这是我的表结构:
-- categories
+----+-------------+
| id | name |
+----+-------------+
| 1 | political |
| 2 | cultural |
| 3 | social |
| 4 | historical |
+----+-------------+
-- tags
+----+-------------+-----------+-------------+-----------------+
| id | name | parent_id | category_id | total_used_num |
+----+-------------+-----------+-------------+-----------------+
| 1 | president | NULL | 1 | 43 |
| 2 | society | NULL | 3 | 345 |
| 3 | Trump | 1 | 1 | 5 |
| 4 | book | 5 | 2 | 5473 |
| 5 | library | NULL | 2 | 433 |
+----+-------------+-----------+-------------+-----------------+
这是我的查询:
SELECT t1.name,
CONCAT(t2.name, ',', cat.name) t_p
FROM tags t1
LEFT JOIN tags t2
ON t1.parent_id = t2.id
INNER JOIN categories cat
ON t1.category_id = cat.id
ORDER BY total_used_num DESC,
t1.id
/* output
+----+-------------+--------------------------+
| 1 | president | NULL |
| 2 | society | NULL |
| 3 | Trump | president,political |
| 4 | book | library,cultural |
| 5 | library | NULL |
+----+-------------+--------------------------+
看到了吗? CONCAT
与 NULL
的结果是 NULL
。无论如何,我怎样才能避免这种情况呢?我还想将逗号 ,
作为条件。如果两个字段都不为 NULL
,则该逗号应该存在。这是预期结果:
+----+-------------+--------------------------+
| 1 | president | political |
| 2 | society | social |
| 3 | Trump | president,political |
| 4 | book | library,cultural |
| 5 | library | cultural |
+----+-------------+--------------------------+
我怎样才能做到这一点?
最佳答案
只需使用concat_ws()
:
SELECT t1.name,
CONCAT_WS(',', t2.name, cat.name) as t_p
FROM tags t1 LEFT JOIN
tags t2
ON t1.parent_id = t2.id INNER JOIN
categories cat
ON t1.category_id = cat.id
ORDER BY total_used_num DESC, t1.id ;
关于mysql - 如何判断JOIN的结果是否为NULL?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/45025521/