MySQL的合并，从多个字段

Question

获取COUNT说我有一个表：

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，这是跟踪父子层次，我想有多少孩子没有父母都有。下面是输出我想：

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

我想算合并LEV1，LEV2，LEV3和lev4领域指望有多少ID的所​​有在所有这些领域。

我阅读了关于UNION ALL的内容，但似乎无法弄清楚它是如何运作的。我在想自己加入一个联盟？

Answer 1

对于每个levN列，您需要一个LEFT JOIN针对子查询，该列将返回不同的级别，并为该列计数。然后他们全部加入并加入到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

Answer 2

我可以帮你部分存在，但我并没有显示计数为零东西。 （另外，正如@RaphaëlAlthaus指出的那样，父母1在您的数据中有6位不计数）。

sqlite> .schema 
CREATE TABLE tmp (
user int, 
parent int, 
l1 int, 
l2 int, 
l3 int, 
l4 int 
); 
sqlite> select * from tmp; 
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 
sqlite> select who,count(who) from 
    ...> (select l1 as who from tmp union all 
    ...> select l2 as who from tmp union all 
    ...> select l3 as who from tmp union all 
    ...> select l4 as who from tmp) 
    ...> where who <> 0 
    ...> group by who; 
1,6 
2,4 
4,3 
5,1 
sqlite>