Let me describe briefly the table structures:
Customer Table
id | name | address_line_one | address_line_two | contact_no_one
SaleInvoice Table
id | id_Customer (Foreign Key) | invoice_no
If I have to print a Sale invoice, I have to use the Customer information (like name, address) from the Customer table.
Assume that after a year, some customer data changes (like name or address), and I update the new data in my customer table. Now, if the customer asks for an old invoice, it will be printed with the new customer data which shall be legally wrong.
Does that mean, I have to create
name_customer
address_line_one_customer
...
and all these fields in the Sale Invoice table too?
If yes, is there a better way to get data from these fields in Customer table to the Sale Invoice table then to write a SQL query to get the values and then set the values?
This is really up to you. In some cases, where it is a legal document, you will save all the details so that you can always bring it up the way it was created. Alternatively if you are producing pdf invoices then save them to be 100% sure.
The other alternative is to create a CustomerHistory table, so that past versions are always saved with a date range, so that you can go back to the old version.
It depends on the use cases, but those are your main options.
It sounds like a problem easily solved by placing the Employee table in version normal form (VNF). This is actually just a flavor of 2nf but done in a way that provides the ability to query current data and past data using the same query.
A datetime parameter is used to provide the distinction. When the value is set to NOW, the current data is returned. When the value is set to a specific datetime value in the past, the data that was current at that date and time is returned.
A brief discussion of the particulars can be found here. That answer also contains links to more information if you think it is something that would work for you.
Related
I'm looking for the best way to store this information. Not every course has an expiry date.
The easiest way I've found so far is:
tblEmployee
-----------
ID (pk)
Expiry1
Expiry2
tblCourseCatalog
----------------
CourseID(pk)
Name
For every course in tblCourseCatalog, a new Expiry is created in tblEmployee to match tblCourseCatalog.CourseID.
I tried to have:
tblCourseExpiryDates
--------------------
EmployeeID (pk) 1:1 with tblEmployee.ID
FirstAid
UnderWaterBasketWeaving
Anytime a new course was added to tblCourseCatalog, a new column was added to tblCourseExpiryDates to match. This became tricky when trying to query some info. Does my current way (Expiry in tblEmployee) change things much from having tblCourseExpiryDates? to me, having a Expiry2 column is a waste if tblCourseCatalog.CourseID=2 (UnderWaterBasketWeaving) does not expire.
The standard normalised way to store something like this is to have a table where every row looks at just one course employee combination, holds any data that is specific to just that combination, and is usually called something like EmployeeCourse (or CourseEmployee)
EmployeeID
CourseID
ExpiryDate
You only put records in this table where you actually have a date that is valid. For a course that has no dates, no employee would ever get a record. If a given employee has never done a course, they get not record. If you want to remove a date, you can remove the record, or just remove the date but leave the record (I'd probably remove it). If you want to add a new course, you just put a record into the Course table and you're done - you don't have to change anything else.
When you need to look up the record, you need to join to the table with an outer join, so that you get any records in the main table that have no CourseEmployee record.
The downside of this normalised data is that it does make it harder to get a list of all the expiry dates for an employee in one row of output - this is where pivot tables come in (and I'm not sure how they work in access).
If you want to read more about this, look up database normalisation.
I want to find record of particular table whose particular field is modified yesterday and also find current value of field and old value.
For example, I have company table with fields id, name and address. I want to track name field and find current value and old value if it is modified. I will run this at end of the day.
In web2py, If we edit record then two rows are created in archive table, first one contains old data and second one is current record. Can I assume that old row and current record row are always back to back? or There can be row between them? did web2py uses locks while writing into archive table? Please explain how archive entries are created.
Please help!
Locks or no locks, I don't think you should use query on just the name field and then go back one row and get the old name. This might create problems for you if the name is not unique (which usually isn't).
A better way to do this is to store the id of the company table in your archive table and use that to query the old name.
So your archive table should now look like this,
| id | name | company_id |
Then using a query on the archive table you can get all rows for a given company_id:
rows = (db.archive.company_id == company_id).select()
And, to get the old name you will have to do something like,
old_name = rows[len(rows) - 1].name
This will return you the last name that you were originally looking for. I hope this helps.
I have a table Salary with a column PersonalId and a table Person with a column Name.
In the first table salary data will saved with a PersonalId which relates it to the Person table. In salary bill all data will gather together and Person name will be referenced from Person table.
After 1 year a specific person name will change from Michael to Maic. Now I want the last year salaries bill remain with previous person name Michael and the new salaries bill generate by new name Maic.
How we can do that?
It could depend on what type of operation you need to to most and on how much people change their name, because the number of joins you may need to make could vary a lot.
keep a field in Person that points to the next Person which is a change of name
keep another key in Person that varies only for the physical person
keep a limited number of names in Person that someone could dispose of, with an index of the current name
in another table you keep the relations between the various name of the Person
It could depend on what rules of normalization you follow, for now I'm not thinking about that.
Anyway, with the first case you don't need to change Salary, but to reconstruct the identity of a Person you need multiple requests or at least a stored procedure.
In the second case you still don't need to change Salary because you add a field to Person, but to get all the Salary entries for that physical person you'll need some work, again probably a stored procedure to get the added field and then something that joins all the Salary entries.
The third maybe is the simplest, but also the limited one, and you need in Salary another field that tells the index of the name to use in that entry.
The last case gives you a stable identity, but it may need some work because of the added table, and still there are multiple implementations. You could have salary reference that table instead of Person, or you could consult that table only when you need all the data, but you cannot reference its primary key from Salary because it would not permit to discriminate the name.
Lunadir's right in a certain way -- but all of those approaches are complex, and of rather great difficulty.
The other way -- simpler, and perhaps more correct & robust -- is to keep NAME and PAID_DATE columns in Salary or SalaryPaid, and write the actual name & date paid at the time the payment is made.
Good old batch-processing style -- and it has the benefit of actually capturing the key financial facts, of what payment was made & what name it was made to, which are the actual auditable transaction history.
Do you pay each Salary entry individually, or in bunch (PaySlip or SalaryPaid)? Put the NAME column wherever you record the actual payment & timestamp it occurred.
I am very new to Access and I am working on a database and I need help coming up with a solution:
I am recording data from a bunch of asphalt laying crews. Each crew has a record with a field for production and equipment. Each crew has varying types of equipment and varying quantities of equipment. Therefore, I would need to create a new table for the type and quantity of equipment every time I enter a new record... can someone please help me come up with a solution?
You do not need a new table for each record, you just need a properly set up table. Let us say:
Crews table
CrewID
Location
Etc
CrewMembers table
MemberID
Etc
CrewEquipment table
CrewID
EquipmentID
DateIn
DateOut
Etc
Equipment table
EquipmentID
Details
Etc
You might like to read http://r937.com/relational.html
With the above set-up, you can have a Crew form with subforms for members and equipment. You can get an idea from this create form to add records in multiple tables
Creating new table everytime is not solution, you should clear some RDBMS concept like normalization first. Create separate table for
crew member (which include crew member id, his name, salary/wages
List item per hour) equipments (which include equipments id, operation cost per hour etc)
Shift (can be separated by date and shift time etc)
Then create proper relationship between tables and this way you can create proper relational database system. so finish some basic tutorial first then start development.
The invoice database design, might look something like this...
http://www.databaseanswers.org/data_models/invoices_and_payments/index.htm
Now If the user decides to change/revise the product code/description
It will change the previous order and invoice produce code/description :(
What do you do? Copy the product code description to the invoice table instead?
You basically have two options:
either you make your Products table "time-enabled" (also known as "temporal database"), e.g. you keep the "previous" state of your individual product in your table, and you give every entry a ValidFrom / ValidTo pair of dates. That way, if you change your product, you get a new entry, and the previous one remains untouched, referenced from those invoices that used it; only the ValidTo date for the product gets updated
or:
you could copy the products (at least those bits you need for your invoice) to the invoice - that'll make sure you always know what the product looked like when you created the invoice - but this will cause lots of data duplication (not recommended)
See this other Stackoverflow question on temporal databases as another input, and also check out this article on Simple-Talk: Database Design: A Point in Time Architecture