Created
August 27, 2019 13:06
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Median in MongoDB
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| db.Transactions.aggregate([ | |
| { | |
| "$group": { | |
| "_id": null, | |
| "count": { | |
| "$sum": 1 | |
| }, | |
| "values": { | |
| "$push": "$INSERT_VALUE_TO_GET_MEDIAN_OF_HERE" | |
| } | |
| } | |
| }, | |
| { | |
| "$unwind": "$values" | |
| }, | |
| { | |
| "$sort": { | |
| "values": 1 | |
| } | |
| }, | |
| { | |
| "$project": { | |
| "values": 1, | |
| "count": 1, | |
| "midpoint": { | |
| "$divide": [ | |
| "$count", | |
| 2 | |
| ] | |
| } | |
| } | |
| }, | |
| { | |
| "$project": { | |
| "values": 1, | |
| "count": 1, | |
| "midpoint": 1, | |
| "high": { | |
| "$ceil": "$midpoint" | |
| }, | |
| "low": { | |
| "$floor": "$midpoint" | |
| } | |
| } | |
| }, | |
| { | |
| "$group": { | |
| "_id": null, | |
| "values": { | |
| "$push": "$values" | |
| }, | |
| "high": { | |
| "$avg": "$high" | |
| }, | |
| "low": { | |
| "$avg": "$low" | |
| } | |
| } | |
| }, | |
| { | |
| "$project": { | |
| "beginValue": { | |
| "$arrayElemAt": ["$values", "$high"] | |
| }, | |
| "endValue": { | |
| "$arrayElemAt": ["$values", "$low"] | |
| } | |
| } | |
| } | |
| ]); |
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Hello. This is wrong (or, at least, somewhat confusing) and does not work for collections of both odd and even lengths. I think, as is, it only works for even lengths but regardless, here is the working code that returns the median of any field in a collection.
db.test.aggregate([ { "$group":{ "_id":null, "count":{ "$sum":1 }, "values":{ "$push":"$INSERT_VALUE_TO_GET_MEDIAN_OF_HERE" } } }, { "$unwind":"$values" }, { "$sort":{ "values":1 } }, { "$project":{ "values":1, "count":1, "midpoint":{ "$divide":[ "$count", 2 ] } } }, { "$project":{ "values":1, "count":1, "midpoint":1, "high":{ "$floor":{ "$subtract":[ "$midpoint", 0.5 ] } }, "low":{ "$floor":"$midpoint" } } }, { "$group":{ "_id":null, "values":{ "$push":"$values" }, "high":{ "$avg":"$high" }, "low":{ "$avg":"$low" } } }, { "$project":{ "values":1, "beginValue":{ "$arrayElemAt":[ "$values", "$high" ] }, "endValue":{ "$arrayElemAt":[ "$values", "$low" ] } } }, { "$project":{ "median":{ "$cond":{ "if":{ "$eq":[ { "$mod":[ { "$size":"$values" }, 2 ] }, 0 ] }, "then":{ "$avg":[ "$beginValue", "$endValue" ] }, "else":"$endValue" } } } } ]);Hope this helps :)