When we are joining two data frames why `FROM` statement doesn't need both data frames names?

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#1

My question is the same as in the title: when we are joining two data frames why FROM statement doesn’t need both data frames names?

for example:

SELECT c.*, f.name country_name FROM facts f
INNER JOIN cities c ON c.facts_id = f.id
LIMIT 5;

Why does from has just facts instead of facts and cities?
Does it matter which one I choose (when I choose just one)?

#2

The simple answer: FROM doesn’t need two dataframes. Consider the simplest of queries:

SELECT *
FROM cities;

This is a perfectly valid query and FROM needs only one dataframe. It’s only when we want to JOIN two dataframes that we actually need the name of another dataframe. Therefore we name the second dataframe only when we use a JOIN statement and the name of the second dataframe is part of the JOIN statement.

Yes, it will matter which one you choose if you’re only choosing one – you will need to choose the one that has the features you’re looking to query. ie the features you place in the SELECT statement.

#3

…so when I join two, it doesn’t matter which I choose in FROM statement? Am I understand this particular case correctly?

#4

The only time order doesn’t matter is when doing an inner join. For all other types of joins, it will matter which is in the FROM statement and which one is in the JOIN statement.

Check out this stackoverflow post for a more in depth explanation.