The field selection within an SQL query can be translated into a corresponding view definition, either by adding the fields to the emitted key (if the value is also used for selection in a WHERE clause), or into the emitted value, if the data is separate from the required query parameters.

For example, to get the sales data by country from each stored document using the following map() function:

function(doc, meta) {
  emit([doc.city, doc.sales], null);
}

If you want to output information that can be used within a reduce function, this should be specified in the value generated by each emit() call. For example, to reduce the sales figures the above map() function could be rewritten as:

function(doc, meta) {
  emit(doc.city, doc.sales);
}

In essence this does not produce significantly different output (albeit with a simplified key), but the information can now be reduced using the numerical value.

If you want to output data or field values completely separate to the query values, then these fields can be explicitly output within the value portion of the view. For example:

function(doc, meta) {
  emit(doc.city, [doc.name, doc.sales]);
}

If the entire document for each item is required, load the document data after the view has been requested through the client library. For more information on this parameter and the performance impact, see Section 9.5.5, “View Writing Best Practice”.