I currently have a MySQL table called Section, and each entry in Section has a bunch of rows in the News table. In each News entry, I have a bunch of rows in Comments table.
It's a many to one relationship between News and Section, and similarly many to one between Comments and News. This is a unidirectional relationship from the children.
This is probably very easy, but I'm new to HQL, how would I query for all the Comments for a particular Section? Is it a bunch of joins of recursive selects?
I apologize I don't know what this is called. Is it cascading?
The concept of cascading is about updating/inserting/removing a group of related entities , querying has nothing to do with the cascading in JPA /Hibernate
In your case , you can use fetch join to issue a single JOIN SQL to get all the children for a given parent .
The difference between fetch join and normal join is that fetch join will also cause all children objects to be initialized along with their parent in a single select such that you can avoid the potential N+1 problem when you loop through the children of the parent.
You can issue the fetch join using something like the following syntax:
from Section as section
left join fetch section.news as new
left join fetch new.comments
Related
I have a scenario where I have the following tables:
Inventories
delivery_items
deliveries
I seek a query where, having the inventory id, I get the delivery_item(fk_inventory),
which then I get the delivery from the (fk_delivery).
Manually, I go to the delivery_items table, then I search for the fk_inventory that matches the id from the inventory that I'm looking for,
then I get the fk_delivery, and get the delivery.
But I need to run a report on 15k+ items.
How to write a query where from a list of inventory ids I can get to the delivery following the relationship that I mentioned above?
There are many sites on writing SQL queries, differentiating between a normal (inner) join vs outer join, left join, right join, subqueries, etc. What you are looking to do is probably best (due to all inventory items in question) is simple joins.
Try to think of it this way, and maybe do it this way. Have a sheet of paper, one representing each table and write the columns on it.
Now, visually looking at the available tables, put them next to each other based on how they are related. Note the column in table A that is the foreign key to the next table. Then again, from the second to the third.
Once you have this done (or even if just mentally), you can SEE how they are related. This is the basis of the FROM clause
select *
from
YourFirstTable yft
JOIN YourSecondTable yst
on yft.WhateverKey = yst.MatchingKeyColumn
JOIN YourThirdTable ytt
on yst.KeyToThirdTable = ytt.KeyInThisTable
Now that you have all your relationships established, you can always declare the individual columns you want from those respective tables. Easier to use with the aliases such as I provided here via yft, yst, ytt representing the first, second and third tables. Use aliases appropriate to your tables such as i=inventories, di = delivery_items, d = deliveries.
Then add whatever FILTERING conditions you want. If the condition is based on the FIRST Table such as yft above, that would go into the WHERE clause such as
where
yft.SomeColumn = 'blah'
If the filtering criteria is specific to your second or third table, just add that to the JOIN / ON condition so it stays with the table and you know contextually it is associated HERE. It makes it easier when you are getting into LEFT JOINs.
from
YourFirstTable yft
JOIN YourSecondTable yst
on yft.WhateverKey = yst.MatchingKeyColumn
AND yst.SecondTableColumn = 'someOtherValue'
AND yst.SomeOtherColumn = 'somethingElse'
So now, the engine can go through all inventory items, to the corresponding details, to the actual deliveries without having to do individual searches each time which would be painful to trace / run / and performance.
Something really bugs me and im not sure what is the "correct" approach.
If i make a select to get contacts from my database there are a decent amount of joins involved.
It will look something like this (around 60-70 columns):
SELECT *
FROM contacts
LEFT JOIN company
LEFT JOIN person
LEFT JOIN address
LEFT JOIN person_communication
LEFT JOIN company_communication
LEFT JOIN categories
LEFT JOIN notes
company and person are 1:1 cardinality so its straight forward.
But "address", "communication" and "categories" are 1:n cardinality.
So depending on the amount of rows in the 1:n tables i will get a lot of "double" rows (I don't know whats the real term for that, the rows are not double i know that the address or phone number etc is different). For myself as a contact, a fairly filled contact, i get 85 rows back.
How do you guys work with that?
In my PHP application i always wrote some "Data-Mapper" where the array key was the "contact.ID aka primary" and then checked if it exists and then pushed the additional data into it. Also PHP is not really type strict what makes it easy.
Now I'm learning GO(golang) and i thought screw that LOOOONG select and data mapping just write selects for all the 1:n.... yeah no, not enough connections to load a table full of contacts. I know that i can increase the connections but the error seems to imply that this would be the wrong way.
I use the following driver: https://github.com/go-sql-driver/mysql
I also tried GROUP_CONCAT but then i running in trouble parsing it back.
Do i have to do my mapping approach again or is there some nice solution out there? I found it quite dirty at points tho?
The solution is simple: you need to execute more than one query!
The cause of all the "duplicate" rows is that you're generating a result called a Cartesian product. You are trying to join to several tables with 1:n relationships, but each of these has no relationship to the other, so there's no join condition restricting them with respect to each other.
Therefore you get a result with every combination of all the 1:n relationships. If you have 3 matches in address, 5 matches in communication, and 5 matches in categories, you'd get 3x5x5 = 75 rows.
So you need to run a separate SQL query for each of your 1:n relationships. Don't be afraid—MySQL can handle a few queries. You need them.
i have a MySQL statement which works - i can get the records requested - movies.* & groups.name.
$stmt= $mysqli->query("SELECT DISTINCT ebooks.*, groups.name FROM ebooks
INNER JOIN ebooks_groups ON ebooks.uuid = ebooks_groups.ebookuuid
INNER JOIN groups_users ON ebooks_groups.groupuuid = groups_users.groupuuid
INNER JOIN groups ON groups_users.groupuuid = groups.uuid
WHERE useruuid=".$get_useruuid."
ORDER BY groups.name");
1/ However i need to grab another column from the groups table - namely groups.uuid
i tried
SELECT DISTINCT movies.*, groups.* FROM movies, groups
&
SELECT DISTINCT movies.*, groups.name, groups.uuid FROM movies, groups
but it retrieved no records.
2/ Then I had another look at my original code - ... FROM movies ... - how is this even working if i'm not selecting FROM movies, groups tables?
AFAIK, this is pure MySQL. PHP or not doesn't come into play.
First to understand is the implicit join:
Explicit vs implicit SQL joins
That understanding should solve at least half of your problem.
Secondly, I'd never code a SELECT * without a very good reason (and there's few). It makes much more sense to select just the columns you need instead of getting them all and even if you need all that are currently there, if you work on the database model later on, there might be more (or less!!) columns in the database and it'll be much harder to detect that your code needs updating if you don't have them explicitly listed.
For the rest I build my SQL queries slowly step by step. That helps a lot to debugging your queries esp. as you have the actual tables and some sample data ...
[That should solve your other half of the question]
Suppose I have a table that acts as an inventory of my house - inventory_items if you will. inventory_items contains everything I own, but only the most general information (i.e fields that will apply to everything I own, like a name, purchase date).
I then wish to have a separate table for electronics_data which is an inventory item, but has special information to store (lets say serial_number, wattage) and another for furniture_data which contains furniture specific information (number_of_legs, material).
In all instances, items in electronics_data will have a matching item in inventory_items linked by an id field. The same is true of furniture_data.
If I now wish to show a list of my inventory items, but include specific information from the child tables, logically I think to load the inventory_data, find out what type of item this is, and load the right information from the right table. I can think of two better ways:
1) Create a foreign key relationship between inventory_items and electronics_data - thus loading all items will get me all of my child data too. But, not all items in inventory_items will have a matching item in electronics_data so does this mean a foreign key can't work?
2) Create a view which loads the extra tables if a matching item exists in them, and load the view in my application. If I have lots of different 'types' of data, will this make my view unnecessarily slow (checking everything) and actually defeat the object of the view in the first place?
These are general questions - particularly 2) I would imagine is very data dependent.
Thanks!
1) Foreign keys will work, since the specialised tables are the child tables, so you need to make sure that each record in the child table has a corresponding record in the overall inventory_items table. The reverse is not necessarily true.
2) The view can left join the child tables on the inventory_items table. If the fields used in the join are indexed in all tables, then the operation is not that resource intensive. The biggest catch could be how you build the view, if you have lots of specialised child tables. But this is probably a wider application design question anyway (if you are looking at your electronic devices, then you probably do not want to see the fields from the furniture items table - in these specialised views I would use inner join, not left join).
well it will make your life easier if you could join the tables when extracting data. There are a lot of ways to join tables, in your case if all your tables have an I.D column then you could use an 'Equijoin' This is how you could do so
SELECT inventory_items.name, electronics_data.wattage, furniture_data.material
FROM inventory_items, electronics_data, furniture_data
WHERE inventory_items.i.d=electronics_data.i.d=furniture_data.id;
so with a join like this you can add as many columns as you wish but make sure to highlight the table they are from and in the 'WHERE' clause show where they are equal otherwise it wont return any data
I have posted an fairly detailed response to a similar question here, even how to define the views you mention. Note that the code shown in the view definition is for illustration only. It will not show the most efficient way to write it. Better ways should be fairly straight-forward, however.
A word about view performance. Take a view that joins very large tables in such a way that the query
select * from <view>
takes a long time, say 30 minutes. The query
select * from <view> where <criteria>
could take fractions of a second. In most modern DBMSs, the where criteria is merged with the existing query in the view definition to execute the query. It does not execute the view definition and then do the filtering. So test view performance with actual queries not "data dump" queries.
Usually linq is using an left outer join for its queries but on some cases it decides to use inner join instead.
I have a situation where that decision results in wrong results since the second table doesn't always have suitable records and that removes the records from the first table.
I'm using a linqdatasource over a dbml where the relevant tables are identical but one holds historical records removed from the first. both have the same primary key. and I'm using a dataloadoption to load both tables at once with out round trips.
Would you explain why linq decided to use an inner join here?
Thanks
No, unfortunately you are incorrect. The LINQ Join operator always does an inner join. http://www.hookedonlinq.com/JoinOperator.ashx
If you want to do a left outer join, you need to use a combination of select or foreach. See these examples:
http://solidcoding.blogspot.com/2007/12/left-outer-join-in-linq.html
http://social.msdn.microsoft.com/Forums/en-US/linqprojectgeneral/thread/c139313e-d745-4e1d-b3dc-ab355507eb48
or google it, there are a ton of examples out there.
It is better practice not to use the join operator where possible, and rely on traversing the relationships set up in the data context between the entities.
from r in Rabbits
select
{
r.Name
r.Parent.Name
}
This will automatically traverse the parent relationship and decide on the appropriate sql to run.
Using .DefaultIfEmpty() appropriately will mean that any NULL entries are added, hence it is translated into the appropriate outer join. This can be used with the join operator by including an 'into'.
JOIN and LEFT JOIN equivalent in LINQ