Mongodb aggregation statistics

Grouping statistics

• Count how many are different name The data of 【 {$group: {“_id”: “$name”}} 】
• Each same name How many times has it appeared , Every time , Once 【 count: {$sum: 1} 】

db.event.aggregate(
[{
    
	$group: {
    
		"_id": "$name",
		count: {
    $sum: 1}
	}
}]
)

• Count how many are different name The data of 【 {$group: {“_id”: “$name”}} 】
• Each same name How many times has it appeared , Every time , Count ten times 【 count: {$sum: 10} 】

db.event.aggregate(
[{
    
	$group: {
    
		"_id": "$name",
		count: {
    $sum: 10}
	}
}]
)

Do not group statistics

• Count how many name The data of 【 {$group: {“_id”:null}} 】
• Each same name How many times has it appeared , Every time , Once 【 count: {$sum: 1} 】

db.event.aggregate(
[{
    
	$group: {
    
		"_id": null,
		count: {
    $sum: 1}
	}
}]
)

• All in the statistical table age Total of 【 {$group: {“_id”:null}} 】
• Each same name How many times has it appeared , Every time , Once 【 count: {$sum: “$age”} 】

db.event.aggregate(
[{
    
	$group: {
    
		"_id": null,
		count: {
    $sum: "$age"} 
	}
}]
)

Be careful

1. Not in groups ：“_id” Set to null
2. When $ When the symbol appears in the numerical position , Must be added single / Double quotes , Such as {$sum: “$age”} in $age Put quotes