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MongoDB 查询简介

MongoDB 查询简介

作者: 只是甲 | 来源:发表于2020-11-20 14:29 被阅读0次

    备注:
    MongoDB 4.2 版本

    测试数据:

    -- 类似Oracle里scott下的emp表
db.emp.insert([
{EMPNO:7369,ENAME:'SMITH',JOB:'CLERK',MGR:7902,HIREDATE:new Date(1980,12,17),SAL:800,DEPTNO:20,dname:'RESEARCH',loc:'DALLAS'},
{EMPNO:7499,ENAME:'ALLEN',JOB:'SALESMAN',MGR:7698,HIREDATE:new Date(1981,2,20),SAL:1600,COMM:300,DEPTNO:30,dname:'SALES',loc:'CHICAGO'},
{EMPNO:7521,ENAME:'WARD',JOB:'SALESMAN',MGR:7698,HIREDATE:new Date(1981,2,22),SAL:1250,COMM:500,DEPTNO:30,dname:'SALES',loc:'CHICAGO'},
{EMPNO:7566,ENAME:'JONES',JOB:'MANAGER',MGR:7839,HIREDATE:new Date(1981,4,2),SAL:2975,DEPTNO:20,dname:'RESEARCH',loc:'DALLAS'},
{EMPNO:7654,ENAME:'MARTIN',JOB:'SALESMAN',MGR:7698,HIREDATE:new Date(1981,9,28),SAL:1250,COMM:1400,DEPTNO:30,dname:'SALES',loc:'CHICAGO'},
{EMPNO:7698,ENAME:'BLAKE',JOB:'MANAGER',MGR:7839,HIREDATE:new Date(1981,5,1),SAL:2850,DEPTNO:30,dname:'SALES',loc:'CHICAGO'},
{EMPNO:7782,ENAME:'CLARK',JOB:'MANAGER',MGR:7839,HIREDATE:new Date(1981,6,9),SAL:2450,DEPTNO:10,dname:'ACCOUNTING',loc:'NEW YORK'},
{EMPNO:7839,ENAME:'KING',JOB:'PRESIDENT',HIREDATE:new Date(1981,11,17),SAL:5000,DEPTNO:10,dname:'ACCOUNTING',loc:'NEW YORK'},
{EMPNO:7844,ENAME:'TURNER',JOB:'SALESMAN',MGR:7698,HIREDATE:new Date(1981,2,20),SAL:1500,COMM:0,DEPTNO:30,dname:'SALES',loc:'CHICAGO'},
{EMPNO:7900,ENAME:'JAMES',JOB:'CLERK',MGR:7698,HIREDATE:new Date(1981,8,9),SAL:950,DEPTNO:30,dname:'SALES',loc:'CHICAGO'},
{EMPNO:7902,ENAME:'FORD',JOB:'ANALYST',MGR:7566,HIREDATE:new Date(1981,12,3),SAL:3000,DEPTNO:20,dname:'RESEARCH',loc:'DALLAS'},
{EMPNO:7934,ENAME:'MILLER',JOB:'CLERK',MGR:7782,HIREDATE:new Date(1982,1,23),SAL:1300,DEPTNO:10,dname:'ACCOUNTING',loc:'NEW YORK'},
{DEPTNO:40,dname:'OPERATIONS',loc:'BOSTON'}
])

    一.MongoDB查询介绍

    作用 SQL MongoDB
    所有记录 select * from emp db.emp.find()
    empno=6379 select * from emp where empno=7369 db.emp.find({EMPNO:7369})
    字段筛选 select empno,ename from emp where empno=7369 db.emp.find({EMPNO:7369},{EMPNO:1,ENAME:1})
    排序 select * from emp where deptno = 10 order by ename db.emp.find({DEPTNO:10}).sort({ENAME:1})
    比大小 select * from emp where deptno > 20 db.emp.find({'DEPTNO':{$gt:20}})
    正则(模糊匹配) select * from emp where ename like 'S%' db.emp.find({ENAME:/^S/})
    or操作 select * from emp where deptno = 10 or ename = 'SMITH' db.emp.find({$or :[{DEPTNO:10},{ENAME:'SMITH'}]})
    仅返回1条 select * from emp limit 1 db.user.findOne()
    count聚合 select count(ename) from emp db.emp.find({ENAME:{'$exists': true}}).count()
    查询计划 explain select * from emp where empno = 7369 db.user.find({EMPNO:7369}).explain()

    2.1 所有记录

    测试记录:

    > db.emp.find()
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b3"), "EMPNO" : 7369, "ENAME" : "SMITH", "JOB" : "CLERK", "MGR" : 7902, "HIREDATE" : ISODate("1981-01-16T16:00:00Z"), "SAL" : 800, "DEPTNO" : 20, "dname" : "RESEARCH", "loc" : "DALLAS" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b4"), "EMPNO" : 7499, "ENAME" : "ALLEN", "JOB" : "SALESMAN", "MGR" : 7698, "HIREDATE" : ISODate("1981-03-19T16:00:00Z"), "SAL" : 1600, "COMM" : 300, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b5"), "EMPNO" : 7521, "ENAME" : "WARD", "JOB" : "SALESMAN", "MGR" : 7698, "HIREDATE" : ISODate("1981-03-21T16:00:00Z"), "SAL" : 1250, "COMM" : 500, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b6"), "EMPNO" : 7566, "ENAME" : "JONES", "JOB" : "MANAGER", "MGR" : 7839, "HIREDATE" : ISODate("1981-05-01T16:00:00Z"), "SAL" : 2975, "DEPTNO" : 20, "dname" : "RESEARCH", "loc" : "DALLAS" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b7"), "EMPNO" : 7654, "ENAME" : "MARTIN", "JOB" : "SALESMAN", "MGR" : 7698, "HIREDATE" : ISODate("1981-10-27T16:00:00Z"), "SAL" : 1250, "COMM" : 1400, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b8"), "EMPNO" : 7698, "ENAME" : "BLAKE", "JOB" : "MANAGER", "MGR" : 7839, "HIREDATE" : ISODate("1981-05-31T16:00:00Z"), "SAL" : 2850, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b9"), "EMPNO" : 7782, "ENAME" : "CLARK", "JOB" : "MANAGER", "MGR" : 7839, "HIREDATE" : ISODate("1981-07-08T16:00:00Z"), "SAL" : 2450, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995ba"), "EMPNO" : 7839, "ENAME" : "KING", "JOB" : "PRESIDENT", "HIREDATE" : ISODate("1981-12-16T16:00:00Z"), "SAL" : 5000, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995bb"), "EMPNO" : 7844, "ENAME" : "TURNER", "JOB" : "SALESMAN", "MGR" : 7698, "HIREDATE" : ISODate("1981-03-19T16:00:00Z"), "SAL" : 1500, "COMM" : 0, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995bc"), "EMPNO" : 7900, "ENAME" : "JAMES", "JOB" : "CLERK", "MGR" : 7698, "HIREDATE" : ISODate("1981-09-08T16:00:00Z"), "SAL" : 950, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995bd"), "EMPNO" : 7902, "ENAME" : "FORD", "JOB" : "ANALYST", "MGR" : 7566, "HIREDATE" : ISODate("1982-01-02T16:00:00Z"), "SAL" : 3000, "DEPTNO" : 20, "dname" : "RESEARCH", "loc" : "DALLAS" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995be"), "EMPNO" : 7934, "ENAME" : "MILLER", "JOB" : "CLERK", "MGR" : 7782, "HIREDATE" : ISODate("1982-02-22T16:00:00Z"), "SAL" : 1300, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995bf"), "DEPTNO" : 40, "dname" : "OPERATIONS", "loc" : "BOSTON" }
> 
>

    2.2 过滤记录

    测试记录:

    > db.emp.find({empno:7369})
> 
> db.emp.find({EMPNO:7369})
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b3"), "EMPNO" : 7369, "ENAME" : "SMITH", "JOB" : "CLERK", "MGR" : 7902, "HIREDATE" : ISODate("1981-01-16T16:00:00Z"), "SAL" : 800, "DEPTNO" : 20, "dname" : "RESEARCH", "loc" : "DALLAS" }
>

    2.3 字段筛选

    测试记录:

    > db.emp.find({EMPNO:7369},{EMPNO:1,ENAME:1})
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b3"), "EMPNO" : 7369, "ENAME" : "SMITH" }
>

    2.4 排序

    测试记录:

    > db.emp.find({DEPTNO:10}).sort({ENAME:1})
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b9"), "EMPNO" : 7782, "ENAME" : "CLARK", "JOB" : "MANAGER", "MGR" : 7839, "HIREDATE" : ISODate("1981-07-08T16:00:00Z"), "SAL" : 2450, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995ba"), "EMPNO" : 7839, "ENAME" : "KING", "JOB" : "PRESIDENT", "HIREDATE" : ISODate("1981-12-16T16:00:00Z"), "SAL" : 5000, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995be"), "EMPNO" : 7934, "ENAME" : "MILLER", "JOB" : "CLERK", "MGR" : 7782, "HIREDATE" : ISODate("1982-02-22T16:00:00Z"), "SAL" : 1300, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
>

    2.5 比大小

    测试记录:

     > db.emp.find({'DEPTNO':{$gt:20}})
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b4"), "EMPNO" : 7499, "ENAME" : "ALLEN", "JOB" : "SALESMAN", "MGR" : 7698, "HIREDATE" : ISODate("1981-03-19T16:00:00Z"), "SAL" : 1600, "COMM" : 300, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b5"), "EMPNO" : 7521, "ENAME" : "WARD", "JOB" : "SALESMAN", "MGR" : 7698, "HIREDATE" : ISODate("1981-03-21T16:00:00Z"), "SAL" : 1250, "COMM" : 500, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b7"), "EMPNO" : 7654, "ENAME" : "MARTIN", "JOB" : "SALESMAN", "MGR" : 7698, "HIREDATE" : ISODate("1981-10-27T16:00:00Z"), "SAL" : 1250, "COMM" : 1400, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b8"), "EMPNO" : 7698, "ENAME" : "BLAKE", "JOB" : "MANAGER", "MGR" : 7839, "HIREDATE" : ISODate("1981-05-31T16:00:00Z"), "SAL" : 2850, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995bb"), "EMPNO" : 7844, "ENAME" : "TURNER", "JOB" : "SALESMAN", "MGR" : 7698, "HIREDATE" : ISODate("1981-03-19T16:00:00Z"), "SAL" : 1500, "COMM" : 0, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995bc"), "EMPNO" : 7900, "ENAME" : "JAMES", "JOB" : "CLERK", "MGR" : 7698, "HIREDATE" : ISODate("1981-09-08T16:00:00Z"), "SAL" : 950, "DEPTNO" : 30, "dname" : "SALES", "loc" : "CHICAGO" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995bf"), "DEPTNO" : 40, "dname" : "OPERATIONS", "loc" : "BOSTON" }
>

    2.6 正则(模糊匹配)

    测试记录:

    > db.emp.find({ENAME:/^S/}) 
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b3"), "EMPNO" : 7369, "ENAME" : "SMITH", "JOB" : "CLERK", "MGR" : 7902, "HIREDATE" : ISODate("1981-01-16T16:00:00Z"), "SAL" : 800, "DEPTNO" : 20, "dname" : "RESEARCH", "loc" : "DALLAS" }
>

    2.7 or操作

    测试记录:

    > db.emp.find({$or :[{DEPTNO:10},{ENAME:'SMITH'}]})
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b3"), "EMPNO" : 7369, "ENAME" : "SMITH", "JOB" : "CLERK", "MGR" : 7902, "HIREDATE" : ISODate("1981-01-16T16:00:00Z"), "SAL" : 800, "DEPTNO" : 20, "dname" : "RESEARCH", "loc" : "DALLAS" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995b9"), "EMPNO" : 7782, "ENAME" : "CLARK", "JOB" : "MANAGER", "MGR" : 7839, "HIREDATE" : ISODate("1981-07-08T16:00:00Z"), "SAL" : 2450, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995ba"), "EMPNO" : 7839, "ENAME" : "KING", "JOB" : "PRESIDENT", "HIREDATE" : ISODate("1981-12-16T16:00:00Z"), "SAL" : 5000, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
{ "_id" : ObjectId("5fa4b2eaeae97236d30995be"), "EMPNO" : 7934, "ENAME" : "MILLER", "JOB" : "CLERK", "MGR" : 7782, "HIREDATE" : ISODate("1982-02-22T16:00:00Z"), "SAL" : 1300, "DEPTNO" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
> 
>

    2.8 仅返回1条

    测试记录:

    > db.emp.findOne()
{
        "_id" : ObjectId("5fa4b2eaeae97236d30995b3"),
        "EMPNO" : 7369,
        "ENAME" : "SMITH",
        "JOB" : "CLERK",
        "MGR" : 7902,
        "HIREDATE" : ISODate("1981-01-16T16:00:00Z"),
        "SAL" : 800,
        "DEPTNO" : 20,
        "dname" : "RESEARCH",
        "loc" : "DALLAS"
}
>

    2.9 count聚合

    测试记录:

    > db.emp.find({ENAME:{'$exists': true}}).count()
12
>

    2.10 查询计划

    测试记录:

    > db.user.find({EMPNO:7369}).explain()
{
        "queryPlanner" : {
                "plannerVersion" : 1,
                "namespace" : "zqs.user",
                "indexFilterSet" : false,
                "parsedQuery" : {
                        "EMPNO" : {
                                "$eq" : 7369
                        }
                },
                "winningPlan" : {
                        "stage" : "EOF"
                },
                "rejectedPlans" : [ ]
        },
        "serverInfo" : {
                "host" : "10-31-1-124",
                "port" : 27017,
                "version" : "4.2.10",
                "gitVersion" : "88276238fa97b47c0ef14362b343c5317ecbd739"
        },
        "ok" : 1
}
>

    2.11 sum

    MongoDB 中聚合(aggregate)主要用于处理数据(诸如统计平均值，求和等)，并返回计算后的数据结果

    语法:

    db.collection.aggregate( [ <stage1>, <stage2>, ... ] )
    表达式 描述 实例
    $sum 计算总和
    $avg 计算平均值
    $min 获取集合中所有文档对应值得最小值
    $max 获取集合中所有文档对应值得最大值
    $push 在结果文档中插入值到一个数组中
    $addToSet 在结果文档中插入值到一个数组中，但不创建副本
    $first 根据资源文档的排序获取第一个文档数据
    $last 根据资源文档的排序获取最后一个文档数据

    2.11.1 分组汇总

    代码:

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",emp_count : {$sum: 1}}}
])
-- 等价于下面的sql
select deptno,count(*) from emp group by deptno

db.emp.aggregate([
{$group : {_id : "$DEPTNO",sum_sal : {$sum: '$SAL'}}}
])
-- 等价于下面的sql
select deptno,sum(sal) from emp group by deptno

    测试记录:

    > db.emp.aggregate([
... {$group : {_id : "$DEPTNO",emp_count : {$sum: 1}}}
... ])
{ "_id" : 30, "emp_count" : 6 }
{ "_id" : 20, "emp_count" : 3 }
{ "_id" : 40, "emp_count" : 1 }
{ "_id" : 10, "emp_count" : 3 }
> 

> db.emp.aggregate([
... {$group : {_id : "$DEPTNO",sum_sal : {$sum: '$SAL'}}}
... ])
{ "_id" : 20, "sum_sal" : 6775 }
{ "_id" : 40, "sum_sal" : 0 }
{ "_id" : 30, "sum_sal" : 9400 }
{ "_id" : 10, "sum_sal" : 8750 }
>

    2.11.2 avg

    代码:

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",avg_sal : {$avg: '$SAL'}}}
])
-- 等价于下面这个sql
select deptno,avg(sal) avg_sal from emp

    测试记录:

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",avg_sal : {$avg: '$SAL'}}}
])

    2.11.3 min

    代码：

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",min_sal : {$min: '$SAL'}}}
])
-- 等价于下面的sql
select deptno,min(sal) as min_sal from emp group by deptno

    测试记录:

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",min_sal : {$min: '$SAL'}}}
])

    2.11.4 max

    代码：

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",max_sal : {$max: '$SAL'}}}
])
-- 等价于下面的sql
select deptno,max(sal) as max_sal from emp group by deptno

    测试记录:

    > db.emp.aggregate([
... {$group : {_id : "$DEPTNO",max_sal : {$max: '$SAL'}}}
... ])
{ "_id" : 20, "max_sal" : 3000 }
{ "_id" : 40, "max_sal" : null }
{ "_id" : 30, "max_sal" : 2850 }
{ "_id" : 10, "max_sal" : 5000 }
>

    2.11.5 push

    代码：

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",emps : {$push: '$ENAME'}}}
])
-- 等价于mysql里如下sql
select deptno,group_concat(ename) from emp group by deptno;

    测试记录:

    > db.emp.aggregate([
... {$group : {_id : "$DEPTNO",emps : {$push: '$ENAME'}}}
... ])
{ "_id" : 20, "emps" : [ "SMITH", "JONES", "FORD" ] }
{ "_id" : 40, "emps" : [ ] }
{ "_id" : 30, "emps" : [ "ALLEN", "WARD", "MARTIN", "BLAKE", "TURNER", "JAMES" ] }
{ "_id" : 10, "emps" : [ "CLARK", "KING", "MILLER" ] }
>

    2.11.6 addToSet

    代码：

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",emps : {$addToSet: '$ENAME'}}}
])
-- 等价于下面的sql
select  deptno,group_concat(ename)
from (select distinct deptno,ename from emp) e
group by deptno

    测试记录:

    > db.emp.aggregate([
... {$group : {_id : "$DEPTNO",emps : {$addToSet: '$ENAME'}}}
... ])
{ "_id" : 20, "emps" : [ "SMITH", "JONES", "FORD" ] }
{ "_id" : 40, "emps" : [ ] }
{ "_id" : 30, "emps" : [ "BLAKE", "TURNER", "JAMES", "WARD", "ALLEN", "MARTIN" ] }
{ "_id" : 10, "emps" : [ "KING", "CLARK", "MILLER" ] }
>

    2.11.7 first

    代码：

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",first_emp : {$first: '$ENAME'}}}
])
-- 这个用sql实现略微复杂

    测试记录:

    > db.emp.aggregate([
... {$group : {_id : "$DEPTNO",first_emp : {$first: '$ENAME'}}}
... ])
{ "_id" : 20, "first_emp" : "SMITH" }
{ "_id" : 40, "first_emp" : null }
{ "_id" : 30, "first_emp" : "ALLEN" }
{ "_id" : 10, "first_emp" : "CLARK" }

    2.11.8 last

    代码：

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",last_emp : {$last: '$ENAME'}}}
])
-- 这个用sql实现略微复杂

    测试记录:

    > db.emp.aggregate([
... {$group : {_id : "$DEPTNO",last_emp : {$last: '$ENAME'}}}
... ])
{ "_id" : 30, "last_emp" : "JAMES" }
{ "_id" : 20, "last_emp" : "FORD" }
{ "_id" : 40, "last_emp" : null }
{ "_id" : 10, "last_emp" : "MILLER" }

    2.11.9 聚合汇总用例

    代码:

    db.emp.aggregate([
{$group : {_id : "$DEPTNO",
            emp_count : {$sum: 1},
            avg_sal : {$avg: '$SAL'},
            min_sal : {$min: '$SAL'},
            max_sal : {$max: '$SAL'},
            emps1 : {$push: '$ENAME'},
            emps2 : {$addToSet: '$ENAME'},
            first_emp : {$first: '$ENAME'},
            last_emp : {$last: '$ENAME'}         
 }}
])

    测试记录；

    > db.emp.aggregate([
... {$group : {_id : "$DEPTNO",
...             emp_count : {$sum: 1},
...             avg_sal : {$avg: '$SAL'},
...             min_sal : {$min: '$SAL'},
...             max_sal : {$max: '$SAL'},
...             emps1 : {$push: '$ENAME'},
...             emps2 : {$addToSet: '$ENAME'},
...             first_emp : {$first: '$ENAME'},
...             last_emp : {$last: '$ENAME'}         
...  }}
... ])
{ "_id" : 30, "emp_count" : 6, "avg_sal" : 1566.6666666666667, "min_sal" : 950, "max_sal" : 2850, "emps1" : [ "ALLEN", "WARD", "MARTIN", "BLAKE", "TURNER", "JAMES" ], "emps2" : [ "BLAKE", "TURNER", "JAMES", "WARD", "ALLEN", "MARTIN" ], "first_emp" : "ALLEN", "last_emp" : "JAMES" }
{ "_id" : 20, "emp_count" : 3, "avg_sal" : 2258.3333333333335, "min_sal" : 800, "max_sal" : 3000, "emps1" : [ "SMITH", "JONES", "FORD" ], "emps2" : [ "SMITH", "FORD", "JONES" ], "first_emp" : "SMITH", "last_emp" : "FORD" }
{ "_id" : 40, "emp_count" : 1, "avg_sal" : null, "min_sal" : null, "max_sal" : null, "emps1" : [ ], "emps2" : [ ], "first_emp" : null, "last_emp" : null }
{ "_id" : 10, "emp_count" : 3, "avg_sal" : 2916.6666666666665, "min_sal" : 1300, "max_sal" : 5000, "emps1" : [ "CLARK", "KING", "MILLER" ], "emps2" : [ "KING", "CLARK", "MILLER" ], "first_emp" : "CLARK", "last_emp" : "MILLER" }
>

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