About MongoDB group

Test data is inserted into the group of MongoDB: for (vari1; i20; I ++) {varnumi % 6; db. test. insert ({_ id: I, name: user _ + I, age: num});} 1. queries the database by common groups. test. group ({key: {age: true}, initial: {num: 0}, $ reduce: function (doc, prev) {prev. num ++}); db. runCom

Test data is inserted into the group of MongoDB: for (var I = 1; i20; I ++) {var num = I % 6; db. test. insert ({_ id: I, name: user _ + I, age: num});} 1. queries the database by common groups. test. group ({key: {age: true}, initial: {num: 0}, $ reduce: function (doc, prev) {prev. num ++}); db. runCom

About MongoDB group

Insert test data first:

For (var I = 1; I <20; I ++ ){

Var num = I % 6;

Db. test. insert ({_ id: I, name: "user _" + I, age: num });

}

1. query common groups

Db. test. group ({key: {age: true}, initial: {num: 0}, $ reduce: function (doc, prev ){

Prev. num ++

}});

Db. runCommand ({group:

{

Ns: "test ",

Key: {age: true },

Initial: {num: 0 },

$ Reduce: function (doc, prev ){

Prev. num ++}

}

});

2. filter and then group

Db. test. group ({key: {age: true}, initial: {num: 0}, $ reduce: function (doc, prev ){

Prev. num ++

},

Condition: {age :{$ gt: 2 }}

});

Db. runCommand ({group:

{

Ns: "test ",

Key: {age: true },

Initial: {num: 0 },

$ Reduce: function (doc, prev ){

Prev. num ++ },

Condition: {age :{$ gt: 2 }}

}

});

Common $ where queries:

Db. test. find ({$ where: function (){

Return this. age> 2;

}

});

Group join $ where Query

Db. test. group ({key: {age: true}, initial: {num: 0}, $ reduce: function (doc, prev ){

Prev. num ++

},

Condition: {$ where: function (){

Return this. age> 2;

}

}

});

3. grouping with function return values

// Note that the $ keyf specified function must return an object.

Db. test. group ({$ keyf: function (doc) {return {age: doc. age };}, initial: {num: 0}, $ reduce: function (doc, prev ){

Prev. num ++

}

});

Db. runCommand ({group:

{

Ns: "test ",

$ Keyf: function (doc) {return {age: doc. age };},

Initial: {num: 0 },

$ Reduce: function (doc, prev ){

Prev. num ++}

}

});

4. Use the Terminator

Db. test. group ({$ keyf: function (doc) {return {age: doc. age };}, initial: {num: 0}, $ reduce: function (doc, prev ){

Prev. num ++

},

Finalize: function (doc) {doc. count = doc. num; delete doc. num ;}

});

Db. runCommand ({group:

{

Ns: "test ",

$ Keyf: function (doc) {return {age: doc. age };},

Initial: {num: 0 },

$ Reduce: function (doc, prev ){

Prev. num ++ },

Finalize: function (doc) {doc. count = doc. num; delete doc. num ;}

}

});

MapReduce

// Insert test data first

For (var I = 1; I <21; I ++)

{

Db. test. insert ({_ id: I, name: 'mm' + I });

}

// Mapreduce

Db. runCommand (

{

Mapreduce: 'test ',

Map: function () {emit (this. name. substr (0, 3), this );},

Reduce: function (key, vals) {return vals [0] ;}, // Note: vals is an Object rather than an array

Out: 'wq'

});

Note:

1. mapreduce groups emit functions based on the first parameter called by the map function.

2. Only when one key matches multiple documents according to the grouping key group, the key and document set are handed over to the reduce function for processing. For example:

Db. runCommand (

{

Mapreduce: 'test ',

Map: function () {emit (this. name. substr (0, 3), this );},

Reduce: function (key, vals) {return 'wq ';},

Out: 'wq'

});

Run the mapreduce command to view the wq table data:

Db. wq. find ()

{"_ Id": "mm1", "value": "wq "}

{"_ Id": "mm2", "value": "wq "}

{"_ Id": "mm3", "value": {"_ id": 3, "name": "mm3 "}}

{"_ Id": "mm4", "value": {"_ id": 4, "name": "mm4 "}}

{"_ Id": "mm5", "value": {"_ id": 5, "name": "mm5 "}}

{"_ Id": "mm6", "value": {"_ id": 6, "name": "mm6 "}}

{"_ Id": "mm7", "value": {"_ id": 7, "name": "mm7 "}}

{"_ Id": "mm8", "value": {"_ id": 8, "name": "mm8 "}}

{"_ Id": "mm9", "value": {"_ id": 9, "name": "mm9 "}}