MongoDB Common Commands

Query whether a field exists

Db.student.findOne ({name:{$exists: true}})

Db.student.findOne ({' department.name ': {$exists: true}})

Db.student.findOne ({' department.name ': {$exists: false}})

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Determine if the Map object is empty, note that the size can not be judged by $GT, $LT, and the index is used to determine

Db.student.find ({class: {$not: {$size: 0}}}). Limit (2)

Db.student. Find ({' class.0 ': {$exists: 1}})

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Determine the size of an array

Db.student. Find ({' class ': {$size: 3}})

Db.student.find ({$where: "This.class.length < 3"})

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Formatted output

Select top 1 *from student;

Db.student.findOne ()

Select top 1 *from student where age>15;

Db.student.findOne ({age:{$gt: 15}})

Select Class,department from student where name= ' shell ' and gender=true;

Db.student.findOne ({name:shell,gender:true},{class:1,department:1})

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The number of cursor jump queries and qualified returns

Db.department.find (). Skip (10)

Select top Ten *from department;

Db.department.find (). Limit (10)

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The value returns the specified field, the first map writes the filter condition, the second map writes the returned field, and the default returns _ID, if it is not necessary to set the _id:0

Db.student.find ({},{name:1,id:1}

Db.student.find ({},{name:1,id:1,_id:0}

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Total queries

Db.student.find (). Count ()

Db.student.find ({age:{$lt:}}). Count ()

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Update and delete a field: Delete the first class and modify the update time to the current time

Db.student.update ({id:10005},{$unset: {' class.0 ': 1}, $set: {Updated:numberlong (New Date (). ValueOf ()/1000)}})

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MongoDB Common Commands