Prompt
Answer
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):
- Table1: Contains main data we want to output.
- Table2: Contains related data for the first join.
- 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
Documents Emission:
emit([doc._id, 0], { table: 'Table1', data: doc });
- Purpose: Emits documents from
Table1
to output in the reduce function.
- Purpose: Emits documents from
Reduce Function Initialization:
var result = { Table1: null, Table2: [], Table3: [] };
- Purpose: Initializes the result object to store data from all tables.
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
, andTable3
.
- Purpose: Populates the result object with the data from
Output Specification:
return result.Table1;
- Purpose: Ensures that the output consists only of data from
Table1
.
- Purpose: Ensures that the output consists only of data from
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. ```
Description
Learn how to create a CouchDB view to join three tables and output data exclusively from the first table using map-reduce functions and the _lookup method.
More Database Query Writers
![Apache Flink Database Query Writer](https://fs.enterprisedna.co/library/icons/flink.png)
![Apache Pig Database Query Writer](https://fs.enterprisedna.co/library/icons/apache_pig.png)
![Azure Data Factory Database Query Writer](https://fs.enterprisedna.co/library/icons/azure-data-factory.png)
![C/C++ Database Query Writer](https://fs.enterprisedna.co/library/icons/c_c_plus.png)
![CouchDB Database Query Writer](https://fs.enterprisedna.co/library/icons/couchdb.png)
![DAX Database Query Writer](https://fs.enterprisedna.co/library/icons/power-bi.png)
![Excel Database Query Writer](https://fs.enterprisedna.co/library/icons/excel.png)
![Firebase Database Query Writer](https://fs.enterprisedna.co/library/icons/firebase.png)
![Google BigQuery Database Query Writer](https://fs.enterprisedna.co/library/icons/google_bigquery.png)
![Google Sheets Database Query Writer](https://fs.enterprisedna.co/library/icons/google_sheets.png)
![GraphQL Database Query Writer](https://fs.enterprisedna.co/library/icons/graphql.png)
![Hive Database Query Writer](https://fs.enterprisedna.co/library/icons/hive.png)
![Java Database Query Writer](https://fs.enterprisedna.co/library/icons/java.png)
![JavaScript Database Query Writer](https://fs.enterprisedna.co/library/icons/javascript.png)
![Julia Database Query Writer](https://fs.enterprisedna.co/library/icons/julia.png)
![Lua Database Query Writer](https://fs.enterprisedna.co/library/icons/lua.png)
![M (Power Query) Database Query Writer](https://fs.enterprisedna.co/library/icons/m.png)
![MATLAB Database Query Writer](https://fs.enterprisedna.co/library/icons/matlab.png)
![MongoDB Database Query Writer](https://fs.enterprisedna.co/library/icons/mongodb.png)
![Oracle Database Query Writer](https://fs.enterprisedna.co/library/icons/oracle.png)
![PostgreSQL Database Query Writer](https://fs.enterprisedna.co/library/icons/postgresql.png)
![Power BI Database Query Writer](https://fs.enterprisedna.co/library/icons/power-bi.png)
![Python Database Query Writer](https://fs.enterprisedna.co/library/icons/python.png)
![R Database Query Writer](https://fs.enterprisedna.co/library/icons/r.png)
![Redis Database Query Writer](https://fs.enterprisedna.co/library/icons/redis.png)
![Regex Database Query Writer](https://fs.enterprisedna.co/library/icons/regex.png)
![Ruby Database Query Writer](https://fs.enterprisedna.co/library/icons/ruby.png)
![SAS Database Query Writer](https://fs.enterprisedna.co/library/icons/sas.png)
![Scala Database Query Writer](https://fs.enterprisedna.co/library/icons/scala.png)
![Shell Database Query Writer](https://fs.enterprisedna.co/library/icons/shell.png)
![SPSS Database Query Writer](https://fs.enterprisedna.co/library/icons/spss.png)
![SQL Database Query Writer](https://fs.enterprisedna.co/library/icons/sql.png)
![SQLite Database Query Writer](https://fs.enterprisedna.co/library/icons/sqlite.png)
![Stata Database Query Writer](https://fs.enterprisedna.co/library/icons/stata.png)
![Tableau Database Query Writer](https://fs.enterprisedna.co/library/icons/tableau.png)
![VBA Database Query Writer](https://fs.enterprisedna.co/library/icons/vba.png)