MySql：按父组然后子组

Question

I have a parent-child table-structure like:

CREATE TABLE t (
 `id` int(10) unsigned NOT NULL AUTO_INCREMENT,  PRIMARY KEY (`id`),
 `parent` int(10) unsigned DEFAULT NULL, INDEX parent(`parent`),
 `attr` tinyint(1) NOT NULL DEFAULT '0'
);

INSERT INTO t SET id=1, parent=null, attr=1;
INSERT INTO t SET id=2, parent=1, attr=1;
INSERT INTO t SET id=3, parent=1, attr=1;
INSERT INTO t SET id=4, parent=null, attr=0;
INSERT INTO t SET id=5, parent=4, attr=1;
INSERT INTO t SET id=6, parent=4, attr=1;
INSERT INTO t SET id=7, parent=null, attr=1;
INSERT INTO t SET id=8, parent=null, attr=0;

I want to query for "attr=1". The result should be:

• If a parent is found, output the parent and no child

• If a child is found and the parent is not found, show the found childs

So with the test-data above the result should be the ids 1, 5, 6, 7.

I tried it with somehting like:

SELECT id, parent FROM t WHERE attr=1 GROUP BY COALESCE(parent, id);

But this only finds ids 1, 5, 7 and not id 6.

Here 4 will not be taken as the attr = 0

This is only a simplified test-structure, so it would be great if it's possible to avoid joins or sub-selects if possible to keep the query fast.

Answer 1

You might be looking for something like this, using self-join

SELECT COALESCE(parent.id, child.id)
FROM t child
left join t parent on (parent.id = child.parent and  parent.attr = 1 )
WHERE child.attr = 1 
GROUP BY COALESCE(parent.id, child.id)

Answer 2

I have tried with the following and could get the result as expected. You might consider taking a look at it as well.

select id, parent from t
    where attr = 1
    and (parent not in 
        (select id from t where parent is NULL and attr = 1)
        or parent is NULL);

Answer 3

I created your table and tested this, and it returns the rows you describe:

SELECT
    t1.id,
    t1.parent
FROM t as t1
LEFT JOIN t as t2
ON t1.parent = t2.id
WHERE (COALESCE(t1.attr,0) = 1 OR COALESCE(t2.attr,0) = 1)
    AND COALESCE(t2.attr,0) <> COALESCE(t1.attr,0)

Answer 4

select distinct
       case when t.parent is not null and p.attr = 1 then t.parent
            else t.id 
        end id
from t left join t p
on t.parent = p.id
where t.attr = 1