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    <p>MongoDB <a href="http://www.mongodb.org/display/DOCS/Aggregation#Aggregation-Group">group by</a> is very limited in most cases, for instance</p> <pre><code>- the result set must be lesser than 10000 keys. - it will not work in sharded environments </code></pre> <p>So its better to use map reduce. so the query would be like this</p> <p><strike> map = function() { emit({a:true,b:true},{count:1}); }</strike> </p> <pre><code>reduce = function(k, values) { var result = {count: 0}; values.forEach(function(value) { result.count += value.count; }); return result; } </code></pre> <p>and then</p> <pre><code>db.list.mapReduce(map,reduce,{out: { inline : 1}}) </code></pre> <p>Its a untested version. let me know if it works</p> <p><strong>EDIT:</strong></p> <p>The earlier map function was faulty. Thats why you are not getting the results. it should have been</p> <pre><code>map = function () { emit({a:this.a, b:this.b}, {count:1}); } </code></pre> <p><strong>Test data:</strong></p> <pre><code>&gt; db.multi_group.insert({a:1,b:2}) &gt; db.multi_group.insert({a:2,b:2}) &gt; db.multi_group.insert({a:3,b:2}) &gt; db.multi_group.insert({a:1,b:2}) &gt; db.multi_group.insert({a:3,b:2}) &gt; db.multi_group.insert({a:7,b:2}) &gt; db.multi_group.mapReduce(map,reduce,{out: { inline : 1}}) { "results" : [ { "_id" : { "a" : 1, "b" : 2 }, "value" : { "count" : 2 } }, { "_id" : { "a" : 2, "b" : 2 }, "value" : { "count" : 1 } }, { "_id" : { "a" : 3, "b" : 2 }, "value" : { "count" : 2 } }, { "_id" : { "a" : 7, "b" : 2 }, "value" : { "count" : 1 } } ], "timeMillis" : 1, "counts" : { "input" : 6, "emit" : 6, "reduce" : 2, "output" : 4 }, "ok" : 1, } </code></pre> <p><strong>EDIT2:</strong></p> <p>Complete solution including applying having count >= 2</p> <pre><code>map = function () { emit({a:this.a, b:this.b}, {count:1,_id:this._id}); } reduce = function(k, values) { var result = {count: 0,_id:[]}; values.forEach(function(value) { result.count += value.count; result._id.push(value._id); }); return result; } &gt;db.multi_group.mapReduce(map,reduce,{out: { replace : "multi_result"}}) &gt; db.multi_result.find({'value.count' : {$gte : 2}}) { "_id" : { "a" : 1, "b" : 2 }, "value" : { "_id" : [ ObjectId("4f0adf2884025491024f994c"), ObjectId("4f0adf3284025491024f994f") ], "count" : 2 } } { "_id" : { "a" : 3, "b" : 2 }, "value" : { "_id" : [ ObjectId("4f0adf3084025491024f994e"), ObjectId("4f0adf3584025491024f9950") ], "count" : 2 } } </code></pre>
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    1. COi checked, but its not return the correct result. it was return total count of the collection. Actually what i am expecting means, I want to know how many duplicate entry is found in this collection with the combination a & b value
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    2. CO@Kumaran, there is an error in my map function. i have updated the new function. that will work.. check it
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    3. CO@Kumaran, check the final edit. thats the equivalent of sql group by and having using mapreduce.
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