Mysql 使用SELECT-JOIN从同一个表中获取两个值

我有三张桌子——材料、活动和单元。 物料和活动表都通过包含

unit\u id

属性与units表相关。如何通过单个查询同时获取活动单位和物料单位

这是我当前的查询：

SELECT u.username, m.name, a.name, un.name
FROM users u
JOIN materials m
ON (m.id = u.material_id)
JOIN activities a
ON (a.id = u.activity_id)
JOIN units un
ON (un.id = m.unit_id OR un.id = a.unit_id)

---

但是我只得到一个单位，我想得到材料的单位和活动的单位。如何实现这一点？

加入

units

表两次

SELECT u.username, 
       m.name as m_name, unm.name as m_unit,
       a.name as a_name, una.name as a_unit
FROM users u
JOIN materials m ON m.id = u.material_id
JOIN activities a ON a.id = u.activity_id
JOIN units unm ON unm.id = m.unit_id
JOIN units una ON una.id = a.unit_id

---

加入

单位

表两次

SELECT u.username, 
       m.name as m_name, unm.name as m_unit,
       a.name as a_name, una.name as a_unit
FROM users u
JOIN materials m ON m.id = u.material_id
JOIN activities a ON a.id = u.activity_id
JOIN units unm ON unm.id = m.unit_id
JOIN units una ON una.id = a.unit_id

---

您可以使用子查询进行此操作，它在以下情况下非常方便：

SELECT u.username, 
       (SELECT name FROM m WHERE m.id = u.material_id), 
       (SELECT name FROM a WHERE a.id = u.activity_id)
   FROM users

---

您可以使用子查询进行此操作，它在以下情况下非常方便：

SELECT u.username, 
       (SELECT name FROM m WHERE m.id = u.material_id), 
       (SELECT name FROM a WHERE a.id = u.activity_id)
   FROM users