Mongodb 表达式命令笔记📒

• #mongodb

本文为阅读 表达式命令官方文档 后做的笔记。

目录：

• 日期类型获取年月日时分秒周数值
• $concat
• $dateToString
• $ifNull
• $toDate

日期类型获取年月日时分秒周数值

create_time 是个日期类型，create_time: ISODate(‘2025-09-16T19:51:22.460+00:00’)

db.your_collection.aggregate([
    {
        $project: {
            year: { $year: "$create_time" },    // 2025。年份是 2025
            month: { $month: "$create_time" },  // 9。月份是 9
            day: { $dayOfMonth: "$create_time" }, // 16。当月的 16 号
            hour: { $hour: "$create_time" },    // 19。小时数
            minutes: { $minute: "$create_time" },   // 51。分钟数
            seconds: { $second: "$create_time" },   // 22。秒数
            milliseconds: { $millisecond: "$create_time" }, // 460。毫秒数
            dayOfYear: { $dayOfYear: "$create_time" },  // 259。一年中第 259 天
            dayOfWeek: { $dayOfWeek: "$create_time" },  // 3。国外周日是第一天，虽然数字是3，其实是周二
            week: { $week: "$create_time" },     // 37。位于第 37 周
            isoWeek: { $isoWeek: '$create_time' }, // 38。ISO 标准的周数，ISO 标准是周一算第一天，所以是 38
            isoDayOfWeek: { $isoDayOfWeek: "$create_time" }, // 2。一周中第 2 天，ISO 标准是周一算第一天，所以是周二
            isoWeekYear: { $isoWeekYear: '$create_time' },  // 2025。规则是日期所在周的周四的年份，比如 '2024-12-30'的 isoWeekYear 是 2025，因为 '2024-12-30' 这一天所在周的周四位于 2025 年
        }
    }
])

db.inventory.insertMany( [
   { _id : 1, item : "ABC1", quarter: "13Q1", description : "product 1" },
   { _id : 2, item : "ABC2", quarter: "13Q4", description : "product 2" },
   { _id : 3, item : "XYZ1", quarter: "14Q2", description : null }
] )

db.inventory.aggregate(
    [
        { $project: { itemDescription: { $concat: [ "$item", " - ", "$description" ] } } }
    ]
)

{ _id : 1, itemDescription : "ABC1 - product 1" }
{ _id : 2, itemDescription : "ABC2 - product 2" }
{ _id : 3, itemDescription : null }

{
    create_date: {
        $dateToString: {
            date: '$create_time',
            format: "%Y-%m-%d",
        }
    }
}

db.inventory.insertMany( [
   { _id: 1, item: "buggy", description: "toy car", quantity: 300 },
   { _id: 2, item: "bicycle", description: null, quantity: 200 },
   { _id: 3, item: "flag" }
] )

db.inventory.aggregate(
   [
      {
         $project: {
            item: 1,
            description: { $ifNull: [ "$description", "Unspecified" ] }
         }
      }
   ]
)

{ _id: 1, item: "buggy", description: "toy car" }
{ _id: 2, item: "bicycle", description: "Unspecified" }
{ _id: 3, item: "flag", description: "Unspecified" }

{
    create_time: {
        $toDate: '$create_date'
    }
}

db.details.aggregate([
    {
        $project: {
            create_date: 1,
            week: {
                $isoWeek: {
                    $toDate: '$create_date'
                }
            }
        }
    }
])