MongoDB Learning (iv)

One, Mongodb projection

Projection means that you select only the data for the field you want, rather than selecting the data for the entire document field. (That is, return the specified field)

The second parameter of the Find method is the Set field display parameter 1 shows the current field 0 hides the current field

Find ({},{key:1})

Two, MongoDB limit record number

Limit (number) returns the number bar data

Skip (number) Skips the result of the query to the number bar data returned

Third, MongoDB sort

Sort ({key:1}) sorted ascending by key field 1 Ascending-1 descending

Iv. MongoDB Set Index

Improve query efficiency, you can create a single index to create a composite index

Key: Name of the field that created the index 1 ascending-1 Creating an index in descending order

Ensureindex ({key:1},{unique:true}) CREATE index

Background: Create an index in the background. So that building an index does not prevent other database activities

Unique: Create a unique index

Name: Index names

Five, MongoDB aggregation query

Aggregate ()

$sum summation

$avg mean value

$min Minimum value
$max Maximum Value
$push adding values to the array can be repeated
$addToSet adds a value to the array, but does not create duplicates

MongoDB Learning (iv)