Find detailed explanation

Query an array equivalently:

Db.getCollection ("test") .find ({tags: ["good", "book", "it", "program"]})

(/ / even the order must be the order in which it is stored, which cannot be changed)

Query a value in the array:

Db.getCollection ("test") .find ({tags: "good"})

Query an array of four elements:

Db.getCollection ("test") .find ({tags: {$size: 4}})

Query whether there is a specified field or no specified field and the specified field is null:

Db.getCollection ("test") .insert ([{_ id: 2222, toy: null}, {_ id: 1112}]); db.getCollection ("test"). Find ({_ id: 2222, toy: null}); db.getCollection ("test"). Find (toy: null); / / error db.getCollection ("test"). Find ({_ id: 2222, toy: {$exists: true}}) / / find the current db.getCollection ("test"). Find ({toy: {$exists: true}); / / find the current db.getCollection ("test"). Find ({toy: {$exists: false}}); / / find all the objects that do not have the field toy

Find the return value cursor:

Db.getCollection ("test") .find () .forEach (function (item) {print (item.name, item.price, item.tags);})

Limit,skip method:

Db.getCollection ("test"). Find (). Limit (1) db.getCollection ("test"). Find (). Skip (2)

Query for $in:

Db.getCollection ("test") .find ({_ id: {$in: {12, objectId ("56970120abt538296thg0y6")}}); / / find documentation records where _ id equals 12 or objectId ("56970120abt538296thg0y6")

($in is used for different documents to specify the same Key for or condition matching, and $or can specify multiple Key or condition matching.)

Interval search:

Db.getCollection ("test") .find ({price: {$gt: 3, $lt: 33}}); / / query for values with a price range greater than 3 kids 33. Can be used in document numeric fields or array fields