I'm not entirely sure how to phrase what I mean. Let me try: Is there a way to select all elements that cascade by a reference field?
For instance, I have the rows:
parentRef | Reference | Data
------------------------------
aContainer | mainObj | "Parent"
mainObj | secondObj | "Child 1"
secondObj | thirdObj | "Child 2"
nonExistent | blankObj | "Don't select me!"
And I want to select mainObj, secondObj, thirdObj in a single statement when I only know one parentRef: "aContainer". Is this possible?
I could do this by having my code perform many queries: select...where parentRef = 'aContainer', then select...where parentRef = 'mainObj', etc, but I really don't want to hammer my DB with many queries, primarily for speed.
Edit: Tree Queries! That's the search term I needed.
Oracle can do Tree Queries, have a look at START WITH and CONNECT BY
If I understand you right, you want something like a correlated query. This will allow you to get only items that have a parent reference in the table. an example would look something like the following (although I might have reversed the logic):
select
parentRef
, Reference
, Data
from mytable parentTable
where Reference in (
select
reference
from mytable childTable
where childTable.reference = parenttable.parentref)
Related
How to get a unique, identical value of a table?
For example, if there are tables like 't_aa', 't_bb', 't_cc', I want a result like below.
id | table_name
-------------------
1 | 't_aa'
2 | 't_bb'
3 | 't_cc'
What I exactly want is to get a specific, and unique number from the name of tables.
I have tried
SELECT * FROM information_schema.tables;
-- or
SELECT * FROM pg_catalog.pg_tables;
but this doesn't provide any identical numbers to me.
I hope there is some way to get results like above by using some lines of query,
but if I really have to make a new table for this, that could be okay as an alternative.
please help me, thank you
-- edit
I need numbers because I will use it as an advisory lock key for some reasons.
ThIs is it:
SELECT table_name,ROW_NUMBER () OVER (
ORDER BY table_name
) as id FROM information_schema.tables;
I've been working on a SQL query which needs to pull a value with a two-column key, where one of the columns may be null.And if it's null, I want to pick that value only if there is no row with the specific key
So.
CUSTOM_____PLAN_____COST
VENDCO_____LMNK_____50
VENDCO_____null_____25
BALLCO_____null_____10
I'm trying to run a query that will pull this into one field, i.e., the value of VENDCO at 50, and the value of BUYCO at 10, ignoring the VENDCO row with 25. This would be as part of a joined subquery, so I can't use the actual keys of VENDCO/BUYCO etc. Essentially, pick the cost value with the plan if it exists, but the one where it's null if the plan is not there.
It might also be worthwhile to point out that if I "select * from table where PLAN is null" I don't get results -- I have to select where PLAN=''. I'm not sure if that indicates anything weird about the data.
Hope I'm making myself clear.
I think that not exists should do what you want:
select t.*
from mytable t
where
plan is not null
or not exists (
select 1 from mytable t1 where t1.custom = t.custom and t1.plan is not null
)
Basically this gives priority to rows where plan is not null in groups sharing the same custom.
Demo on DB Fiddle:
CUSTOM | PLAN | COST
:----- | :--- | ---:
VENDCO | LMNK | 50
BALLCO | null | 10
I'm having a bit of a hard time trying to piece this together. I'm not adept with databases or complex queries.
The Database
I'm using the latest MariaDB release.
I have a database table configuration like so, representing a hierarchical data structure:
|----------------------|
| fieldsets |
|----+-----------------|
| id | parent_field_id |
|----+-----------------|
| 1 | NULL |
| 2 | 1 |
|----------------------|
|-------------------------|
| fields |
|----+--------------------|
| id | parent_fieldset_id |
|----+--------------------|
| 1 | 1 |
| 2 | 1 |
|-------------------------|
The Problem
I'm trying to piece together a recursive query. I need to select every fieldset in a given hierarchy. For example, in the above, stripped-down example, I want to select fieldset of id = 1, and every descendant fieldset.
The IDs of the next rung down in any given level in the hierarchy are obtained only via columns of a secondary table.
The table fieldsets contains no column by which I can directly get all child fieldsets. I need to get all fields that are a child of a given fieldset, and then get any fieldsets that are a child of that field.
A Better Illustration of the Problem
This query does not work because of the reported error: "Restrictions imposed on recursive definitions are violated for table all_fieldsets"
However, it really illustrates what I need to do in order to get all descendant fieldsets in the hierarchy (remember, a fieldset does not contain the column for its parent fieldset, since a fieldset cannot have a fieldset as a direct parent. Instead, a fieldset has a parent_field_id which points to a row in the fields table, and that row in the fields table correspondingly has a column named parent_fieldset_id which points to a row back in the fieldsets table, which is considered the parent fieldset to a fieldset, just an indirect parent.
WITH RECURSIVE all_fieldsets AS (
SELECT fieldsets.* FROM fieldsets WHERE id = 125
UNION ALL
SELECT fieldsets.* FROM fieldsets
WHERE fieldsets.parent_field_id IN (
SELECT id FROM fields f
INNER JOIN all_fieldsets afs
WHERE f.parent_fieldset_id = afs.id
)
)
SELECT * FROM all_fieldsets
My Attempt
The query I have thus far (which does not work):
WITH RECURSIVE all_fieldsets AS (
SELECT fieldsets.* FROM fieldsets WHERE id = 125
UNION
SELECT fieldsets.* FROM fieldsets WHERE fieldsets.id IN (SELECT fs.id FROM fieldsets fs LEFT JOIN fields f ON f.id = fs.parent_field_id WHERE f.parent_fieldset_id = fieldsets.id)
)
SELECT * FROM all_fieldsets
My Research
I'm also having a hard time finding an example which fits my use-case. There's so many results for hierarchical structures that involve one table having only relations to itself, not via a secondary table, as in my case. It's difficult when you do not know the correct terms for certain concepts, and any layman explanation seems to yield too many tangential search results.
My Plea
I would be enormously grateful to all who can point out where I'm going wrong, and perhaps suggest the outline of a query that will work.
The main problem I see with your current code is that the recursive portion of the CTE (the query which appears after the union) is not selecting from the recursive CTE, when it should be. Consider this updated version:
WITH RECURSIVE all_fieldsets AS (
SELECT * FROM fieldsets WHERE id = 125
UNION ALL
SELECT f1.*
FROM fieldsets f1
INNER JOIN all_fieldsets f2
ON f1.parent_field_id = f2.id
)
SELECT *
FROM all_fieldsets;
Note that the join in the recursive portion of the CTE relates a given descendant record in fieldsets to its parent in the CTE.
I got home from work, and I just could not set this down!
But, out of that came a solution.
I highly recommend reading this answer about recursive queries to get a better idea of how they work, and what the syntax means. Quite brilliantly explained: How to select using WITH RECURSIVE clause
The Solution
WITH RECURSIVE all_fieldsets AS (
SELECT * FROM fieldsets fs
WHERE id = 59
UNION ALL
SELECT fs.* FROM fieldsets fs
INNER JOIN all_fieldsets afs
INNER JOIN fields f
ON f.parent_fieldset_id = afs.id
AND fs.parent_field_id = f.id
)
SELECT * FROM all_fieldsets
I had to use joins to get the information from the fields table, in order to get the next level in the hierarchy, and then do this recursively until there is an empty result in the recursive query.
I want to create a tree structure of categories and need to find a proper way to store it into the database. Think of the following animal tree, which pretty accurately describes how it should look like:
My question now is whether chaining those entries within the same table is a good idea or not. SQLite doesn't allow me to add a FOREIGN KEY constraint to a value in the same table, so I have to make sure manually that I don't create inconsistencies. This is what I currently plan to have:
id | parent | name
---+--------+--------
1 | null | Animal
2 | 1 | Reptile
3 | 2 | Lizard
4 | 1 | Mammal
5 | 4 | Equine
6 | 4 | Bovine
parent references to an id in the same table, going up all the way until null is found, which is the root. Is this a bad pattern? And if so, what are common alternatives to put a tree structure into a relational database?
If your version of SQLite supports recursive CTE, then this is one option:
WITH RECURSIVE cte (n) AS (
SELECT id FROM yourTable WHERE parent IS NULL
UNION ALL
SELECT t1.id
FROM yourTable t1
INNER JOIN cte t2
ON t1.parent = t2.n AND t1.name NOT LIKE '%Lizard%'
)
SELECT *
FROM yourTable
WHERE id IN cte;
This is untested, but the check on t1.name in the recursive portion of the above CTE (hopefully) should stop the recursion as soon we reach a record which matches the name in the LIKE expression. In the case of searching for Lizard, the recursion should stop one level above Lizard, meaning that every record above it in the hierarchy should be returned.
I have the follwing given two tables which can not be changed.
1: DataTypes
+----------------------+-----------------------+
| datatypename(String) | datatypetable(String) |
+----------------------+-----------------------+
Example data:
+-----------+------------+
| CycleTime | datalong |
+-----------+------------+
| InjTime1 | datadouble |
+-----------+------------+
2: datalong_1 (data model does not matter here)
I want to make a query now that reads the datatypetable attribute from the datatypes table, adds the String "_1" to it and selects all content from it.
I imagined it, from a programmatic perspective, to look something similar to this statement which obviously doesn't work yet:
SELECT * FROM
(SELECT datatypetable FROM datatypes WHERE datatypename = 'CycleTime') + '_1'
How can I make this happen in SQL using HSQLDB?
Thanks to Leonidas199x I know now how to get in the '_1' in but how do I tell the FROM statement that the subselect is not a new table I want to read from but instead the name of an existing table I want to read from.
SELECT * FROM
(SELECT RTRIM(datatypetable)+'_1' FROM datatypes WHERE datatypename = 'CycleTime')
According to this question which is identical to mine this is not possible:
using subquery instead of the tablename
:(
Can you explain your data model in a little more detail? I am not sure I understand exactly what it is you are looking to do.
If you are wanting to add _1 to the 'datatypename', you can use:
SELECT datatypename+'_1'
FROM datatypes