# Table Definition¶

DataJoint models data as sets of entities with shared attributes, often visualized as tables with rows and columns. Each row represents a single entity and the values of all of its attributes. Each column represents a single attribute with a name and a datatype, applicable to entity in the table. Unlike rows in a spreadsheet, entities in DataJoint don’t have names or numbers: they can only be identified by the values of their attributes. Defining a table means defining the names and datatypes of the attributes as well as the constraints to be applied to those attributes. Both MATLAB and Python use the same syntax define tables.

For example, the following code in defines the table User, that contains users of the database:

The table definition is contained in the first block comment in the class definition file. Note that although it looks like a mere comment, the table definition is parsed by DataJoint. This solution is thought to be convenient since MATLAB does not provide convenient syntax for multiline strings.

%{
# database users
username : varchar(20)   # unique user name
---
first_name : varchar(30)
last_name  : varchar(30)
role : enum('admin', 'contributor', 'viewer')
%}
classdef User < dj.Manual
end


This defines the class User that creates the table in the database and provides all its data manipulation functionality.

## Table creation on the database server¶

Users do not need to do anything special to have the table created in the database. The table is created upon the first attempt to use the class for manipulating its data (e.g. inserting or fetching entities).

## Changing the definition of an existing table¶

Once the table is created in the database, the definition string has no further effect. In other words, changing the definition string in the class of an existing table will not actually update the table definition. To change the table definition, one must first drop the existing table. This means that all the data will be lost, and the new definition will be applied to create the new empty table.

Therefore, in the initial phases of designing a DataJoint pipeline, it is common to experiment with variations of the design before populating it with substantial amounts of data.

It is possible to modify a table without dropping it. This topic is covered separately.

## Reverse-engineering the table definition¶

DataJoint objects provide the describe method, which displays the table definition used to define the table when it was created in the database. This definition may differ from the definition string of the class if the definition string has been edited after creation of the table.

### Examples¶

s = describe(lab.User)


Furthermore, DataJoint provides the syncDef method to update the classdef file definition string for the table with the definition in the actual table:

syncDef(lab.User)    % updates the table definition in file +lab/User.m