You can designate the primary key for a table by modifying the Model file. In the column object, add the #key after the data type element, separated by a comma.
Note: When designating a new primary key, you can query the INFORMATION_SCHEMA.SYSTEM_SAMPLING_STATUS system table for a list of potential primary key candidates. See "Determining the primary key" for more information.

In the following example, the employeeID column has been designated the primary key for this table.

{
"my_table":{
           "#path":[
               "https://example.com/employees"
          ],
          "employeeID":"VarChar(32),#key",
          "position_title":"VarChar(46)",
          "start_year":"Integer",
           }
}

You an also create a composite primary key by using the #key element to designate multiple columns in a definition. For example, the values of the employeeID and position columns act as a composite key in the following: 

{
"my_table":{
           "#path":[
               "https://example.com/employees"
          ],
          "employeeID":"VarChar(32),#key",
          "position":"Integer",#key,
          "position_title":"VarChar(46)",
          "start_year":"Integer",
           }
}