连接表，其中将空值包含在THIRD表中

Question

我正在研究电子词典，我有2张桌子：

第一张表：条款

+----+--------------+
| id | term_text    |
+----+--------------+
| 1  | hello        |
| 2  | how are you? |
| 3  | hola         |
+----+--------------+

第二张表：字典

<pre>
+-----------+------------+
| term_id_1 | term_id_2  |
+-----------+------------+
| 1         | 3          | /* hello - hola
| 2         | NULL       | /* how are you? - NULL
+-----------+------------+
</pre>

我想这样显示：

<pre>


+--------------+-------------+
| term_text_1  | term_text_2 |
+--------------+-------------+
| hello        | hola        | 
| how are you? | NULL        | 
+--------------+-------------+

</pre>

我正尝试使用以下查询获取数据：

<pre>
SELECT t1.term_text_1, t2.term_text_2
FROM dictionary d, term t1, term t2
WHERE d.term_id_1 = t1.id
AND d.term_id_2 = t2.id
</pre>

但是我只得到值不为null的地方！

Answer 1

您可以使用LEFT JOIN

SELECT t1.term_text as term_text_1, t2.term_text as term_text_2
FROM dictionary d
LEFT JOIN term t1 ON d.term_id_1 = t1.id
LEFT JOIN term t2 ON d.term_id_2 = t2.id

Demo

Answer 2

您需要两次将dictionary与term一起加入：

select 
  t1.term_text term_text_1,
  t2.term_text term_text_2
from dictionary 
left join term t1 on t1.id = term_id_1 
left join term t2 on t2.id = term_id_2 

请参见demo。
结果：

| term_text_1  | term_text_2 |
| ------------ | ----------- |
| hello        | hola        |
| how are you? |             |