hi i am creating a project where i am actually having 3 related tables which are connected to one table like below
table 1
id
name
table 2
id
tb1_id
random_thing
table 3
id
tb1_id
random_thing
i can not basically go with an option where i can create a row in table1 first and then tb2,tb3 . client wants everything to be done on single button . so i am creating a new blank row whenever the page is called and getting the new tb1_id and then linking everything and go with single button but the problem is i can delete unused rows like 2-3 days later but thats ridiculous so is there any other best practices to get over situations such as this?
Edit
Explanation with an example will be really helpful , i am good to go with any database or any language just example has to be good so i can understand how its done. sorry but i am one of those guys who hates theory and love practicals :d
The best practice is to use explicit foreign key relationships and transactions.
So, the basic idea is:
Begin a transaction.
Insert a row in table 1 with the name.
Get the id of the newly created row, ideally using a returning or output clause (depending on the database).
Insert a row in table 2.
Insert a row in table 3.
Commit the transaction.
When using transactions, just be careful to rollback the transaction if it does not complete for any reason.
As for deleting rows, you can have "cascading delete" options on the foreign key definitions, so if the parent row is deleted then the related rows are also deleted.
Some databases (notably Postgres) offer some functionality where you can put all this into a single statement using CTEs that modify the data. The idea is still the same, just easier to code.
I should note that there are perfectly reasonable alternatives. For instance, you could create a view on the "data" columns of the three tables and create insert/update/delete triggers on the view. Personally, I find that hiding this functionality in triggers makes it more difficult to understand and maintain. I think that is a personal opinion and this is also a reasonable approach.
Related
I have seen something like this asked a number of times but not quite in this configuration. I have a table that has a one to many relation.
Let’s say I have a computer table and a parts table. The user enters a generic info in the computer table then selects parts that are stored in the parts table with a relationship to the computer table of computerId. So the original write is a simple insert. Now let’s say the user select the computer again and changes the part on the pc, adds some new, removes some, and updates a few. Then the user hits save to save the changes. I run a simple update on the computer table but now the issue with the parts table.
Would it be better to delete all the records from the parts table for the computer Id and then do a clean insert of all the parts selected.
Or Run some method that would look at the existing parts in the table and where the part has been updated update the record, where the part no longer exists do a delete, and then insert the remaining parts?
Clearly the simple solution is to delete all and then insert all.
The down side of this SQL traffic, locks, and table fragmentation.
If it is small table and only few concurrent users then fine.
In a high volume environment I do the following
There is no update - that is just an ignore
- delete items gone
- ignore any items not changed
- insert new items
And you can do that in one pass two/three statements.
Or you could define a stored procedure.
Do the delete before the insert to clear space first.
You can get real fancy and use an update for delete / insert but that just gets more complex than it is worth in my mind. You would still have an insert or a delete if the item count is not the same.
delete comp_part
where compID = #compID and partID not in (....);
Insert is a little more tricky:
You can to it with a series of inserts and if you have a PK just let the insert fail
The other way is to create a #table and use it for both the delete and insert
This is only worth the hassle if you have a REALLY busy table.
It all depends upon the business model, if you would want to track the transaction than its not a good option to delete it. If you have all your old transactions with your customers than it would be beneficial for tracking purposes., Your CustomerID would be Primarykey and you can have another Unique key as PartOrderID which will be a unique value for each insert.
Hope this helps
Really you should have three tables. Product, Part, and ProductPart; the ProductPart table would store the association of "this product has these parts". As far as updating, the simplest thing would be to delete all ProductParts for a given Product and re-insert the records you want.
I design my database incorrectly, should I fix this while its in development?
"user" table is suppose to have a 1.1 relationship with "userprofile" table
however the actual design the "user" table has a 1.* relationship with "userprofile" table.
Everything works! but should it be fixed anyways?
Do one thing
User Table
Userid(p)
UserName
othercol..
UserProfile
id(p)
UserId(f) - and unique
othercol..
hope this way you can easily fix the isse
Make the user_id in the user_profile table unique and its fixed.
If it's a 1:1 relationship and you often are bringing back records from "user" table and "userprofile" together then you might consider just merging them into one table.
Yes, fix this with a unique index on the FK field. The reason why you need to fix it now is that you can't control how badly people are going to insert data over time when the database is not set up correctly with controls that do not allow the behavior you do not want.
The first time you havea a duplicated record inserted into the child table, you might break a lot of code. With no unique index, the chances of a second record getting inserted can be quite high. You can say, you'll control this at the application level but that is usaully a poor choice as there is no guaranteee that other applications, bulk inserts etc aren't gong to happen that circumvent the application. Putting things right as soon as you can in a database design is critical. It becomes really hard to fix a poor design when there are a lot of records in the database.
#pranay
User Table
Userid(p)
UserName
othercol..
UserProfile
id(p)
UserId(f) - and unique
othercol..
Is that normally how you do it(above)? or do you do this(below)?
User Table
Userid(p)
UserName
othercol..
UserProfile
id(p) <--- userid
othercol..
I have a SQL Server as backend and use ms access as frontend.
I have two tables (persons and managers), manager is derived from persons (a 1:1 relation), thus i created a view managersFull which is basically a:
SELECT *
FROM `managers` `m`
INNER JOIN `persons` `p`
ON `m`.`id` = `p`.`id`
id in persons is autoincrementing and the primary key, id in managers is the primary key and a foreign key, referencing persons.id
now i want to be able to insert a new dataset with a form in ms access, but i can’t get it to work. no error message, no status line, nothing. the new rows aren’t inserted, and i have to press escape to cancel my changes to get back to design view in ms access.
i’m talking about a managers form and i want to be able to enter manager AND person information at the same time in a single form
my question is now: is it possible what i want to do here? if not, is there a “simple” workaround using after insert triggers or some lines of vba code?
thanks in advance
The problem is that your view is across several tables. If you access multiple tables you could update or insert in only one of them.
Please also check the MSDN for more detailed information on restrictions and on proper strategies for view updates
Assuming ODBC, some things to consider:
make sure you have a timestamp field in the person table, and that it is returned in your managers view. You also probably need the real PK of the person table in the manager view (I'm assuming your view takes the FK used for the self-join and aliases it as the ID field -- I wouldn't do that myself, as it is confusing. Instead, I'd use the real foreign key name in the managers view, and let the PK stand on its own with its real name).
try the Jet/ACE-specific DISTINCTROW predicate in your recordsource. With Jet/ACE back ends, this often makes it possible to insert into both tables when it's otherwise impossible. I don't know for certain if Jet will be smart enough to tell SQL Server to do the right thing, though.
if neither of those things works, change your form to use a recordsource based on your person table, and use a combo box based on the managers view as the control with which you edit the record to relate the person to a manager.
Ilya Kochetov pointed out that you can only update one table, but the work-around would be to apply the updates to the fields on one table and then the other. This solution assumes that the only access you have to these two tables is through this view and that you are not allowed to create a stored procedure to take care of this.
To model and maintain two related tables in access you don’t use a query or view that is a join of both tables. What you do is use a main form, and drop in a sub-form that is based on the child table. If the link master and child setting in the sub-form is set correctly, then you not need to write any code and access will insert the person’s id in the link field.
So, don’t use a joined table here. Simply use a form + sub-form setup and you be able to edit and maintain the data and the data in the related child table.
This means you base the form on the table, and not a view. And you base the sub-form on the child table. So, don't use a view here.
I am putting together a schema for a database. The goal of the database is to track applications in our department. I have a repeated problem that I am trying to solve.
For example, I have an "Applications" table. I want to keep track if any application uses a database or a bug tracking system so right now I have fields in the Applications table called
Table: Applications
UsesDatabase (bit)
Database_ID (int)
UsesBugTracking (bit)
BugTracking_ID (int)
Table: Databases:
id
name
Table: BugTracking:
id
name
Should I consolidate the "uses" column with the respective ID columns so there is only one bug tracking column and only one database column in the applications table?
Any best practice here for database design?
NOTE: I would like to run reports like "Percent of Application that use bug tracking" (although I guess either approach could generate this data.)
You could remove the "uses" fields and make the id columns nullable, and let a null value mean that it doesn't use the feature. This is a common way of representing a missing value.
Edit:
To answer your note, you can easily get that statistics like this:
select
count(*) as TotalApplications,
count(Database_ID) as UsesDatabase,
count(BugTracking_ID) as UsesBugTracking
from
Applications
Why not get rid of the two Use fields and simply let a NULL value in the _ID fields indicate that the record does not use that application (bug tracking or database)
Either solution works. However, if you think you may want to occasionally just get a list of applications which do / do not have databases / bugtracking consider that having the flag fields reduces the query by one (or two) joins.
Having the bit fields is slightly denormalized, as you have to keep two fields in sync to keep one piece of data updated, but I tend to prefer them for cases like this for the reason I gave in the prior paragraph.
Another option would be to have the field nullable, and put null in it for those entries which do not have DBs / etc, but then you run into problems with foreign key constraints.
I don't think there is any one supreme right way, just consider the tradeoffs and go with what makes sense for your application.
I would use 3 tables for the objects: Application, Database, and BugTracking. Then I would use 2 join tables to do 1-to-many joins: ApplicationDatabases, and ApplicationBugTracking.
The 2 join tables would have both an application_id and the id of the other table. If an application used a single database, it would have a single ApplicationDatabases record joining them together. Using this setup, an application could have 0 database (no records for this app in the ApplicationDatabases table), or many databases (multiple records for this app in the ApplicationDatabases table).
"Should i consolidate the "uses" column"
If I look at your problem statement, then there either is no "uses" column at all, or there are two. In either case, it is wrong of you to speak of "THE" uses column.
May I politely suggest that you learn to be PRECISE when asking questions ?
Yes using null in the foreign key fields should be fine - it seems superfluous to have the bit fields.
Another way of doing it (though it might be considered evil by database people ^^) is to default them to 0 and add in an ID 0 data row in both bugtrack and database tables with a name of "None"... when you do the reports, you'll have to do some more work unless you present the "None" values as they are as well with a neat percentage...
To answer the edited question-
Yes, the fields should be combined, with NULL meaning that the application doesn't have a database (or bug tracker).
I have many tables in my database which are interrelated. I have a table (table one) which has had data inserted and the id auto increments. Once that row has an ID i want to insert this into a table (table three) with another set of ID's which comes from a form(this data will also be going into a table, so it could from from that table), the same form as the data which went into the first table came from.
The two ID's together make the primary key of the third table.
How can I do this, its to show that more than one ID is joined to a single ID for something else.
Thanks.
You can't do that through a trigger as the trigger only has available to it the data that you already inserted not data that is currenlty only residing in your user interface.
Normally how you handle this situation is that you write a stored proc that inserts the meeting, returns the id value (using scope_identity() in SQL Server, but I'm sure other databases would have method to return the auto-generated id as well). Then you would use that value to insert to the other table with the other values you need for that table. You would of course want to wrap the whole thing in a transaction.
I think you can probably do what you're describing (just write the INSERTs to table 3) in the table 1 trigger) but you'll have to put the additional info for the table 3 rows into your table 1 row, which isn't very smart.
I can't see why you would do that instead of writing the INSERTs in your code, where someone reading it can see what's happening.
The trouble with triggers is that they make it easy to hide business logic in the database. I think (and I believe I'm in the majority here) that it's easier to understand, manage, maintain and generally all-round deal with an application where all the business rules exist in the same general area.
There are reasons to use triggers (for propagating denormalised values, for example) just as there are reasons for useing stored procedures. I'm going to assert that they are largely related to performance-critical areas. Or should be.