How to use value from one table as column name in another table in SQL query

Question

Let's say I have two tables:

Flights looks like:

flight_num	dep_apt_code	arr_apt_code	date
34	ATL	JFK	2022-06-01
48	IAD	SFO	2022-06-02

and Weather:

date	ATL	IAD	JFK	SFO
2022-06-01	cloudy	windy	rainy	sunny
2022-06-02	sunny	rainy	rainy	windy

where the names of the columns correspond to values in two of the columns in the Flights table.

Currently, I want to reference the column corresponding to Flights.dep_apt_code in Weather to create a table like:

date	flight_num	dep	arr	weather
2022-06-01	34	ATL	JFK	cloudy
2022-06-02	48	IAD	SFO	rainy

but I haven't been able to figure it out. I'm not the biggest expert on advanced queries, so even if someone can just point me to resources that may help me figure this out, it would be greatly appreciated.

I tried something like:

SELECT
    F.date,
    F.flight_num,
    F.dep_apt_code as dep,
    F.arr_apt_code as arr,
    W.F.dep as weather
FROM Flights as F JOIN Weather as W
WHERE F.date = W.date;

but obviously that doesn't work, I just don't know syntactically how to do it.

How does one reference a column using a value from another table?

Normalize your structure, and the query will be trivial. of course, you may use CASE operator... — Akina
– Akina, Commented Jun 13, 2022 at 17:16

Bill Karwin · Accepted Answer · 2022-06-13 17:22:47Z

SQL requires all identifiers are fixed in your expressions at the time the query is parsed. But you could do a CASE expression like this:

SELECT
    F.date,
    F.flight_num,
    F.dep_apt_code as dep,
    F.arr_apt_code as arr,
    CASE F.dep_apt_code 
      WHEN 'ATL' THEN W.ATL 
      WHEN 'IAD' THEN W.IAD 
      WHEN 'JFK' THEN W.JFK
      WHEN 'SFO' THEN W.SFO 
    END AS dep_weather
FROM Flights AS F JOIN Weather AS W ON F.date = W.date;

The comment above says that you should normalize your structure. This means to store the weather per city on individual rows instead of in columns on one row.

date	apt_code	weather
2022-06-01	ATL	cloudy
2022-06-01	IAD	windy
2022-06-01	JFK	rainy
2022-06-01	SFO	sunny
2022-06-02	ATL	sunny
2022-06-02	IAD	rainy
2022-06-02	JFK	rainy
2022-06-02	SFO	windy

Then you can get the value by joining to the row that matches both the date and the airport code:

SELECT
    F.date,
    F.flight_num,
    F.dep_apt_code as dep,
    F.arr_apt_code as arr,
    W.weather AS dep_weather
FROM Flights AS F JOIN Weather AS W ON F.date = W.date AND F.dep_apt_code = W.apt_code;

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