mongodb根据子项中的指标查找最小或最大值

Posted 2021-01-01 青冥绿水

假设students集合中有这样的数据：

{
        "_id" : 1,
        "name" : "Aurelia Menendez",
        "scores" : [
                {
                        "type" : "exam",
                        "score" : 60.06045071030959
                },
                {
                        "type" : "quiz",
                        "score" : 52.79790691903873
                },
                {
                        "type" : "homework",
                        "score" : 71.76133439165544
                },
                {
                        "type" : "homework",
                        "score" : 34.85718117893772
                }
        ]
},
{
        "_id" : 3,
        "name" : "Bao Ziglar",
        "scores" : [
                {
                        "type" : "exam",
                        "score" : 71.64343899778332
                },
                {
                        "type" : "quiz",
                        "score" : 24.80221293650313
                },
                {
                        "type" : "homework",
                        "score" : 1.694720653897219
                },
                {
                        "type" : "homework",
                        "score" : 42.26147058804812
                }
        ]
},
{
        "_id" : 2,
        "name" : "Corliss Zuk",
        "scores" : [
                {
                        "type" : "exam",
                        "score" : 67.03077096065002
                },
                {
                        "type" : "quiz",
                        "score" : 6.301851677835235
                },
                {
                        "type" : "homework",
                        "score" : 20.18160621941858
                },
                {
                        "type" : "homework",
                        "score" : 66.28344683278382
                }
        ]
}

需要找出每个学生成绩最小值，可使用如下语句：

db.students.aggregate([{$project: {minScore: {$min: "$scores.score"}}}])

语句里面的minScore等于是一个别名，可以使用其它的。结果如下：

{ "_id" : 1, "minScore" : 34.85718117893772 }
{ "_id" : 3, "minScore" : 1.694720653897219 }
{ "_id" : 2, "minScore" : 6.301851677835235 }
{ "_id" : 4, "minScore" : 19.21886443577987 }
{ "_id" : 5, "minScore" : 10.53058536508186 }
{ "_id" : 7, "minScore" : 42.48780666956811 }
{ "_id" : 6, "minScore" : 16.58341639738951 }
{ "_id" : 8, "minScore" : 14.63969941335069 }
{ "_id" : 9, "minScore" : 12.47568017314781 }
{ "_id" : 0, "minScore" : 1.463179736705023 }
{ "_id" : 12, "minScore" : 14.78936520432093 }
{ "_id" : 13, "minScore" : 78.18795058912879 }
{ "_id" : 14, "minScore" : 13.66179556675781 }
{ "_id" : 11, "minScore" : 15.81264595052612 }
{ "_id" : 16, "minScore" : 7.772386442858281 }
{ "_id" : 15, "minScore" : 3.311794422000724 }
{ "_id" : 18, "minScore" : 62.12870233109035 }
{ "_id" : 17, "minScore" : 31.15090466987088 }
{ "_id" : 10, "minScore" : 19.31113429145131 }
{ "_id" : 19, "minScore" : 0.6578497966368002 }