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Qualified column name

As explained in Learn SQL in a Month of Lunches

A qualified column name identifies a column by combining its table name and column name, such as table.column.

When a query uses more than one table, identify a column with its table and column names: table.column. This prevents confusion when tables share a column name. section 8.2.1

Joining an order to its customer

SELECT orderheader.CustomerID, customer.FirstName, customer.LastName FROM orderheader JOIN customer ON orderheader.CustomerID = customer.CustomerID WHERE orderheader.OrderID = 1001; section 8.2.1

Here, orderheader.CustomerID and customer.CustomerID tell the database exactly which CustomerID to compare. Writing only CustomerID would be ambiguous because both tables contain that column. Two-part names also make a query easier to read and organize. section 8.2.1

SELECT
    orderheader.CustomerID,
    customer.FirstName,
    customer.LastName
FROM orderheader
JOIN customer
    ON orderheader.CustomerID = customer.CustomerID
WHERE orderheader.OrderID = 1001;

Inspect the complete join query and see qualified names used in SELECT, ON, and WHERE clauses. section 8.2.1

How Learn SQL in a Month of Lunches covers it

How deep it goes
Explains table.column syntax, ambiguity errors, readability, and use across SELECT, ON, and WHERE, without descending into database internals.
Examples
Demonstrates joining orderheader and customer to retrieve CustomerID and customer-name values for OrderID 1001.
What you can do after
Apply it via SQL to write clearer joins and avoid ambiguous column-reference errors.
How it connects
Connects qualified column names to SQL JOIN clauses, predicates, SELECT clauses, WHERE clauses, and ambiguous references.
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