【MonogoDB】The high update operation of MongoDB，monogodb

【MonogoDB】The high update operation of MongoDB，monogodbmongodb

In the last four blog, we talked about the high query operation of mongodb. Now in this blog let's know about the high update operation about monodb.

1. Save

as for save(), whose function is to update the existing record and to create the new record.
grammar: db.mycollection.save(x);

2. General Operation

2.1 $inc 

grammar: { $inc : { field : value } }
function： add the value to the certain field. 
example: 

2.2 $Set and $unset

{ $set : { field : value } }  /  { $unset : { field : 1 } } 
function: set the field value, if the field does not exist, then add the new field, and value will support all the type. 

2.3 $push, $pushAll,$addToSet

{ $push : { field : value }  } 
function: if the field is a existing arry, this function will add the value to the end.  if filed doesnot exist, it will add the new field, and then set the field value.  if field do exists but not array, it will throw error. 
$pushAll:  { $pushAll : {field : value_array } } 
this function is the same as $push, but only the value_array is array.
$addToSet: { $addToSet : { field : value } }

2.4   $pop and $pull $pullAll

2.5 $rename

{ $rename : { old_friend_name: new_field_name}}

本来就选A，楼主疑惑的是不是from the top of the high building?其实不是这里的from, from where 的from是来源于see from，因为有定语从句所以提前到先行词前面，变成现在见到的A选项，原来的顺序是：
you could see the whole city from the top of the high building.
这个解释可以不？
 

当分句的主语和主句一样时，就可以省略掉，所以when分句完整的应该是：

When it is compared with the size of the whole earth