php - MySQL - 如何访问表中第 2 级的所有节点

Question

我正在为具有嵌套类别的电子商务网站创建一个数据库，并且我正在使用修改后的预购遍历算法。我的问题是如何访问第 2 级中的所有节点，即文章、投资组合、联系人

Answer 1

文章没有明确告诉你如何从一个级别获取所有节点。但是，如果您仔细阅读它，它会告诉您如何做更多 -> 获取每个类别的深度计数。然后，您所要做的就是按该深度进行过滤。

SELECT node.name, (COUNT(parent.name) - 1) AS depth
FROM nested_category AS node, nested_category AS parent
WHERE node.lft BETWEEN parent.lft AND parent.rgt
GROUP BY node.name
HAVING depth = 1
ORDER BY node.lft;

编辑（发生了什么）：

为了使用表格的lft和rgt列，nested_category我们应该选择表格两次。

SELECT *
FROM nested_category AS node, nested_category AS parent

如果您检查此查询，您会发现对于nested_category我们再次选择所有行中的每一行。所以我们现在想要的是从第一个表（我们称为 的表AS node）中删除它们不是其子表的所有行parent。这就是我们过滤的原因WHERE node.lft BETWEEN parent.lft AND parent.rgt

我想提一下这个查询：

SELECT *
FROM nested_category AS node, nested_category AS parent
WHERE node.lft BETWEEN parent.lft AND parent.rgt
ORDER BY node.lft;

等于

SELECT *
FROM nested_category AS node
LEFT JOIN nested_category AS parent ON (node.lft BETWEEN parent.lft AND parent.rgt)
ORDER BY node.lft;

所以现在我们有所有孩子和他们父母的+ 1（因为我们过滤的方式，每个都child属于它自己）

+-------------+----------------------+-----+-----+-------------+----------------------+------+------+
| category_id | name                 | lft | rgt | category_id | name                 | lft  | rgt  |
+-------------+----------------------+-----+-----+-------------+----------------------+------+------+
|           1 | ELECTRONICS          |   1 |  20 |           1 | ELECTRONICS          |    1 |   20 |
|           2 | TELEVISIONS          |   2 |   9 |           1 | ELECTRONICS          |    1 |   20 |
|           2 | TELEVISIONS          |   2 |   9 |           2 | TELEVISIONS          |    2 |    9 |
|           3 | TUBE                 |   3 |   4 |           1 | ELECTRONICS          |    1 |   20 |
|           3 | TUBE                 |   3 |   4 |           3 | TUBE                 |    3 |    4 |
|           3 | TUBE                 |   3 |   4 |           2 | TELEVISIONS          |    2 |    9 |
|           4 | LCD                  |   5 |   6 |           2 | TELEVISIONS          |    2 |    9 |
|           4 | LCD                  |   5 |   6 |           1 | ELECTRONICS          |    1 |   20 |
|           4 | LCD                  |   5 |   6 |           4 | LCD                  |    5 |    6 |
|           5 | PLASMA               |   7 |   8 |           1 | ELECTRONICS          |    1 |   20 |
|           5 | PLASMA               |   7 |   8 |           5 | PLASMA               |    7 |    8 |
|           5 | PLASMA               |   7 |   8 |           2 | TELEVISIONS          |    2 |    9 |
|           6 | PORTABLE ELECTRONICS |  10 |  19 |           6 | PORTABLE ELECTRONICS |   10 |   19 |
|           6 | PORTABLE ELECTRONICS |  10 |  19 |           1 | ELECTRONICS          |    1 |   20 |
|           7 | MP3 PLAYERS          |  11 |  14 |           7 | MP3 PLAYERS          |   11 |   14 |
|           7 | MP3 PLAYERS          |  11 |  14 |           1 | ELECTRONICS          |    1 |   20 |
|           7 | MP3 PLAYERS          |  11 |  14 |           6 | PORTABLE ELECTRONICS |   10 |   19 |
|           8 | FLASH                |  12 |  13 |           1 | ELECTRONICS          |    1 |   20 |
|           8 | FLASH                |  12 |  13 |           8 | FLASH                |   12 |   13 |
|           8 | FLASH                |  12 |  13 |           6 | PORTABLE ELECTRONICS |   10 |   19 |
|           8 | FLASH                |  12 |  13 |           7 | MP3 PLAYERS          |   11 |   14 |
|           9 | CD PLAYERS           |  15 |  16 |           1 | ELECTRONICS          |    1 |   20 |
|           9 | CD PLAYERS           |  15 |  16 |           9 | CD PLAYERS           |   15 |   16 |
|           9 | CD PLAYERS           |  15 |  16 |           6 | PORTABLE ELECTRONICS |   10 |   19 |
|          10 | 2 WAY RADIOS         |  17 |  18 |           1 | ELECTRONICS          |    1 |   20 |
|          10 | 2 WAY RADIOS         |  17 |  18 |          10 | 2 WAY RADIOS         |   17 |   18 |
|          10 | 2 WAY RADIOS         |  17 |  18 |           6 | PORTABLE ELECTRONICS |   10 |   19 |
+-------------+----------------------+-----+-----+-------------+----------------------+------+------+

下一步 - 获取深度计数。为了做到这一点，我们必须按每个孩子分组（示例使用GROUP BY node.name但它也可以在每个组上完成node.category_id并计算parents- 1的数量(COUNT(parent.name) - 1) AS depth（也可以parent.category_id代替使用）

这样做

SELECT node.*, (COUNT(parent.category_id) - 1) AS depth
FROM nested_category AS node
LEFT JOIN nested_category AS parent ON (node.lft BETWEEN parent.lft AND parent.rgt)
GROUP BY node.category_id
ORDER BY node.lft;

我们得到这个

+-------------+----------------------+-----+-----+-------+
| category_id | name                 | lft | rgt | depth |
+-------------+----------------------+-----+-----+-------+
|           1 | ELECTRONICS          |   1 |  20 |     0 |
|           2 | TELEVISIONS          |   2 |   9 |     1 |
|           3 | TUBE                 |   3 |   4 |     2 |
|           4 | LCD                  |   5 |   6 |     2 |
|           5 | PLASMA               |   7 |   8 |     2 |
|           6 | PORTABLE ELECTRONICS |  10 |  19 |     1 |
|           7 | MP3 PLAYERS          |  11 |  14 |     2 |
|           8 | FLASH                |  12 |  13 |     3 |
|           9 | CD PLAYERS           |  15 |  16 |     2 |
|          10 | 2 WAY RADIOS         |  17 |  18 |     2 |
+-------------+----------------------+-----+-----+-------+

现在是最后一步，说我们只想要这些深度 = 1 的记录（这里使用HAVING depth = 1.HAVING是因为它是在聚合之后应用的（因此它可以过滤聚合））

SELECT node.*, (COUNT(parent.category_id) - 1) AS depth
FROM nested_category AS node
LEFT JOIN nested_category AS parent ON (node.lft BETWEEN parent.lft AND parent.rgt)
GROUP BY node.category_id
HAVING depth = 1
ORDER BY node.lft;

+-------------+----------------------+-----+-----+-------+
| category_id | name                 | lft | rgt | depth |
+-------------+----------------------+-----+-----+-------+
|           2 | TELEVISIONS          |   2 |   9 |     1 |
|           6 | PORTABLE ELECTRONICS |  10 |  19 |     1 |
+-------------+----------------------+-----+-----+-------+

我希望它现在更清楚。如果我犯了一些错误，再次为我糟糕的英语感到抱歉。