Mongodb query and projection operators

One 、 Compare

mongodb Common comparison query operators ：

name	describe
$eq	Match the value equal to the specified value , namely =
$ne	Matches all values that are not equal to the specified value , namely !=
$gt	Match values greater than specified , namely >
$gte	Match values greater than or equal to the specified value , namely >=
$lt	Match values less than specified , namely <
$lte	Match values less than or equal to the specified value , namely <=
$in	Matches any value specified in the array
$nin	Does not match any of the values specified in the array

	db.people.find( {
     "name": "xiaoming" } );			// :  be equal to 
	db.people.find( {
     "name": {
     $eq: "xiaoming"} } );	//$eq  be equal to 
	db.people.find( {
     "name": {
     $ne: "xiaoming"} } );	//$ne  It's not equal to 

	db.people.find( {
     "age": {
     $gt: 18 } } ); 	//gt  Greater than 18
	db.people.find( {
     "age": {
     $gte: 18 } } );	//gte  Greater than or equal to 18
	
	db.people.find( {
     "age": {
     $lt: 30 } } );	//lt  Less than 30
	db.people.find( {
     "age": {
     $lte: 30 } } );	//lte  Less than or equal to 30

	db.people.find( {
     qty: {
     $in: [ 5, 15 ] } } );
	db.people.find( {
     qty: {
     $nin: [ 5, 15 ] } } );

Two 、 Logic

name	describe
$and	Join query clauses logically AND All documents that match both clauses will be returned .
$not	Reverse the effect of a query expression , And return the document that does not match the query expression .
$nor	Join query clauses logically NOR All documents that do not match the two clauses will be returned .
$or	Join query clauses logically OR All documents that meet the conditions of any clause will be returned .

2.1 $and

2.2 $not

2.3 $nor

2.4 $or

3、 ... and 、 Elements

name	describe
$exists	Match the document with the specified field .
$type	If the field is of the specified type , Then select a document .

3.1 $exists

3.2 $type

Four 、 assessment

5、 ... and 、 Geographical space

6、 ... and 、 Array

7、 ... and 、 Bitwise

8、 ... and 、 notes

Nine 、 Projection operator

To be added ...