Drill down to query expression

($where,$regex is not recommended! Slow)

Query expression:

1: the simplest query expression

{filed:value}, which refers to querying documents whose field column has a value of value

2: $ne--! = query expression

{field: {$nq:value}}

Function-check documents where the value of filed column is not equal to value

3: $nin-- > not in

4: $all

Syntax: {field: {$all: [v1Magne v2..]}}

Indicates that the field column is an array and contains at least v1 and v2 values

5: $exists

Syntax: {field: {$exists:1}}

Function: query out documents with field fields

6: $nor

{$nor, [condition 1, condition 2]}

It means that a document that does not meet all conditions is returned as true.

7: use regular expressions to query products that start with "Nokia"

Example: db.goods.find ({goods_name:/ Nokia. * /}, {goods_name:1})

8: query with $where expression

Example: db.goods.find ({$where:'this.cat_id! = 3 & & this.cat_id! = 11'})

9: $regex

Note: when querying with $where, mongodb is an object that converts the binary data of the bson structure into the json structure, and then compares whether the object's properties satisfy the expression. Slow speed