I have a table a with primary key id and a table b that represents a specialized version of a (it has all the same characteristics to track as a does, plus some specific to its b-ness--the latter are all that are stored in b). If I decide to represent this by having b's primary key be also a foreign key to a.id, what's the proper terminology for b in relation to a?
A real world example might be a person table with student and teacher add-on tables. A student might also be a teacher (a TA for example) but they're both the same person.
I would call it a 'child table' of a but I already use that as a synonym for 'detail table', like lines on a purchase order, for example.
Your design sounds like Concrete Table Inheritance.
I'd call table B a concrete table that extends table A.
The relationship is one-to-one.
Other answers have suggested storing only the columns specific to the extended table. This design would be called Class Table Inheritance.
Ok this is sort of off topic but first things first, why does B have all of A's columns? It should only have the added columns, ESPECIALLY if you are referencing A with a foriegn key.
"Add on" records are usually called "Detail(s)"
For example, lets say my Table A is "Cars" my Table B would be "CarDetails"
As Neil N said, you shouldn't have the columns in both places if you're referencing table A in table B through a foreign key.
What your describing sounds a bit like a parallel to inheritance in object oriented programming. Personally, I don't use any specific naming convention in this case. I name A what it is and I name B what it is. For example, I might have:
CREATE TABLE People
(
people_id INT NOT NULL,
first_name VARCHAR(40) NOT NULL,
last_name VARCHAR(40) NOT NULL,
...
CONSTRAINT PK_People PRIMARY KEY CLUSTERED (people_id)
)
GO
CREATE TABLE My_Application_Users
(
people_id INT NOT NULL,
user_name VARCHAR(20) NOT NULL,
security_level INT NOT NULL,
CONSTRAINT PK_My_Application_Users PRIMARY KEY CLUSTERED (people_id),
CONSTRAINT UI_My_Application_Users_user_name UNIQUE (user_name)
)
GO
This is just an example, so please don't tell me that my name columns are too long or too short or that they should allow NULLs, etc. ;)
what's the proper terminology for b in relation to a?
Table B is a child of Table A (the parent), because in order for a record to exist in the child, it must first exist in the parent.
Tables should be modeled based on either having one-to-many or many-to-one relationships depending on the context, and of those options they can be either optional or required. Tables that link two sets of lists together will relate to other tables in a many-to-one fashion for every table involved. For example, users, groups, and user_groups_xref - the user_groups_xref can support numerous specific user instances of a user records, and the same relationship to the groups table.
There's no point in one-to-one relationships - these should never be allowed to exist because it should only be one table.
Related
We have 2 tables with a 1:1 relationship.
1 table should reference the other, typically one would use a FK relationship.
Since there is a 1:1 relationship, we could also directly use the same Guid in both tables as primary key.
Additional info: the data is split into 2 tables since the data is rather separate, think "person" and "address" - but in a world where there is a clear 1:1 relationship between the 2.
As per the tags I was suggested I assume this is called "shared primary key".
Would using the same Guid as PK in 2 tables have any ill effects?
To consolidate info from comments into answer...
No, there are no ill effects of two tables sharing PK.
You will still need to create a FK reference from 2nd table, FK column will be the same as PK column.
Though, your example of "Person" and "Address" in 1:1 situation is not best suited. Common usage of this practice is entities that extend one another. For example: Table "User" can hold common info on all users, but tables "Candidate" and "Recruiter" can each expand on it, and all tables can share same PK. Programming language representation would also be classes that extends one another.
Other (similar) example would be table that store more detailed info than the base table like "User" and "UserDetails". It's 1:1 and no need to introduce additional PK column.
Code sample where PK is also a FK:
CREATE TABLE [User]
(
id INT PRIMARY KEY
, name NVARCHAR(100)
);
CREATE TABLE [Candidate]
(
id INT PRIMARY KEY FOREIGN KEY REFERENCES [User](id)
, actively_looking BIT
);
CREATE TABLE [Recruiter]
(
id INT PRIMARY KEY
, currently_hiring BIT
, FOREIGN KEY (id) REFERENCES [User](id)
);
PS: As mentioned GUID is not best suited column for PK due to performance issues, but that's another topic.
This may seem like a simple question, but I am stumped:
I have created a database about cars (in Oracle SQL developer). I have amongst other tables a table called: Manufacturer and a table called Parentcompany.
Since some manufacturers are owned by bigger corporations, I will also show them in my database.
The parentcompany table is the "parent table" and the Manufacturer table the "child table".
for both I have created columns, each having their own Primary Key.
For some reason, when I inserted the values for my columns, I was able to use the same value for the primary key of Manufacturer and Parentcompany
The column: ManufacturerID is primary Key of Manufacturer. The value for this is: 'MBE'
The column: ParentcompanyID is primary key of Parentcompany. The value for this is 'MBE'
Both have the same value. Do I have a problem with the thinking logic?
Or do I just not understand how primary keys work?
Does a primary key only need to be unique in a table, and not the database?
I would appreciate it if someone shed light on the situation.
A primary key is unique for each table.
Have a look at this tutorial: SQL - Primary key
A primary key is a field in a table which uniquely identifies each
row/record in a database table. Primary keys must contain unique
values. A primary key column cannot have NULL values.
A table can have only one primary key, which may consist of single or
multiple fields. When multiple fields are used as a primary key, they
are called a composite key.
If a table has a primary key defined on any field(s), then you cannot
have two records having the same value of that field(s).
Primary key is table-unique. You can use same value of PI for every separate table in DB. Actually that often happens as PI often incremental number representing ID of a row: 1,2,3,4...
For your case more common implementation would be to have hierarchical table called Company, which would have fields: company_name and parent_company_name. In case company has a parent, in field parent_company_name it would have some value from field company_name.
There are several reasons why the same value in two different PKs might work out with no problems. In your case, it seems to flow naturally from the semantics of the data.
A row in the Manufacturers table and a row in the ParentCompany table both appear to refer to the same thing, namely a company. In that case, giving a company the same id in both tables is not only possible, but actually useful. It represents a 1 to 1 correspondence between manufacturers and parent companies without adding extra columns to serve as FKs.
Thanks for the quick answers!
I think I know what to do now. I will create a general company table, in which all companies will be stored. Then I will create, as I go along specific company tables like Manufacturer and parent company that reference a certain company in the company table.
To clarify, the only column I would put into the sub-company tables is a column with a foreign key referencing a column of the company table, yes?
For the primary key, I was just confused, because I hear so much about the key needing to be unique, and can't have the same value as another. So then this condition only goes for tables, not the whole database. Thanks for the clarification!
I'm working on a web app and I have to design it's database. There's a part that didn't come very straightforward to me, so after some thinking and research I came with multiple ideas. Still neither seems completely suitable, so I'm not sure which one to implement and why.
The simplified problem looks as follows:
I have a table Teacher. There are 2 types of teachers, according to the relations with their Fields and Subjects:
A Teacher that's related to a Field, the Field is obligatory related to a Category
A Teacher that's not related to a Field, but directly to a Category
My initial idea was to have two nullable foreign keys, one to the table Field, and the other to the table Category. But in this case, how can I make sure that exactly one is null, and the other one is not?
The other idea is to create a hierarchy, with two types of Teacher tables derived from the table Teacher (is-a relation), but I couldn't find any useful tutorial on this.
I'm developing the app using Django with SQLite db
OK, your comment made it much clearer:
If a Teacher belongs to exactly one category, you should keep this in the Teacher's table directly:
Secondly each teacher belongs to "one or zero" fields. If this is sure for ever you should use a nullable FieldID column. This is set or remains empty.
Category (CategoryID, Name, ...)
Field (FieldID,Name,...)
Teacher (TeacherID,FieldID [NULL FK],CategoryID [NOT NULL FK], FirstName, Lastname, ...)
Remark: This is almost the same as my mapping table of the last answer. The only difference is, that you'll have a strict limitation with your "exactly one" or "exactly none or one"... From my experience I'd still prefer the open approach. It is easy to enforce your rules with unique indexes including the TeacherID-column. Sooner or later you'll probably have to re-structure this...
As you continue, one category is related to "zero or more" fields. There are two approaches:
Add a CategoryID-column to the Field-table (NOT NULL FK). This way you define a field several times with differing CategoryIDs (combined unique index!). A category's fields list you'll get simply by asking the Field-table for all fields with the given CategoryID.
Better in my eyes was a mapping table CategoryField. If you enforce a unique FieldID you'll get for sure, that no field is mapped twice. And add a unique index on the combination of CategoryID and FieldID...
A SELECT could be something like this (SQL Server Syntax, untested):
SELECT Teacher.TeacherID
,Teacher.FieldID --might be NULL
,Teacher.CategoryID --never NULL
,Teacher.[... Other columns ...]
,Field.Name --might be NULL
--The following columns you pick from the right source,
--depending on the return value of the LEFT JOIN to Field and the related "catField"
--the directly joined "Category" (which is never NULL) is the "default"
,ISNULL(catField.CategoryID,Category.CategoryID) AS ResolvedCategoryID
,ISNULL(catField.Name,Category.Name) AS ResolvedCategoryName
,[... Other columns ...]
FROM Teacher
INNER JOIN Category ON Teacher.CategoryID=Category.CategoryID --never NULL
LEFT JOIN Field ON Teacher.FieldID=Field.FieldID --might be NULL
LEFT JOIN Category AS catField ON Field.CategoryID=catField.CategoryID
This was the answer before the EDIT:
I try to help you even if the concept is not absolutely clear to me
Teacher-Table: TeacherID, person's data (name, address...), ...
Category-Table: CategoryID, category title, ...
Field-Tabls: FieldID, field title, ...
You say, that fields are bound to a category in all cases. If this is the same category in all cases, you should set the category as a FK-column in the Field-Table. If there is the slightest chance, that a field's category could differ with the context, you should not...
Same with teachers: If a teacher is ever bound to one single category set a FK-column within the Teacher-table, otherwise don't.
The most flexible you'll be with at least one mapping table:
(SQL Server Syntax)
CREATE TABLE TeacherFieldCategory
(
--A primary key to identify this row. This is not needed actually, but it will serve as clustered key index as a lookup index...
TeacherFieldCategoryID INT IDENTITY NOT NULL CONSTRAINT PK_TeacherFieldCategory PRIMARY KEY
--Must be set
,TeacherID INT NOT NULL CONSTRAINT FK_TeacherFieldCategory_TeacherID FOREIGN KEY REFERENCES Teacher(TeacherID)
--Field may be left NULL
,FieldID INT NULL CONSTRAINT FK_TeacherFieldCategory_FieldID FOREIGN KEY REFERENCES Field(FieldID)
--Must be set. This makes sure, that a teacher ever has a category and - if the field is set - the field will have a category
,CategoryID INT NOT NULL CONSTRAINT FK_TeacherFieldCategory_CategoryID FOREING KEY REFERENCES Category(CategoryID)
);
--This unique index will ensure, that each combination will exist only once.
CREATE UNIQUE INDEX IX_TeacherFieldCategory_UniqueCombination ON TeacherFieldCategory(TeacherID,FieldID,CategoryID);
It could be a better concept to have a mapping table FieldCategory and this table mapped to the mapping table above through a foreign key. Doing so you could avoid invalid field-category combinations.
Hope this helps...
I have implemented the following ways of storing relational topology:
1.A general junction relation table:
Table: Relation
Columns: id parent_type parent_id parent_prop child_type child_id child_prop
On which joins are not generally capable of being executed against by most sql engines.
2.Relation specific junction tables
Table: Class2Student
Columns: id parent_id parent_prop child_id child_prop
On which joins are capable of being executed against.
3.Storing lists/string maps of related objects in a text field on both bidirectional objects.
Class: Class
Class properties: id name students
Table columns: id name students_keys
Rows: 1 "history" [{type:Basic_student,id:1},{type:Advanced_student,id:3}]
To enable joins by the sql engines, it would be possible to write a custom module which would be made even easier if the contents of students_keys was simply [1,3], ie that a relation was to the explicit Student type.
The questions are the following in the context of:
I fail to see what the point of a junction table is. For example, I fail to see that any problems the following arguments for a junction table claim to relieve, actually exist:
Inability to logically correctly save a bidirectional relations (eg
there is no data orphaning in bidirectional relations or any
relations with a keys field, because one recursively saves and one can enforce
other operations (delete,update) quite easily)
Inability to join effectively
I am not soliciting opinions on your personal opinions on best practices or any cult-like statements on normalization.
The explicit question(s) are the following:
What are the instances where one would want to query a junction table that is not provided by querying a owning object's keys field?
What are logical implementation problems in the context of computation provided by the sql engine where the junction table is preferable?
The only implementation difference with regards to a junction table vs a keys fields is the following:
When searching for a query of the following nature you would need to match against the keys field with either a custom indexing implementation or some other reasonable implementation:
class_dao.search({students:advanced_student_3,name:"history"});
search for Classes that have a particular student and name "history"
As opposed to searching the indexed columns of the junction table and then selecting the approriate Classes.
I have been unable to identify answers why a junction table is logically preferable for quite literally any reason. I am not claiming this is the case or do I have a religious preference one way or another as evidenced by the fact that I implemented multiple ways of achieving this. My problem is I do not know what they are.
The way I see it, you have have several entities
CREATE TABLE StudentType
(
Id Int PRIMARY KEY,
Name NVarChar(50)
);
INSERT StudentType VALUES
(
(1, 'Basic'),
(2, 'Advanced'),
(3, 'SomeOtherCategory')
);
CREATE TABLE Student
(
Id Int PRIMARY KEY,
Name NVarChar(200),
OtherAttributeCommonToAllStudents Int,
Type Int,
CONSTRAINT FK_Student_StudentType
FOREIGN KEY (Type) REFERENCES StudentType(Id)
)
CREATE TABLE StudentAdvanced
(
Id Int PRIMARY KEY,
AdvancedOnlyAttribute Int,
CONSTRIANT FK_StudentAdvanced_Student
FOREIGN KEY (Id) REFERENCES Student(Id)
)
CREATE TABLE StudentSomeOtherCategory
(
Id Int PRIMARY KEY,
SomeOtherCategoryOnlyAttribute Int,
CONSTRIANT FK_StudentSomeOtherCategory_Student
FOREIGN KEY (Id) REFERENCES Student(Id)
)
Any attributes that are common to all students have columns on the Student table.
Types of student that have extra attributes are added to the StudentType table.
Each extra student type gets a Student<TypeName> table to store its specific attributes. These tables have an optional one-to-one relationship with Student.
I think that your "straw-man" junction table is a partial implementation of an EAV anti-pattern, the only time this is sensible, is when you can't know what attributes you need to model, i.e. your data will be entirely unstructured. When this is a real requirment, relational databases start to look less desirable. On those occasions consider a NOSQL/Document database alternative.
A junction table would be useful in the following scenario.
Say we add a Class entity to the model.
CREATE TABLE Class
(
Id Int PRIMARY KEY,
...
)
Its concievable that we would like to store the many-to-many realtionship between students and classes.
CREATE TABLE Registration
(
Id Int PRIMARY KEY,
StudentId Int,
ClassId Int,
CONSTRAINT FK_Registration_Student
FOREIGN KEY (StudentId) REFERENCES Student(Id),
CONSTRAINT FK_Registration_Class
FOREIGN KEY (ClassId) REFERENCES Class(Id)
)
This entity would be the right place to store attributes that relate specifically to a student's registration to a class, perhaps a completion flag for instance. Other data would naturally relate to this junction, pehaps a class specific attendance record or a grade history.
If you don't relate Class and Student in this way, how would you select both, all the students in a class, and all the classes a student reads. Performance wise, this is easily optimised by indices on key columns.
When a many-to-many realtionships exists without any attributes I agree that logically, the junction table needn't exist. However, in a relational database, junction tables are still a useful physical implmentaion, perhaps like this,
CREATE TABLE StudentClass
(
StudentId Int,
ClassId Int,
CONSTRAINT PK_StudentClass PRIMARY KEY (ClassId, StudentId),
CONSTRAINT FK_Registration_Student
FOREIGN KEY (StudentId) REFERENCES Student(Id),
CONSTRAINT FK_Registration_Class
FOREIGN KEY (ClassId) REFERENCES Class(Id)
)
this allows simple queries like
// students in a class?
SELECT StudentId
FROM StudentClass
WHERE ClassId = #classId
// classes read by a student?
SELECT ClassId
FROM StudentClass
WHERE StudentId = #studentId
additionaly, this enables a simple way to manage the relationship, partially or completely from either aspect, that will be familar to relational database developers and sargeable by query optimisers.
Just trying to figure out the best way to design my table for the following scenario:
I have several areas in my system (documents, projects, groups and clients) and each of these can have comments logged against them.
My question is should I have one table like this:
CommentID
DocumentID
ProjectID
GroupID
ClientID
etc
Where only one of the ids will have data and the rest will be NULL or should I have a separate CommentType table and have my comments table like this:
CommentID
CommentTypeID
ResourceID (this being the id of the project/doc/client)
etc
My thoughts are that option 2 would be more efficient from an indexing point of view. Is this correct?
Option 2 is not a good solution for a relational database. It's called polymorphic associations (as mentioned by #Daniel Vassallo) and it breaks the fundamental definition of a relation.
For example, suppose you have a ResourceId of 1234 on two different rows. Do these represent the same resource? It depends on whether the CommentTypeId is the same on these two rows. This violates the concept of a type in a relation. See SQL and Relational Theory by C. J. Date for more details.
Another clue that it's a broken design is that you can't declare a foreign key constraint for ResourceId, because it could point to any of several tables. If you try to enforce referential integrity using triggers or something, you find yourself rewriting the trigger every time you add a new type of commentable resource.
I would solve this with the solution that #mdma briefly mentions (but then ignores):
CREATE TABLE Commentable (
ResourceId INT NOT NULL IDENTITY,
ResourceType INT NOT NULL,
PRIMARY KEY (ResourceId, ResourceType)
);
CREATE TABLE Documents (
ResourceId INT NOT NULL,
ResourceType INT NOT NULL CHECK (ResourceType = 1),
FOREIGN KEY (ResourceId, ResourceType) REFERENCES Commentable
);
CREATE TABLE Projects (
ResourceId INT NOT NULL,
ResourceType INT NOT NULL CHECK (ResourceType = 2),
FOREIGN KEY (ResourceId, ResourceType) REFERENCES Commentable
);
Now each resource type has its own table, but the serial primary key is allocated uniquely by Commentable. A given primary key value can be used only by one resource type.
CREATE TABLE Comments (
CommentId INT IDENTITY PRIMARY KEY,
ResourceId INT NOT NULL,
ResourceType INT NOT NULL,
FOREIGN KEY (ResourceId, ResourceType) REFERENCES Commentable
);
Now Comments reference Commentable resources, with referential integrity enforced. A given comment can reference only one resource type. There's no possibility of anomalies or conflicting resource ids.
I cover more about polymorphic associations in my presentation Practical Object-Oriented Models in SQL and my book SQL Antipatterns.
Read up on database normalization.
Nulls in the way you describe would be a big indication that the database isn't designed properly.
You need to split up all your tables so that the data held in them is fully normalized, this will save you a lot of time further down the line guaranteed, and it's a lot better practice to get into the habit of.
From a foreign key perspective, the first example is better because you can have multiple foreign key constraints on a column but the data has to exist in all those references. It's also more flexible if the business rules change.
To continue from #OMG Ponies' answer, what you describe in the second example is called a Polymorphic Association, where the foreign key ResourceID may reference rows in more than one table. However in SQL databases, a foreign key constraint can only reference exactly one table. The database cannot enforce the foreign key according to the value in CommentTypeID.
You may be interested in checking out the following Stack Overflow post for one solution to tackle this problem:
MySQL - Conditional Foreign Key Constraints
The first approach is not great, since it is quite denormalized. Each time you add a new entity type, you need to update the table. You may be better off making this an attribute of document - I.e. store the comment inline in the document table.
For the ResourceID approach to work with referential integrity, you will need to have a Resource table, and a ResourceID foreign key in all of your Document, Project etc.. entities (or use a mapping table.) Making "ResourceID" a jack-of-all-trades, that can be a documentID, projectID etc.. is not a good solution since it cannot be used for sensible indexing or foreign key constraint.
To normalize, you need to the comment table into one table per resource type.
Comment
-------
CommentID
CommentText
...etc
DocumentComment
---------------
DocumentID
CommentID
ProjectComment
--------------
ProjectID
CommentID
If only one comment is allowed, then you add a unique constraint on the foreign key for the entity (DocumentID, ProjectID etc.) This ensures that there can only be one row for the given item and so only one comment. You can also ensure that comments are not shared by using a unique constraint on CommentID.
EDIT: Interestingly, this is almost parallel to the normalized implementation of ResourceID - replace "Comment" in the table name, with "Resource" and change "CommentID" to "ResourceID" and you have the structure needed to associate a ResourceID with each resource. You can then use a single table "ResourceComment".
If there are going to be other entities that are associated with any type of resource (e.g. audit details, access rights, etc..), then using the resource mapping tables is the way to go, since it will allow you to add normalized comments and any other resource related entities.
I wouldn't go with either of those solutions. Depending on some of the specifics of your requirements you could go with a super-type table:
CREATE TABLE Commentable_Items (
commentable_item_id INT NOT NULL,
CONSTRAINT PK_Commentable_Items PRIMARY KEY CLUSTERED (commentable_item_id))
GO
CREATE TABLE Projects (
commentable_item_id INT NOT NULL,
... (other project columns)
CONSTRAINT PK_Projects PRIMARY KEY CLUSTERED (commentable_item_id))
GO
CREATE TABLE Documents (
commentable_item_id INT NOT NULL,
... (other document columns)
CONSTRAINT PK_Documents PRIMARY KEY CLUSTERED (commentable_item_id))
GO
If the each item can only have one comment and comments are not shared (i.e. a comment can only belong to one entity) then you could just put the comments in the Commentable_Items table. Otherwise you could link the comments off of that table with a foreign key.
I don't like this approach very much in your specific case though, because "having comments" isn't enough to put items together like that in my mind.
I would probably go with separate Comments tables (assuming that you can have multiple comments per item - otherwise just put them in your base tables). If a comment can be shared between multiple entity types (i.e., a document and a project can share the same comment) then have a central Comments table and multiple entity-comment relationship tables:
CREATE TABLE Comments (
comment_id INT NOT NULL,
comment_text NVARCHAR(MAX) NOT NULL,
CONSTRAINT PK_Comments PRIMARY KEY CLUSTERED (comment_id))
GO
CREATE TABLE Document_Comments (
document_id INT NOT NULL,
comment_id INT NOT NULL,
CONSTRAINT PK_Document_Comments PRIMARY KEY CLUSTERED (document_id, comment_id))
GO
CREATE TABLE Project_Comments (
project_id INT NOT NULL,
comment_id INT NOT NULL,
CONSTRAINT PK_Project_Comments PRIMARY KEY CLUSTERED (project_id, comment_id))
GO
If you want to constrain comments to a single document (for example) then you could add a unique index (or change the primary key) on the comment_id within that linking table.
It's all of these "little" decisions that will affect the specific PKs and FKs. I like this approach because each table is clear on what it is. In databases that's usually better then having "generic" tables/solutions.
Of the options you give, I would go for number 2.
Option 2 is a good way to go. The issue that I see with that is you are putting the resouce key on that table. Each of the IDs from the different resources could be duplicated. When you join resources to the comments you will more than likely come up with comments that do not belong to that particular resouce. This would be considered a many to many join. I would think a better option would be to have your resource tables, the comments table, and then tables that cross reference the resource type and the comments table.
If you carry the same sort of data about all comments regardless of what they are comments about, I'd vote against creating multiple comment tables. Maybe a comment is just "thing it's about" and text, but if you don't have other data now, it's likely you will: date the comment was entered, user id of person who made it, etc. With multiple tables, you have to repeat all these column definitions for each table.
As noted, using a single reference field means that you could not put a foreign key constraint on it. This is too bad, but it doesn't break anything, it just means you have to do the validation with a trigger or in code. More seriously, joins get difficult. You can just say "from comment join document using (documentid)". You need a complex join based on the value of the type field.
So while the multiple pointer fields is ugly, I tend to think that's the right way to go. I know some db people say there should never be a null field in a table, that you should always break it off into another table to prevent that from happening, but I fail to see any real advantage to following this rule.
Personally I'd be open to hearing further discussion on pros and cons.
Pawnshop Application:
I have separate tables for Loan, Purchase, Inventory & Sales transactions.
Each tables rows are joined to their respective customer rows by:
customer.pk [serial] = loan.fk [integer];
= purchase.fk [integer];
= inventory.fk [integer];
= sale.fk [integer];
I have consolidated the four tables into one table called "transaction", where a column:
transaction.trx_type char(1) {L=Loan, P=Purchase, I=Inventory, S=Sale}
Scenario:
A customer initially pawns merchandise, makes a couple of interest payments, then decides he wants to sell the merchandise to the pawnshop, who then places merchandise in Inventory and eventually sells it to another customer.
I designed a generic transaction table where for example:
transaction.main_amount DECIMAL(7,2)
in a loan transaction holds the pawn amount,
in a purchase holds the purchase price,
in inventory and sale holds sale price.
This is clearly a denormalized design, but has made programming alot easier and improved performance. Any type of transaction can now be performed from within one screen, without the need to change to different tables.