MySQL 从组合的多个字段中获取 COUNT

Question

假设我有一张 table :

user_id    parent_id    lev1    lev2   lev3    lev4
1          0            0       0      0       0
2          1            1       0      0       0
3          1            1       0      0       0
4          2            2       1      0       0
5          4            4       2      1       0
6          4            4       2      1       0
7          5            5       4      2       1

基本上，这是为了跟踪父子层次结构，我想知道 parent 有多少个 child 。下面是我想要的输出:

parent_id     children
1             5
2             4
3             0
4             3
5             1
6             0
7             0

我想计算组合的 lev1、lev2、lev3 和 lev4 字段，以计算这些字段中总共有多少个 ID。

我阅读了有关 UNION ALL 的信息，但我似乎无法弄清楚它是如何运作的。我在考虑一个带有 self JOIN 的 UNION ALL？

Answer 1

您需要针对每个 levN 列的子查询进行 LEFT JOIN，这会返回不同的级别，并对该列进行计数。然后将它们全部加起来并加入 user_id。

SELECT
  DISTINCT
  user_id,
  /* COALESCE() is needed so NULLs don't ruin the calculation */
  COALESCE(l1count, 0) +
  COALESCE(l2count, 0) +
  COALESCE(l3count, 0) +
  COALESCE(l4count, 0) AS children
FROM
  yourtable
  /* a left join individually against each of the `levN` columns to get the count per value of each */
  LEFT JOIN (SELECT lev1, COUNT(*) AS l1count FROM yourtable GROUP BY lev1) l1 ON yourtable.user_id = l1.lev1
  LEFT JOIN (SELECT lev2, COUNT(*) AS l2count FROM yourtable GROUP BY lev2) l2 ON yourtable.user_id = l2.lev2
  LEFT JOIN (SELECT lev3, COUNT(*) AS l3count FROM yourtable GROUP BY lev3) l3 ON yourtable.user_id = l3.lev3
  LEFT JOIN (SELECT lev4, COUNT(*) AS l4count FROM yourtable GROUP BY lev4) l4 ON yourtable.user_id = l4.lev4

http://sqlfiddle.com/#!2/214a8/16