The primary key element

Primary key specifications determine which columns of the table form the primary key. There can only be one primary key specification per table. In the following example, we see that state and name are the primary key of the table city.

<table id="City" lex="city,cities,town,towns">
    <column id="name" type="text" lex="name,names,named,called"/>
    <column id="state" type="text" lex="where,located in"/>
    <column id="population" type="int" lex="population,populations,..."/>
    <primary_key key="name,state"/>

The DTD definition of primary_key is:

<!ATTLIST primary_key
          key CDATA #REQUIRED
          description CDATA #IMPLIED>

key lists the comma separated identifiers of columns that form the primary key.

Note to support the Edit functionality in the NLI Portal, the primary key must be only a single column.