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$concat(聚合)
定义
举例
请考虑包含以下文档的 inventory
集合:
{ "_id" : 1, "item" : "ABC1", quarter: "13Q1", "description" : "product 1" } { "_id" : 2, "item" : "ABC2", quarter: "13Q4", "description" : "product 2" } { "_id" : 3, "item" : "XYZ1", quarter: "14Q2", "description" : null }
以下操作使用$concat
操作符通过“-”分隔符连接item
字段和description
字段。
db.inventory.aggregate( [ { $project: { itemDescription: { $concat: [ "$item", " - ", "$description" ] } } } ] )
操作返回以下结果:
{ "_id" : 1, "itemDescription" : "ABC1 - product 1" } { "_id" : 2, "itemDescription" : "ABC2 - product 2" } { "_id" : 3, "itemDescription" : null }