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    <p>what corresponds to the "case" SQL statement in the <a href="http://docs.mongodb.org/manual/reference/aggregation/" rel="noreferrer">aggregation framework</a>, is the $cond operator (see <a href="http://docs.mongodb.org/manual/reference/aggregation/cond/" rel="noreferrer">manual</a>). $cond statements can be nested to simulate "when-then" and "else", but I have chosen another approach, because it is easier to read (and to generate, see below): I'll use the $concat operator to write the range string, which then serves as grouping key.</p> <p>So for the given collection:</p> <pre><code>db.xx.find() { "_id" : ObjectId("514919fb23700b41723f94dc"), "name" : "A", "timespent" : 100 } { "_id" : ObjectId("514919fb23700b41723f94dd"), "name" : "B", "timespent" : 200 } { "_id" : ObjectId("514919fb23700b41723f94de"), "name" : "C", "timespent" : 300 } { "_id" : ObjectId("514919fb23700b41723f94df"), "name" : "D", "timespent" : 400 } { "_id" : ObjectId("514919fb23700b41723f94e0"), "name" : "E", "timespent" : 500 } </code></pre> <p>the aggregate (hardcoded) looks like this:</p> <pre><code>db.xx.aggregate([ { $project: { "_id": 0, "range": { $concat: [{ $cond: [ { $lte: ["$timespent", 250] }, "range 0-250", "" ] }, { $cond: [ { $and: [ { $gte: ["$timespent", 251] }, { $lt: ["$timespent", 450] } ] }, "range 251-450", "" ] }, { $cond: [ { $and: [ { $gte: ["$timespent", 451] }, { $lt: ["$timespent", 650] } ] }, "range 450-650", "" ] }] } }}, { $group: { _id: "$range", count: { $sum: 1 } } }, { $sort: { "_id": 1 } }, ]); </code></pre> <p>and the result is:</p> <pre><code>{ "result" : [ { "_id" : "range 0-250", "count" : 2 }, { "_id" : "range 251-450", "count" : 2 }, { "_id" : "range 450-650", "count" : 1 } ], "ok" : 1 } </code></pre> <p>In order to generate the aggregate command, you have to build the "range" projection as a JSON object ( or you could generate a string and then use JSON.parse(string) )</p> <p>The generator looks like this:</p> <pre><code>var ranges = [ 0, 250, 450, 650 ]; var rangeProj = { "$concat": [] }; for (i = 1; i &lt; ranges.length; i++) { rangeProj.$concat.push({ $cond: { if: { $and: [{ $gte: [ "$timespent", ranges[i-1] ] }, { $lt: [ "$timespent", ranges[i] ] }] }, then: "range " + ranges[i-1] + "-" + ranges[i], else: "" } }) } db.xx.aggregate([{ $project: { "_id": 0, "range": rangeProj } }, { $group: { _id: "$range", count: { $sum: 1 } } }, { $sort: { "_id": 1 } }]); </code></pre> <p>which will return the same result as above.</p>
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