Explain which table to choose "FROM" in a JOIN statement

Question

I'm new to SQL and am having trouble understanding why there's a FROM keyword in a JOIN statement if I use dot notation to select the tables.columns that I want. Does it matter which table I choose out of the two? I didn't see any explanation for this in w3schools definition on which table is the FROM table. In the example below, how do I know which table to choose for the FROM? Since I essentially already selected which table.column to select, can it be either?

For example:

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
INNER JOIN Orders
ON Customers.CustomerID=Orders.CustomerID
ORDER BY Customers.CustomerName;

Answer 1

The order doesn't matter in an INNER JOIN.

However, it does matter in LEFT JOIN and RIGHT JOIN. In a LEFT JOIN, the table in the FROM clause is the primary table; the result will contain every row selected from this table, while rows named in the LEFT JOIN table can be missing (these columns will be NULL in the result). RIGHT JOIN is similar but the reverse: rows can be missing in the table named in FROM.

For instance, if you change your query to use LEFT JOIN, you'll see customers with no orders. But if you swapped the order of the tables and used a LEFT JOIN, you wouldn't see these customers. You would see orders with no customer (although such rows probably shouldn't exist).

Answer 2

The from statement refers to the join not the table. The join of table will create a set from which you will be selecting columns.