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2025-04-07 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > Database >
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1. Specify the return key 2, query condition 2.1 comparison operator $lt=
$nephrite =
2.2 query conditions query the names and ages of students aged 25 to 27 db.persons.find ({age: {$gte:25,$lte:27}}, {_ id:0,name:1,age:1}) query the math scores of all students who are not Korean students db.persons.find ({country: {$ne: "Korea"}}, {_ id:0,country:1) Db.persons.find ({country: {$in: ["China", "USA"]}}, {_ id:0,name:1,country:1}) query the name and country db.persons.find ({country: {$nin: ["China", "USA"]}}, {_ id:0,name:1) of students whose nationality is not Chinese or American. Country:1}) 2.4OR queries for information on students whose scores are greater than 85 in Chinese or 90 in English ({$or: [{c: {$gt:85}}, {e: {$gt:90}}]}, {_ id:0,name:1,c:1,e:1}) 2.5NULL finds out that students whose sex is null will first increase the gender of Chinese students by db.persons.update ({country: "China"}, {$set: {sex: "m"}). False,true) perform query operations db.persons.find ({sex: {$in: [null]}}, {_ id:0,country:1}) 2.6 regular query db.persons.find ({name:/zhangsan}, {_ id:0,name:1}) 2.7$ not is used to reverse db.person.find ({name: {$not:/zhang/}}, {_ id:0) Name:1}) 2.8. applications of $all and index query students who like "Mongodb" and "JS" db.persons.find ({books: {$all: ["MONGODB", "JS"]}}, {_ id:0,books:1,name:1}) query information about students whose second book is java db.persons.find ({"books.1": "JAVA"}, {_ id:0,name:1,books:1}) 2.9 query array $size of specified length It cannot be used with comparators. (drawbacks) db.persons.find ({books: {$size:5}}, {_ id:0,name:1}) 2.10 query the 2nd to 4th books on the jim shelf db.persons.find ({name: "jim"}, {books: {$slice: [1m3]}}) 2.11 query the last book db.persons.find ({name: "jim"}, {books: {$slice:-1}, _ id:0) Name:1}) 2.12 document query add educational background document to jim var jim = [{
School: "K"
Score: "A"
}, {
School: "L"
Score: "B"
}, {
School: "J"
Score: "A+"
}]
Db.persons.update ({name: "jim"}, {$set: {school:jim}}) query students who have studied in K db.persons.find ({"school.school": "K"}, {_ id:0,name:1}) db.persons.find ({school: {$elemMatch: {"school": "K"}}, {_ id:0,name:1}) 2.13$ where query age over 22 years old, like reading C++ books We can use $where for complex queries about students who have studied in K School, because he is omnipotent, but we should try to avoid using it less because it will have a performance cost.
Db.persons.find ({"$where": function () {
/ / every document that gets the query result
Var books = this.books
/ / get the school object in the document
Var school = this.school
/ / if age > = 22
If (this.age > 22) {
Var php = null
/ / traversing books
For (var I = 0; I < books.length; iTunes +) {
If (books[ I] = = "C++") {
Php = books [I]
/ / if the school is real
If (school) {
For (var j = 0; j < school.length; jacks +) {
/ / to judge whether to go to school in K or not.
If (school [j] .school = = "K") {
/ / the return is true
Return true
}
}
Break
}
}
}
}
})
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