In Structured Query Language SQL, consider the query SELECT CUSTOMER_T.CUSTOMER_ID, ORDER_T.CUSTOMER_ID, NAME, ORDER_ID FROM CUSTOMER_T, ORDER_T; Which type of join operation between CUSTOMER_T and ORDER_T does this query effectively perform when no WHERE clause is specified?

Introduction / Context:
This question tests your knowledge of join semantics in SQL, especially when tables are listed in the FROM clause without an explicit join condition. Understanding how SQL combines rows from multiple tables is essential when writing queries and debugging unexpected result sets.

Given Data / Assumptions:

- The query selects columns from CUSTOMER_T and ORDER_T.- The FROM clause lists CUSTOMER_T and ORDER_T separated by a comma.- No WHERE clause or explicit join condition is shown.

Concept / Approach:
In SQL, when two tables appear in the FROM clause separated by a comma and there is no join condition in the WHERE clause or using the JOIN keyword, the result is a Cartesian product. A Cartesian join pairs every row from the first table with every row from the second table. Equi joins, natural joins, and outer joins all require some kind of condition that matches columns between tables, which is absent here.

Step-by-Step Solution:
Step 1: Observe that CUSTOMER_T and ORDER_T are listed in the FROM clause without any join condition.Step 2: Recall that the old style join syntax uses a WHERE clause to specify how rows should match between tables.Step 3: Note that since there is no WHERE clause in the given query, no matching condition has been applied.Step 4: When there is no join predicate, SQL forms the Cartesian product of the two tables.Step 5: The Cartesian product is commonly referred to as a Cartesian join.Step 6: Therefore, the correct classification of this query is a Cartesian join.

Verification / Alternative check:
You can verify this by considering a simple example. If CUSTOMER_T has three rows and ORDER_T has four rows, an equi join on CUSTOMER_ID would produce only those combinations where the customer identifiers match. However, the query in the question would produce 3 multiplied by 4 equals 12 rows, because every customer row is paired with every order row. This behavior is by definition the Cartesian product of two sets, confirming that the query represents a Cartesian join.

Why Other Options Are Wrong:
An equi join requires a join condition using equality between columns of the two tables, which is missing here. A natural join also uses matching column names to generate an implicit join condition, but there is no NATURAL JOIN keyword shown. Outer joins extend equi or natural joins to include unmatched rows from one or both tables, again needing a join condition. A self join involves joining a table with itself, but this query involves two different tables, CUSTOMER_T and ORDER_T.

Common Pitfalls:
Developers sometimes forget to include join conditions, which leads to a Cartesian product and an unexpectedly large result set. This mistake can cause performance problems and incorrect data analysis. Always check that each pair of tables in a multi table query has an appropriate join condition unless a Cartesian product is explicitly intended for tasks such as testing or generating combinations.

Final Answer:
The query performs a Cartesian join between CUSTOMER_T and ORDER_T.