Question

请解释下面提到的两个查询之间的区别。我正在获取第一个查询的输出但第二个没有输出。提前谢谢。

表格数据：

> db.inventory.find()
{ "_id" : ObjectId("5911ce218c302726ff998853"), "item" : "journal", "instock" : [ { "warehouse" : "A", "qty" : 5 }, { "warehouse" : "C", "qty" : 15 } ] }
{ "_id" : ObjectId("5911ce218c302726ff998854"), "item" : "notebook", "instock" : [ { "warehouse" : "C", "qty" : 5 } ] }
{ "_id" : ObjectId("5911ce218c302726ff998855"), "item" : "paper", "instock" : [ { "warehouse" : "A", "qty" : 60 }, { "warehouse" : "B", "qty" : 15 } ] }
{ "_id" : ObjectId("5911ce218c302726ff998856"), "item" : "planner", "instock" : [ { "warehouse" : "A", "qty" : 40 }, { "warehouse" : "B", "qty" : 5 } ] }
{ "_id" : ObjectId("5911ce218c302726ff998857"), "item" : "postcard", "instock" : [ { "warehouse" : "B", "qty" : 15 }, { "warehouse" : "C", "qty" : 35 } ] }
{ "_id" : ObjectId("5911ce948c302726ff998858"), "item" : "journal", "instock" : [ { "warehouse" : "A", "qty" : 5 } ] }

查询1：

 > db.inventory.find( { "instock": { $elemMatch: { qty: 5, warehouse: "A" }}} )

输出：

{ "_id" : ObjectId("5911ce218c302726ff998853"), "item" : "journal", "instock" : [ { "warehouse" : "A", "qty" : 5 }, { "warehouse" : "C", "qty" : 15 } ] }
{ "_id" : ObjectId("5911ce948c302726ff998858"), "item" : "journal",    "instock" : [ { "warehouse" : "A", "qty" : 5 } ] }

查询2：

> db.inventory.find( { "instock.qty": { $elemMatch: { $gt: 10, $lte: 20 } } } )

输出：

无输出

Answer 1

来自docs

$ elemMatch运算符匹配包含数组字段的文档至少有一个元素匹配所有指定的查询标准。

注意field应该是一个数组。

查询数组内嵌入字段qty的第二个查询的正确语法是

db.inventory.find( { "instock": { $elemMatch: { qty: { $gt: 10, $lte: 20 } } } } )

您应该有一个类似下面的文档qty作为当前第二个查询返回结果的数组字段。

{ "item" : "journal", "instock" : [ { "qty" : [ 5, 15 ] }, { "warehouse" : "A", "qty" : 5 }, { "warehouse" : "C", "qty" : 15 }] }

在mongoDB中提到的两个查询之间的差异

1 个答案: