MongoDB: $project

1.$project

0. Basic Usage

Run the following command to filter return fields

// Add field, delete field
{
    "$project" : {
        "userId" : "$_id"."$_id" : 0}}Copy the code

1. Mathematical expressions

Function: new field, return the basic operation (add, subtract, multiply, divide, mod) results;

// Multiple values can be added
{
    "$project" : {
        "total" : {
            "$add" : ["$fieldname1"."$fieldname2"."..."]}}}Copy the code

// The first value minus the second value
{
    "$project" : {
        "total" : {
            "$subtract" : ["$fieldname1"."$fieldname2"]}}}Copy the code

// Multiple values can be multiplied
{
    "$project" : {
        "total" : {
            "$multiply" : ["$fieldname1"."$fieldname2"."..."]}}}Copy the code

// The first value divided by the second value
{
    "$project" : {
        "total" : {
            "$divide" : ["$fieldname1"."$fieldname2"]}}}Copy the code

// The first value is mod by the second value
{
    "$project" : {
        "total" : {
            "$mod" : ["$fieldname1"."$fieldname2"]}}}Copy the code

2. String expressions

Function: string operation, clipping, splicing, uppercase, lowercase;

// Clipping param1, starting with param2, param3 bytes (bytes not characters, note in multi-byte encoding);
{
    "$project" : {
        "total" : {
            "$substr" : ["$fieldname".0.1]}}}Copy the code

// Concatenate all params in order (constants, results, other fields)
{
    "$project" : {
        "total" : {
            "$concat" : [
            "abc",
            {"$substr" : ["$fieldname".0.1]},
            "$fieldname"]}}}Copy the code

// Convert the field to lowercase
{
    "$project" : {
        "total" : {
            "$toLower" : "$fieldname"}}}Copy the code

// Uppercase the field
{
    "$project" : {
        "total" : {
            "$toUpper" : "$fieldname"}}}Copy the code

3. Logical expressions

Function: Some logical expressions can be used to control statements;

1. Compare expressions

$fieldName === 0 total = 0;
$fieldName > 0 total = 1;
$fieldName < 0 total = -1;
{
    "$project" : {
        "total" : {
            "$cmp" : ["$fieldname".100]}}}Copy the code

// This book is case sensitive. The latest official mongodb documentation is case insensitive
$fieldName === 0 total = 0;
$fieldName > 0 total = 1;
$fieldName < 0 total = -1;
{
    "$project" : {
        "total" : {
            "$strcasecmp" : ["$fieldname"."asd"]}}}Copy the code

//"$eq" "$ne" "$gt" "$gte" "$lt" "$lte"
// Param1 is compared with param2 and returns true or false
{
    "$project" : {
        "total" : {
            "$lt" : ["$fieldname".10]}}}Copy the code

2. Boolean expressions

//"$and" expr1 expr2... All expressions are true and return true, otherwise false
//"$or" expr1 expr2... Any expression true returns true, otherwise false
//"$not" expr1
{
    "$project" : {
        "total" : {
            "$and": [{"$gt": [ "$qty".100] {},"$lt": [ "$qty".250]}]}}Copy the code

3. Control statements

//"$cond" param1 returns param2 if true, param3 if not. (similar to the JS ternary operator)
{
    "$project" : {
        "total" : {
            "$cond": [{"$gte": [ "$qty".250]},30.20]}}}Copy the code

//"$ifNull" param1 ifNull returns param2, otherwise returns param1
{
    "$project" : {
        "total" : {
            "$ifNull": [ "$description"."Unspecified"]}}}Copy the code

4. More

The official documentation