CouchDB Multi-Table Join View

CouchDB Query to Join Three Tables

Objective

To join three tables in CouchDB and output the result from the first table. Due to the nature of CouchDB (a NoSQL database), traditional SQL joins cannot be used directly. Instead, map-reduce functions, and the _lookup method available since CouchDB 3.x, will be utilized.

Database Structure

Assume we have three tables (collections):

1. Table1: Contains main data we want to output.
2. Table2: Contains related data for the first join.
3. Table3: Contains related data for the second join.

Query Construction

Utilize map-reduce functions to create a view that joins the tables, then output data from the first table.

// Language: JavaScript (CouchDB View)
{
  "_id": "_design/joined_view",
  "views": {
    "join_three_tables": {
      "map": function (doc) {
        // Emit documents from Table1
        if (doc.type === 'Table1') {
          emit([doc._id, 0], { table: 'Table1', data: doc });
        }
        // Emit documents from Table2
        if (doc.type === 'Table2') {
          emit([doc.table1_id, 1], { table: 'Table2', data: doc });
        }
        // Emit documents from Table3
        if (doc.type === 'Table3') {
          emit([doc.table2_id, 2], { table: 'Table3', data: doc });
        }
      }.toString(),
      "reduce": function (keys, values, rereduce) {
        var result = {};
        // Initialize result object with nulls
        result.Table1 = null;
        result.Table2 = [];
        result.Table3 = [];
        // Place documents in result accordingly
        values.forEach(function (value) {
          if (value.table === 'Table1') {
            result.Table1 = value.data;
          } else if (value.table === 'Table2') {
            result.Table2.push(value.data);
          } else if (value.table === 'Table3') {
            result.Table3.push(value.data);
          }
        });
        return result.Table1; // Output only data from Table1
      }.toString()
    }
  }
}

Annotations

1. Documents Emission:

   emit([doc._id, 0], { table: 'Table1', data: doc });

   • Purpose: Emits documents from Table1 to output in the reduce function.

2. Reduce Function Initialization:

   var result = { Table1: null, Table2: [], Table3: [] };

   • Purpose: Initializes the result object to store data from all tables.

3. Data Aggregation:

   if (value.table === 'Table1') {
     result.Table1 = value.data;
   } else if (value.table === 'Table2') {
     result.Table2.push(value.data);
   } else if (value.table === 'Table3') {
     result.Table3.push(value.data);
   }

   • Purpose: Populates the result object with the data from Table1, Table2, and Table3.

4. Output Specification:

   return result.Table1;

   • Purpose: Ensures that the output consists only of data from Table1.

Summary

This query constructs a view in CouchDB to join three tables and then outputs the result exclusively from the first table. The map function emits documents from all three tables, while the reduce function processes the emitted values, merges the related data, and filters the output to include only data from Table1.

For further learning on CouchDB and advanced techniques including joins, exploring resources on Enterprise DNA Platform is recommended. ```