$eq¶
- $eq¶
3.0 新版功能.
Specifies equality condition. The $eq operator matches documents where the value of a field equals the specified value.
{ <field>: { $eq: <value> } }
The $eq expression is equivalent to { field: <value> }.
Behavior¶
Comparison Order¶
For comparison of different BSON type values, see the specified BSON comparison order.
Match a Document Value¶
If the specified <value> is a document, the order of the fields in the document matters.
Match an Array Value¶
If the specified <value> is an array, MongoDB matches documents where the <field> matches the array exactly or the <field> contains an element that matches the array exactly. The order of the elements matters. For an example, see Equals an Array Value.
Examples¶
The following examples query against the inventory collection with the following documents:
{ _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] }
{ _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] }
{ _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] }
{ _id: 4, item: { name: "xy", code: "456" }, qty: 30, tags: [ "B", "A" ] }
{ _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }
Equals a Specified Value¶
The following example queries the inventory collection to select all documents where the value of the qty field equals 20:
db.inventory.find( { qty: { $eq: 20 } } )
The query is equivalent to:
db.inventory.find( { qty: 20 } )
Both queries match the following documents:
{ _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] }
{ _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }
Field in Embedded Document Equals a Value¶
The following example queries the inventory collection to select all documents where the value of the name field in the item document equals "ab". To specify a condition on a field in an embedded document, use the dot notation.
db.inventory.find( { "item.name": { $eq: "ab" } } )
The query is equivalent to:
db.inventory.find( { "item.name": "ab" } )
Both queries match the following document:
{ _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] }
Array Element Equals a Value¶
The following example queries the inventory collection to select all documents where the tags array contains an element with the value "B" [1]:
db.inventory.find( { tags: { $eq: "B" } } )
The query is equivalent to:
db.inventory.find( { tags: "B" } )
Both queries match the following documents:
{ _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] }
{ _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] }
{ _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] }
{ _id: 4, item: { name: "xy", code: "456" }, qty: 30, tags: [ "B", "A" ] }
| [1] | The query will also match documents where the value of the tags field is the string "B". |
Equals an Array Value¶
The following example queries the inventory collection to select all documents where the tags array equals exactly the specified array or the tags array contains an element that equals the array [ "A", "B" ].
db.inventory.find( { tags: { $eq: [ "A", "B" ] } } )
The query is equivalent to:
db.inventory.find( { tags: [ "A", "B" ] } )
Both queries match the following documents:
{ _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] }
{ _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }