Design Pattern to add columns in database table dynamically

Question

The user wants to add new fields in UI dynamically. This new field should get stored in database and they should be allowed to perform CRUD on it.

Now I can do this

Answer 1

This can be approached using a key value system. You create a table with the primary key column(s) of the table you want to annotate, a column for the name of the attribute, and a column for its value. When you user wants to add an attribute (say height) to the record of person 123 you add a row to the new table with the values (123, 'HEIGHT', '140.5').

In general you cast the values to TEXT for storage but if you know all the attributes will be numeric you can choose a different type for the value column. You can also (not recommended) use several different value columns depending on the type of the data.

This technique has the advantage that you don't need to modify the database structure to add new attributes and attributes are only stored for those records that have them. The disadvantage is that querying is not as straightforward as if the columns were all in the main data table.