Question

I have an application that displays tabular information and allows the user to enter and edit it. I have been working on the database design for this and have come across the following problem.

I want to avoid creating an MSSQL table for each table in the application, so I’ve been trying to represent all of the tables together, in two tables.

I have defined a MyAppTable table which has Id INTEGER primary key and a Name CHARACTER VARYING(255) columns.

From there, I have defined a MyAppColumn table which foreign keys against the MyAppTable.Id column and also has a Position INTEGER column to indicate its position in the rendered table.

Finally, I have defined a MyAppRow table which foreign keys against the MyAppColumn.Id column and has a Data BINARY(32) column. It’s a table of small files, so this data type is fitting.

The actual problem is that I cannot guarantee that the MyAppColumn.Position column will be unique for each MyAppTable.

An example of a normal situation:

• MyAppTable with Id 0
• MyAppColumn with Id 0, TableId 0, Position 0
• MyAppColumn with Id 1, TableId 0, Position 1
• MyAppRow with ColumnId 0, Data [something]
• MyAppRow with ColumnId 1, Data [something]
• MyAppTable with Id 1
• MyAppColumn with Id 2, TableId 1, Position 0
• MyAppRow with ColumnId 2, Data [something]

An example of a situation I want to make impossible in the database:

• MyAppTable with Id 0
• MyAppColumn with Id 0, TableId 0, Position 0
• MyAppColumn with Id 1, TableId 0, Position 0

As you can see, I want some way of constraining a pair of columns. The two columns TableId and Position should be UNIQUE but if that were done independently, it would mean two tables could never have columns both at a Position of 0.

How can I accomplish this in Microsoft SQL Server 2012?

Answer 1

The solution to your problem is to not encode metadata in tables but to create actual tables, even if it’s inconvenient or a lot of work.

Some stuff that will bite you in the rear very soon:

• No relationships can be modeled
• No way to index
• No input validation (is that a date or an int or an image?)
• No way to get actual accurate statistics
• Every single query that you write will be a nightmare

Save yourself a lot of time and effort in the future and just make actual tables now in the design phase. This type of meta-data “god table” has been tried repeatedly and never works.

That being said, if you are representing a very limited set of data (like you say, for files only), I would consider a coordinate system.

Keep your MyAppTable as is.

Create a MyAppFile table with fields:

• FileId (int identity)
• MyAppTableId (FK)
• Data (varbinary)
• XPos (int)
• YPos (int)

With a unique constraint on (MyAppTableId, Xpos, Ypos).

This more accurately represents what you are trying to store, will be easier to maintain, and allows you to enforce integrity.