问题
I have this data in a table, for instance,
id name parent parent_id
1 add self 100
2 manage null 100
3 add 10 200
4 manage null 200
5 add 20 300
6 manage null 300
How can I left join or inner join this table itself so I get this result below?
id name parent
2 manage self
4 manage 10
6 manage 20
As you can I that I just want to query the row with the keyword of 'manage' but I want the column parent's data in add's row as the as in manage's row in the result.
Is it possible?
EDIT:
the simplified version of my actual table - system,
system_id parent_id type function_name name main_parent make_accessible sort
31 30 left main Main NULL 0 1
32 31 left page_main_add Add self 0 1
33 31 left page_main_manage Manage NULL 0 2
my actual query and it is quite messy already...
SELECT
a.system_id,
a.main_parent,
b.name,
b.make_accessible,
b.sort
FROM system AS a
INNER JOIN -- self --
(
SELECT system_id, name, make_accessible, sort
FROM system AS s2
LEFT JOIN -- search --
(
SELECT system_id AS parent_id
FROM system AS s1
WHERE s1.function_name = 'page'
) AS s1
ON s1.parent_id = s2.parent_id
WHERE s2.parent_id = s1.parent_id
AND s2.system_id != s1.parent_id
ORDER BY s2.sort ASC
) b
ON b.system_id = a.parent_id
WHERE a.function_name LIKE '%manage%'
ORDER BY b.sort ASC
result I get currently,
system_id main_parent name make_accessible sort
33 NULL Main 0 1
but I am after this,
system_id main_parent name make_accessible sort
33 self Main 0 1
回答1:
You just need to reference the table twice:
select t1.id, t1.name, t2.id, t2.name
from TableA t1
inner join TableA t2
on t1.parent_id = t2.Id
Replace inner with left join if you want to see roots in the list.
UPDATE:
I misread your question. It seems to me that you always have two rows, manage one and add one. To get to "Add" from manage:
select system.*, (select parent
from system s2
where s2.parent_id = system.parent_id
and s2.name = 'add')
AS parent
from system
where name = 'manage'
Or, you might split the table into two derived tables and join them by parent_id:
select *
from system
inner join
(
select * from system where name = 'add'
) s2
on system.parent_id = s2.parent_id
where system.name = 'manage'
This will allow you to use all the columns from s2.
回答2:
Your data does not abide to a child-parent hierarchical structure. For example, your column parent holds the value 10, which is not the value of any id, so a child-parent association is not possible.
In other words, there's nothing that relates the record 2,manage,null to the record 1,add,self, or the record 4,manage,null to 3,add,10, as you intend to do in your query.
To represent hierarchical data, you usually need a table that has a foreign key referencing it's own primary key. So your column parent must reference the column id, then you can express a child-parent relationship between manage and add. Currently, that's not possible.
回答3:
UPDATED: Joining by parent_id, try:
select m.id, m.name, a.parent
from myTable m
join myTable a on m.parent_id = a.parent_id and a.name = 'add'
where m.name = 'manage'
Change the inner join to a left join if there may not be a corresponding add row.
来源：https://stackoverflow.com/questions/9824948/how-to-left-join-or-inner-join-a-table-itself