Let's say we use an ORM like Doctrine and Symfony2 as Framework.
Whe have three entity (A,B,C) related eache other in that fashion:
A --[1-m]-- B
B --[m-1]-- C
Let's now say that C has an attribute (table column) that tell us the correct order of C records (for user display purposes).
Finally, we have an object of A type fetched using ORM.
What's the best approach - if any - for return C object ordered starting from A ? Have I to write a custom query and, so, lose ORM characteristics?
Notice
If you don't get the problem, let's remember that, for retrieve C object, you have to pass through B objects which could be ordered in different way as order field is onto C object and, of course, "copy" order also onto B object isn't a good solution
SELECT a, b, c
FROM Mapping:EntityA a
JOIN a.b b
JOIN b.c c
ORDER BY b.prop1 ASC, c.prop1 ASC
That's all there is to it really. All c's that belong to a b are ordered, while all b's belonging to an a are ordered as well.
Related
I have three entity types (let's call them A, B, C) persisted on three tables of my database.
Each entity type has a relation with the other two entities. Relations are persisted in three tables of the DB as well (let's call them AB, AC, BC), where every record is a couple of IDs of the respective entities.
Relations A-B are one-to-many and are mandatory: every A has at least one relation with a B, every B has a relation with an A.
Relations A-C and B-C are many-to-many and are optional: there can be As without relation with Cs, there can be Bs without relations with Cs.
I cannot change this schema.
I need to build a table with all the As and their related data. Every row of the table must contain only related data or NULLs where there are no relations.
I thought I would be fine with something like:
SELECT * -- let's omit columns for simplicity
FROM AB
LEFT JOIN BC ON BC.IdB = AB.IdB
LEFT JOIN AC ON AC.IdA = AB.IdA AND
AC.IdC = BC.IdC
INNER JOIN A ON A.Id = AB.IdA
INNER JOIN B ON B.Id = AB.IdB
LEFT JOIN C ON C.Id = AC.IdC
and then filtering with a WHERE clause. My problem is I don't get how, which makes me think I am approaching the problem in the wrong way.
Any hint would be appreciated, thank you in advance.
I need a parse/obj-c alternative preferably server side or client side (iOS) to GROUP BY function in sql.
I understand that there is no GROUP BY function in parse. On SQL I would execute:
select count(*), name, organization from table GROUP BY name
And I would get the number of entries for each unique name.
Since I cant do that on Parse, I am stuck after querying the table.
I have an NSArray whose objects are NSDictionaries which I can access by [valueforKey:#"name"] etc
Name Organization
a b
a b
a b
c d
c d
I need a result table such as
Name Organization Count
a b 3
c d 2
Is it possible at all on parse? If not how can I get this done on obj-c?
Lets suppose that I have a table A with couple of columns. I work with tables, where there is no index on the entries, since they are 'historical' tables. I use one specific column, though, to sort of identify my things. Lets call this ID.
If you'd make a query like the one below, sometimes you'd get one line back, other cases a few.
SELECT * FROM A WHERE ID = '<something>'
Lets say I have two more tables, B and C. Both have ID columns, like A.
Also, some of the IDs in A, are also in B OR C. IDs existing in B CANNOT exist in C. And ALL IDs in A EXIST in either B OR C.
B and C contain extra information, which I'd like to join to A at the same SELECT.
My problem is, that they would only provide extra information. I do not want extra lines in my output..
To make it more clear: my selection from A returns a hundred lines of output.
When I left/right/inner join B table, I —probably— will have less lines as output. Same thing with joining C.
AND FINALLY, my question is:
Is there a way to join table B only on those IDs, which exist in B and vice versa? (And it I would want it in the same SELECT.... statement.)
If you don't want extra lines in your output, you could do something like this:
select *
from A join
(select B.*
from B
group by B.id
) B
on A.id = B.id;
This would choose arbitrary values from B for each id and join them back to A. Is this what you want?
Well it seems like you should build some left join between A and two "Select MAX"s: one from table B, the other one from table C.
And if you do not want 'duplicate' IDs from table A, a 'group by' on table A should help.
This is a SQL design question. First, the setup. I have three tables:
A, which is automatically populated based on a query against a linked server. The data in this table cannot be changed;
B, which has just a dozen or so rows, containing the names for collections of As;
AtoB, which is the mapping table by which As are organized into named collections, with foreign keys on both columns;
For example, A contains:
Giraffe
Owl
Tiger
And B contains:
Seattle Zoo
San Jose Zoo
And AtoB contains:
1,1 (Giraffe in Seattle)
2,1 (Owl in Seattle)
3,1 (Tiger in Seattle)
2,2 (Owl in San Jose)
Now, the problem:
I've been asked to include in some of these collections items not found in A. So, I create a table, C, with the same identity and Name columns as A, and populate it. In keeping with the earlier example, let's say C contains:
Dragon
The question is, how do I include items from C in AtoB? What if I need to include a Dragon in the Seattle Zoo?
My first instinct, being naive, was to create a view V containing the union of A and C, and modifying AtoB to be VtoB. That's where my naivety paid off: one cannot create a foreign key to a view.
I suspect that there's a standard, correct means of relating one or more A OR C with a B.
To expand on Arthur Thomas's solution here's a union without the WHERE in the subselects so that you can create a universal view:
SELECT A.Name as Animal, B.Name as Zoo FROM A, AtoB, B
WHERE AtoB.A_ID = A.ID && B.ID = AtoB.B_ID
UNION
SELECT C.Name as Animal, B.Name as Zoo FROM C, CtoB, B
WHERE CtoB.C_ID = C.ID && B.ID = CtoB.B_ID
Then, you can perform a query like:
SELECT Animal FROM zoo_animals WHERE Zoo="Seattle Zoo"
If you can't put a Dragon in A then you will need to create another table and another link table. The problem is creating a unique set of data that needs to be stored (another table) that cannot be the same set as A. Since it isn't the same set then you can no longer use the link table (AtoB) which has foreign keys that ensure that the link is a reference from set A. So you could create a tables like this:
imaginary_creatures
id
name
imaginary_creatures_to_b
imaginary_creatures_id (link to imaginary_creatures table)
b_id (link to zoos table)
Later when you want to get all creatures in a zoo you can do a UNION
SELECT A.Name FROM A where A.ID IN
(SELECT AB.A_ID FROM AtoB AB WHERE B_ID =
(SELECT B.ID FROM B WHERE B.Name = 'Zoo Name'))
UNION
SELECT i.name FROM imaginary_creatures i i.id IN
(SELECT ic.imaginary_creatures_id FROM imaginary_creatures_to_c ic
WHERE ic.b_id = (SELECT B.ID FROM B WHERE B.Name = 'Zoo Name'))
There may be a better way of writing that, but it should work for your purposes.
Arthur Thomas has a good solution, the other possible solution is to add a column to the link table indicating which table (A or C) it is related to. Then enforce the relationships through triggers rather than foreign keys. But really Arthur's solution is the preferred way of doing this sort of thing.
What you want to do is put Dragon in A, and if you want to select ALL records from A regardless of if they have a matching record in AtoB, do a LEFT OUTER JOIN. Something like this:
SELECT * FROM A
LEFT OUTER JOIN AtoB
ON A.id = AtoB.A_ID
Edit: This would only work if you could add your new records to A. I missed the fact that you are not able to. I think Arthur Thomas's solution is what you want.
Truncate the table dept_details
Display the structure of the table emp_details
Convert the first letter of emp_name into capitals.
I know I'm gonna get down votes, but I have to make sure if this is logical or not.
I have three tables A, B, C. B is a table used to make a many-many relationship between A and C. But the thing is that A and C are also related directly in a 1-many relationship
A customer added the following requirement:
Obtain the information from the Table B inner joining with A and C, and in the same query relate A and C in a one-many relationship
Something like:
alt text http://img247.imageshack.us/img247/7371/74492374sa4.png
I tried doing the query but always got 0 rows back. The customer insists that I can accomplish the requirement, but I doubt it. Any comments?
PS. I didn't have a more descriptive title, any ideas?
UPDATE:
Thanks to rcar, In some cases this can be logical, in order to have a history of all the classes a student has taken (supposing the student can only take one class at a time)
UPDATE:
There is a table for Contacts, a table with the Information of each Contact, and the Relationship table. To get the information of a Contact I have to make a 1:1 relationship with Information, and each contact can have like and an address book with; this is why the many-many relationship is implemented.
The full idea is to obtain the contact's name and his address book.
Now that I got the customer's idea... I'm having trouble with the query, basically I am trying to use the query that jdecuyper wrote, but as he warns, I get no data back
This is a doable scenario. You can join a table twice in a query, usually assigning it a different alias to keep things straight.
For example:
SELECT s.name AS "student name", c1.className AS "student class", c2.className as "class list"
FROM s
JOIN many_to_many mtm ON s.id_student = mtm.id_student
JOIN c c1 ON s.id_class = c1.id_class
JOIN c c2 ON mtm.id_class = c2.id_class
This will give you a list of all students' names and "hardcoded" classes with all their classes from the many_to_many table.
That said, this schema doesn't make logical sense. From what I can gather, you want students to be able to have multiple classes, so the many_to_many table should be where you'd want to find the classes associated with a student. If the id_class entries used in table s are distinct from those in many_to_many (e.g., if s.id_class refers to, say, homeroom class assignments that only appear in that table while many_to_many.id_class refers to classes for credit and excludes homeroom classes), you're going to be better off splitting c into two tables instead.
If that's not the case, I have a hard time understanding why you'd want one class hardwired to the s table.
EDIT: Just saw your comment that this was a made-up schema to give an example. In other cases, this could be a sensible way to do things. For example, if you wanted to keep track of company locations, you might have a Company table, a Locations table, and a Countries table. The Company table might have a 1-many link to Countries where you would keep track of a company's headquarters country, but a many-to-many link through Locations where you keep track of every place the company has a store.
If you can give real information as to what the schema really represents for your client, it might be easier for us to figure out whether it's logical in this case or not.
Perhaps it's a lack of caffeine, but I can't conceive of a legitimate reason for wanting to do this. In the example you gave, you've got students, classes and a table which relates the two. If you think about what you want the query to do, in plain English, surely it has to be driven by either the student table or the class table. i.e.
select all the classes which are attended by student 1245235
select all the students which attend class 101
Can you explain the requirement better? If not, tell your customer to suck it up. Having a relationship between Students and Classes directly (A and C), seems like pure madness, you've already got table B which does that...
Bear in mind that the one-to-many relationship can be represented through the many-to-many, most simply by adding a field there to indicate the type of relationship. Then you could have one "current" record and any number of "history" ones.
Was the customer "requirement" phrased as given, by the way? I think I'd be looking to redefine my relationship with them if so: they should be telling me "what" they want (ideally what, in business domain language, their problem is) and leaving the "how" to me. If they know exactly how the thing should be implemented, then I'd be inclined to open the source code in an editor and leave them to it!
I'm supposing that s.id_class indicates the student's current class, as opposed to classes she has taken in the past.
The solution shown by rcar works, but it repeats the c1.className on every row.
Here's an alternative that doesn't repeat information and it uses one fewer join. You can use an expression to compare s.id_class to the current c.id_class matched via the mtm table.
SELECT s.name, c.className, (s.id_class = c.id_class) AS is_current
FROM s JOIN many_to_many AS mtm ON (s.id_student = mtm.id_student)
JOIN c ON (c.id_class = mtm.id_class);
So is_current will be 1 (true) on one row, and 0 (false) on all the other rows. Or you can output something more informative using a CASE construct:
SELECT s.name, c.className,
CASE WHEN s.id_class = c.id_class THEN 'current' ELSE 'past' END AS is_current
FROM s JOIN many_to_many AS mtm ON (s.id_student = mtm.id_student)
JOIN c ON (c.id_class = mtm.id_class);
It doesn't seem to make sense. A query like:
SELECT * FROM relAC RAC
INNER JOIN tableA A ON A.id_class = RAC.id_class
INNER JOIN tableC C ON C.id_class = RAC.id_class
WHERE A.id_class = B.id_class
could generate a set of data but inconsistent. Or maybe we are missing some important part of the information about the content and the relationships of those 3 tables.
I personally never heard a requirement from a customer that would sound like:
Obtain the information from the Table
B inner joining with A and C, and in
the same query relate A and C in a
one-many relationship
It looks like that it is what you translated the requirement to.
Could you specify the requirement in plain English, as what results your customer wants to get?