CouchDB aggregation query
CouchDB is a document oriented NoSQL database that supports querying and aggregating document data. CouchDB uses the MapReduce mechanism to implement aggregate queries. The following are some aggregation queries supported by CouchDB:
1. MapReduce query:
-The Map function is used to map document data to key value pairs.
-The Reduce function is used to aggregate the mapping results.
Example:
script
//Map function
function(doc) {
emit(doc.category, 1);
}
//Reduce function
function(keys, values, rereduce) {
return sum(values);
}
In this example, the Map function groups documents by category field, and the count for each group is 1. The Reduce function accumulates the counts of each group to obtain the final result.
2. Group query:
-Grouping queries are used to group document data according to specified fields and aggregate data within each group.
Example:
script
//Map function
function(doc) {
emit(doc.category, doc.price);
}
//Reduce function
function(keys, values, rereduce) {
return sum(values);
}
In this example, the Map function groups by category field and maps the prices within each group. The Reduce function accumulates the prices of each group to obtain the final result.
3. Filter Query:
-Filter queries are used to filter document data based on specified criteria and perform aggregation operations.
Example:
script
//Map function
function(doc) {
if (doc.category === 'electronics') {
emit(doc._id, doc.price);
}
}
//Reduce function
function(keys, values, rereduce) {
return sum(values);
}
In this example, the Map function filters out documents classified as' electronics' based on conditions and maps their prices. The Reduce function accumulates the prices of these documents to obtain the final result.
4. Bucket query:
-Bucket splitting query is used to divide document data into buckets according to a specified range and aggregate the data within each bucket.
Example:
script
//Map function
function(doc) {
var bucket = Math.floor(doc.price / 100);
emit(bucket, doc.price);
}
//Reduce function
function(keys, values, rereduce) {
return sum(values);
}
In this example, the Map function divides the document into buckets based on the price range and maps the prices within each bucket. The Reduce function accumulates the prices of each barrel to obtain the final result.
Note that the Map and Reduce functions in the example are JavaScript functions that can be defined and used in the Software design description of CouchDB. At the same time, CouchDB's query function can also be used to call and execute these aggregated queries.
