I've checked everything for errors: primary key, uniqueness, and type. Access just doesnt seem to be able to link the 2 fields i have in my database. can someone please take a look?
http://www.jpegtown.com/pictures/jf5WKxKRqehz.jpg
Thanks.
Your relationship diagram shows that you've made the ID fields your primary key in all your tables, but you're not using them for your joins. Thus, they serve absolutely no purpose. If you're not going to use "surrogate keys" (i.e., a meaningless ID number that is generated by the database and is unique to each record, but has absolutely no meaning in regard to the data in your table), then eliminate them. But if you're going to use "natural keys" (i.e., a primary key constructed from a set of real data fields that together are going to be unique for each record), you must have a unique compound index on those fields.
However, there are issues with both approaches:
Surrogate Keys: a surrogate PK makes each record unique. That is you could have a record for David Fenton with ID 1 and a record for David Fenton with ID 2. If it's the same David Fenton, you've got duplicate data, but as far as your database knows, they are unique.
Natural Keys: some types of entities work very well with natural keys. The best such are where there's a single field that identifies the record uniquely. An example would be "employee type," where values might be "associate, manager, etc." In that case, it's a very good candidate for using the natural key instead of adding a surrogate key. The only argument against the natural key in that case is if the data in the candidate natural key is highly volatile (i.e., it changes frequently). While every modern database engine provides "CASCADE UPDATE" functionality (i.e., if the value in the PK field changes, all the tables where that field is a Foreign Key are automatically updated), this imposes a certain amount of overhead and can be problematic. For single-column keys, it's unlikely to be an issue. Now, except for lookup tables, there are very few entities for which a natural key will be a single column. Instead, you have to create a compound index, i.e., an index that spans multiple data fields. In the index dialog in Access table design, you create a compound key by giving it a name in the first column, and then adding multiple rows in the second column (from the dropdown list of fields in your table). The drawback of this is that if any of the fields in your compound unique index are unknown, you won't get uniqueness. That is, if a field has a Null in two records, and the rest of the fields are identical, this won't be counted as a conflict of uniqueness because Null never equals Null. This is because Null doesn't mean "empty" -- it means "Unknown."
Allen Browne has explained everything you need to know about Nulls:
Nulls: Do I Need Them?
Common Errors with Null
In your graphic, you show that you are trying to link the Company table with the PManager table. The latter table has a CompanyID field, and your Company table has a unique index on its ID field, so all you need is a link from the ID field of the Company table to the CompanyID field of the PManager table. For your example to work (which would be useless, since you already have a unique index on the ID field), you'd need to create a unique compound key spanning both ID and ShortName in the Company table.
Additionally, if ShortName is a field that you want to be unique (i.e., you don't want two company records to have the same ShortName), you should add a unique index to it, whether or not you still use the ID field as your primary key. This brings me back to item #1 above, where I described a situation where a surrogate key could lead you to enter duplicate records, because uniqueness is established by the surrogate key along. Any time you choose to use a surrogate key, you must also add a unique compound index on any combination of data fields that needs to be unique (with the caveat about Null fields as outlined in item #2).
If you're thinking "surrogate keys mean more indexes" you're correct, in that you have two unique indexes on the same table (assuming you don't have the Null problem). But you do get substantial ease of use in joining tables in SQL, as well as substantially less duplication of data. Likewise, you avoid the overhead of CASCADE UPDATE. On the other hand, if you're viewing a child table with a natural foreign key, you don't need to join to the parent table to be able to identify the parent record, because the data that identifies that record is right there in the foreign-key fields. That lack of a need for a join can be a major performance gain in certain scenarios (especially for the case where you'd need an outer join because the foreign key can be Null).
This is actually quite a huge topic, and it's something of a religious argument. I'm firmly in the surrogate key camp, but I use natural keys for lookup tables where the key is a single column. I don't use natural keys for any other purpose. That said, where possible (i.e., no Null problems) I also have a unique index on the natural key.
Hope this helps.
Actually you need an index on the name fields, on both sides
However, may I suggest that you have way too many joins? In general there should only be one join from one table to the next. It is rare to have more than one join between tables, and exceedingly rare to have more than two.
Have a look at this link:
http://weblogs.asp.net/scottgu/archive/2006/07/12/Tip_2F00_Trick_3A00_-Online-Database-Schema-Samples-Library.aspx
Notice how all of the tables are joined together by a single relationship?
Each of the fields labeled PK are primary keys. These are AUTONUMBER fields. Each of the fields labeled FK are foreign keys. These are indexed Number fields of type Integer. The Primary Keys are connected to the Foreign Keys in a 1 to many relationship (in most cases).
99% of the time, you won't need any other kind of joins. The trick is to create tables with unique information. There is a lot of repeated information in your database.
A database that is reorganized in this manner is called a "normalized" database. There are lots of good examples of these at http://www.databaseanswers.org/data_models/
Just join on the CompanyID. You could also get rid of the Company field in PManager.
I did the following and the problem was solved (I face the same problem of referential integrity in access).
I exported data from both tables in Access to Excel. Table1
was containing Cust Code and basic information about the company.
Cust Code as Primary key.
Table2 was containing all information about who the
customers associated with that company.
I removed all duplicates from Table2 exported to excel.
Using Vlookup I checked and found that there are 11
customers code not present in Table1.
I added those codes in Access Table. I linked by
referential integrity and Problem was solved.
Also look for foreign key if it does not work.
You need to create an INDEX. Perhaps look for some kind of create index button and create an index on CompanyID
Related
I'm building a new DB using MySQL to store lessons learned across a variety of projects. When we talk about this in the office, we refer to lessons by the Project Number and Lesson Number, i.e. PR12-81, where PR12 refers to the project and 81 refers to the specific lesson within that project. I want the primary key in my DB to have a hyphen in it as well.
When defining a composite key in SQL, I can make it reference the project and lesson but without the hyphen, i.e. PR1281. I've also considered creating a separate column of data type CHAR(1), putting a hyphen in every row and delcaring that the PK is made of 3 columns.
Is there another way that I can specify the primary key to be formatted in the preferred way?
Let your table's primary key be a nonsensical auto-increment number with no "meaning" whatsoever. Then, within that table, define two columns: project_number and lesson_number. If the two need to be unique, define a UNIQUE index encompassing the two fields.
Don't(!) create database keys which embed information into them, even if the business does so. If the business needs to refer to strings like PR12, so be it ... create a column to store the appropriate value, or use a one-to-many table. Use indexes as needed to enforce uniqueness.
Notice(!) that I've now described four columns:
The auto-increment based "actual" primary key, which contains no information.
The project_number column, probably a foreign key to a projects table.
Ditto the lesson_number. (With a UNIQUE composite index if needed.)
The column (or table) which contains "the string that the business uses."
Over time, business practices do change. And someday you just might .. no, you will... ... encounter a "business-used string" that was incorrectly assigned by the human-beings who do such things! Your database design needs to gracefully handle this. The schema I've described is so-called third-normal form. Do a Google-search on "normal forms" if you haven't already.
I have two tables: students and courses, assuming that each student can be in more than one course and that each course can have more than one student.
[Table Students] [Table Courses]
id(PK) id(PK)
name name
age duration
etc... etc...
and what I want to do it is to relate both tables into another table, for example, studying, in which I will store the course or courses that is doing each student. Like this:
[Table studying]
idStudent
idCourse
What I have deduced
I think that idStudent and idCourse should be foreign keys because the information it is stored in students and courses respectively with an unique primary key and to respect the consistency of the database. It cannot exist a relation without information neither of the student nor the course or just without the information of one of them.
I also know that some tables has two primary keys to allow that in the table could exist more than one repeated value of a primary key, but not of both primary keys at the same time.
My questions
These ids (idStudent, idCourse). Have to be primary keys or foreign keys?
Should the table studying has another column with an ID?
Is my deduction in the good way?
P.S: I do not need sql statements, I just need help to clarify my confusion.
Thanks in advance!
These ids (idStudent, idCourse). Have to be primary keys or foreign keys?
You want them to be foreign keys, because the existence of each record on your third table depends on the availability of the first, that is, there cannot be a "Student Course" or a "Course with Students" without either the course or the student. It could (if you don't make those keys) but you would break referential integrity
On the other hand, having FK's is usually a good thing because you make sure that you don't remove dependable records by mistake (which is what the constraint is for on the first place) unless you did something like cascade deleting
Should the table studying has another column with an ID?
No, it does not have to but again, sometimes it is a good practice because some software like Object Relational Mappers, Diagram Software, etc. may rely on the fact that they always needs a by-convention primary key. Some others don't even support composite keys so while it is not mandatory it can help in the future and it does not hurt. Of course this all depends on what you are using the database for and how (pure SQL, which engine you use, if you use it with a framework etc.)
Is my deduction in the good way?
All is relative. But I think your logic is good. My advice is that you always design your data schemas as flexible as you can because if a project grows its harder (and more costly) to do those changes down the road. Invest time on thinking how you may expand your application functionality and think if the schema will adapt to it.
Your deduction is correct.
In fact, you should have a composite primary key consisting of both (idStudent, idCourse) columns, because this tuple is the identifier of row in the table, you do not need additional ID column (of course, you can also take that approach to add additional ID column that would be your primary key, but you do not need it if one student can have one course assigned only once)
To respect the integrity, both columns (separately) should be foreign keys - idStudent should be referencing id column of Students table and idCourse should reference id column of Courses table.
If you like you can make them primary keys on studying table. But this is unnecesary, because relation (role of studying table) is many to many and this kind of table dont need primary keys. You need to know that also when you make them pk (pair of student id and course id) , thats mean that theee could be only one pair of each, thats equivalent to constrain unique - student can take a course only ones. In the future you maybe would like to add to this table start_date and this kind of pk could be a problem, you will need to modify them.
Question
Is there a way to have a many-to-many relationship among 3 tables without the use of automatic incrementers (usually ID), or are ID's required for this?
Why I ask
I have 3 relative tables. Since one-to-one relationships seem to can't happen directly, I made a 4th to do one-to-many relationships to the other 3 tables. However, since there's still a primary key to each table, a value can only be used once in a table, which I don't want to happen.
What I have
Connectors has multiple Pockets which have multiple pins.
The 4th Table is ConnectorFullInfo
There is no requirement that a table have an "automatic incrementer" as a primary key.
But, a familiar pattern is to add a surrogate ID column as primary key on entity tables. The "ideal" primary key will be "anonymous" (carry no meaningful information), "unique" (no duplicate values), "simple" (single column, short simple native datatype), ...
There are a couple of schools of thought on whether it's a good idea to introduce a surrogate key. I will also note that there are those who have been later burned by the decision to use a natural key rather than a surrogate key. And there are those that haven't yet been burned by that decision.
In the case of "association" tables (tables introduced to resolve many-to-many relationships), the combination of the foreign keys can be used as the primary key. I often do this.
BUT, if the association table is itself turns out to be entity table, with it's own attributes, I will introduce a surrogate ID column. As an example, the association between person and club, a person can be a member of multiple clubs, and a club can have multiple members...
club +--< membership >--+ person
When we start adding attributes to membership (such as status, date_joined, office_held, etc... at that point membership isn't just an association table; it's turning into an entity. When I suspect that an association is actually an entity, so I'll add the surrogate ID column.
The other case where I will add a surrogate ID column to an association table is when we want to allow "duplicates", where we want to allow multiple associations. In that case, I will also introduce a surrogate ID column.
Yes you can but,
It is customary to represent a table row by a unique identifier which is the number, its becomes more efficient.
I am currently rebuilding a database which is used to store patient records. In the current database, the primary key for a patient is their name and date of birth, (a single column, ie "John Smith 1970-01-01", it is not composite). This is also a foreign key in many other tables to reference the patients table. I am planning to replace this key with an auto-generated integer key (since there will obviously be duplicate keys one day under the current system). How can I add a new primary key to this table and add appropriate foreign keys on all the other tables? Keep in mind that there is already a very large amount of data (~500,000 records) and these data references cannot be broken.
Thanks!
If up to me..
Add a new future-PK column as a non-null unique index (it must be a KEY, but not necessarily the PK) with auto_increment.
Add the appropriate new-FK columns to all the related tables, these should be initially nullable.
Set the new-FK value to the appropriate future-PK value based on the current-PK/FK relationships. Use an "UPDATE .. JOIN" for this step.
Enable the Referential Integrity Constraints (DRI) on the relevant tables. It only needs to be KEY/FK, not PK/FK, which is why the future-PK can be used. Every existing DRI constraint using the current-PK should likely be updated during this step.
Remove the new-FK column nullability based on modeling requirements.
Remove any residue old-FK columns as they are now redundant data.
Switch the old-PK and the new/future-PK (this can be done in one command and may take awhile to physically reorganize all the rows). Remove the old PK column as applicable, or perhaps simply remove the KEY status.
I would also offline the database during the process, review and test the process (use a testing database for dry-runs), and maintain backups.
The Data-Access Layer and any Views/etc will also need to be updated. These should be done at the same time, again through a review and testing process.
Also, even when adding an auto-increment PK, the table should generally still have an appropriate covering natural key enforced with unique constraints.
I solved the problem using the following method:
1- Assigned added a new primary key to the patients table and assigned unique values to all existing records
2- Created materialized views (without triggers) for each of the referencing tables that included all fields in the referencing table as well as the newly created id field in the patients table (via a join).
3- Deleted the source referencing tables
4- Renamed the materialized views to the names of the original source tables
The materialized views are now the dependent tables.
A reference for materialized views: http://www.fromdual.com/mysql-materialized-views
I have mysql database table for addresses, it contains separate columns for post code / street /town.
How can I make sure a full table entry is unique rather than just one of the columns alone?
Make a composite key on {post code, street, town}.
A key can be primary (PRIMARY KEY) or alternate (UNIQUE constraint).
BTW, can two different towns have the same post code? If not, you don't need the town in the key. In fact, consider normalizing your model by "extracting" the town to a separate table.
mysql allows you to have a unique key for n number of fields or columns. When you declare this column as unique , mysql checks for the constraint for duplication.
For more info read this mysql unique key
"The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns."
But, i suggest the front end validation for making the user comfortable (just incase u had not thought of it ;) )
A database table may have multiple unique keys, so it will reject any INSERT queries that violate the constraints you set in the table.
Simply set the other columns to be unique and you should find that your problem is solved.