airline | 基于D3.js的全球机场，航线动态可视化

This might seem as though it has a simple answer... however... the simple answer as to to why your first query can't give you what you want is that you never told it to group on the FLIGHTS.origin_city and so it doesn't know what to do with that field. You added an Aggregate function of MAX but you also added another field... so now it's not only looking at FLIGHTS.actual_time but also FLIGHTS.origin_city and so now when we group the FLIGHTS.origin_city the query knows you are looking to see the MAX actual_time per each origin_city.

The query doesn't know which city you are needing a MAX(FLIGHTS.actual_time). If you want to know a specific origin_city and its MAX actual_time you can add a filter to the WHERE clause to specify which origin_city you want to see... otherwise by adding the origin_city to the GROUP BY Clause this now tells the query you wish to see the MAX actual_time per each city.

If it's not added to the GROUP BY clause the SQL doesn't know how to use MAX.

Try this: library(shiny) library(tidyverse)

Use Postgres's parameter placeholder instead of sqlx one. Change from

Vim colorschemes typically define styling attributes for basic terminals (term), color terminals (cterm*), and GUI (gui*). Here is an example:

Well, I managed to get your solution by using aggregateByKey function and map to return the desired "schema":

Since you are only practicing and the integrity of the data in the current database doesn't matter I suggest doing the following:

• Delete the db.sqlite3 file in the root of your project.
• Find the migrations folder within your app where the flights models are and delete all of the files inside except for __init__.py.
• Run python manage.py makemigration and python manage.py migrate again.

I find this the easiest method when situations like this occur. Do not do this in situations where you have data in your db you need to keep though.

You are mistakenly double defining variables.

In exports.displayFlights there is no need to again initialize the variables and removing those two lines will solve your problem.

Since, there is no req.body in exports.displayFlights your variables are getting initialized to undefined.

Ok after our discussion here is what you need to do :

• $unwind deconstruct content array
• $group by airline and content and get the total count
• $group by the only airline and construct counts array key-value format
• $arrayToObject convert key-value array to object