4.Mongodb Data Query 2

1.limit &skip (1) limit

• Method Limit (): Used to read a specified number of documents
• Grammar:

Db. Collection name. Find (). Limit (number)

　　

• The number of the parameter indicates how many bars to get the document
• Displays all documents in the collection if no parameters are specified
• Example 1: Query 2 Student Information

Db.stu.find (). Limit (2)

　　

(2) Skip

• Method Skip (): Used to skip a specified number of documents
• Grammar:

Db. Collection name. Find (). Skip (number)

　　

• The number of the parameter indicates the count of skipped records, the default value is 0
• Example 2: Querying student information starting with 3rd

Db.stu.find (). Skip (2)

　　

(3) Use together

• Method Limit () and skip () can be used together, in order of precedence

• Create a data set

for (i=0;i<15;i++) {Db.t1.insert ({_id:i})}

　　

• Query 5th to 8th data

Db.stu.find (). Limit (4). Skip (5) or Db.stu.find (). Skip (5). Limit (4)

　　

2. Projection

• In the returned results of the query, select only the necessary fields instead of selecting the entire field of a document
• such as: A document has 5 fields, need to display only 3, projected 3 of the fields can be
• Grammar:
• parameter is field and value, value 1 indicates display, value 0 does not display

Db. Collection name. Find ({},{field name: 1,...})

　　

• For fields that need to be displayed, set to 1, not set to not display
• Special: For _id columns default is displayed, if not displayed need to explicitly set to 0

Example 1

Db.stu.find ({},{name:1,gender:1})

　　

Example 2

Db.stu.find ({},{_id:0,name:1,gender:1})

　　

3. Sorting

• Method sort (), which is used to sort the result set
• Grammar

Db. Collection name. Find (). Sort ({field: 1,...})

　　

• Parameter 1 is in ascending order
• Parameter-1 is in descending order
• Example 1: Descending according to sex, and then ascending according to age

Db.stu.find (). Sort ({gender:-1,age:1})

　　

4. Number of statistics

• Method count () is used to count the number of document bars in the result set
• Grammar

Db. Collection name. Find ({condition}). Count ()

• can also be associated with a

Db. Collection name. Count ({condition})

　　

• Example 1: Statistics on the number of males

Db.stu.find ({gender:1}). Count ()

　　

• Example 2: Number of males with a statistical age greater than 20

Db.stu.count ({age:{$gt: 20},gender:1})

　　

5. Eliminate duplication

• Method Distinct () to de-weight the data
• Grammar

Db. Collection name. Distinct (' Redo field ', {condition})

　　

• Example 1: Finding gender older than 18 (de-weight)

Db.stu.distinct (' Gender ', {age:{$gt: 18}})

　　

　　

4.Mongodb Data Query 2