as period_name, s.start_time as start_time, s.end_time as end_time, s.teacher_id as teacher_id, u.name as teacher_name, s.classroom_id as classroom_id, c.name as classroom_name
FROM sections s, grades g, periods p, teachers t, users u, classrooms c
WHERE s.grade_id = g.id AND s.period_id = p.id AND s.classroom_id = c.id AND s.teacher_id = t.id AND t.user_id = u.id

在这里，节表中的teacher_id和classroom_id外键可能为空。但是，如果这些外键中的任何一个为空，mysql将从所有相关表中排除整个列。

我该如何更改查询，以便mysql将显示 sections 表中的数据，即使这些键为空。

Answer 1

您需要使用外部联接。

首先，您需要使用自1992年以来可用的现代JOIN语法。使用此语法，您可以使用LEFT [OUTER] JOIN，如下所示：

SELECT
  s.id AS id, s.grade_id as grade_id, g.name as grade_name, s.name AS name, 
  s.scheduled_days as scheduled_days, s.period_id as period_id,
  p.name as period_name, s.start_time as start_time, 
  s.end_time as end_time, s.teacher_id as teacher_id, 
  u.name as teacher_name, s.classroom_id as classroom_id, 
  c.name as classroom_name
FROM sections s
JOIN grades g ON s.grade_id = g.id
JOIN periods p ON s.period_id = p.id
LEFT JOIN teachers t ON s.teacher_id = t.id -- allows nulls for s.teacher_id
LEFT JOIN users u ON t.user_id = u.id -- allows nulls on t.user_id
LEFT JOIN classrooms c ON s.classroom_id = c.id -- allows nulls for s.classroom_id

这样，您将使用WHERE子句仅包括“过滤条件”，而不再包括“联接条件”。阅读和调试起来会容易得多。

MySQL：组合相关表时显示所有可用数据，即使引用外键为空

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