The following code is an example of a(n):

SELECT Customer_T.CustomerID, Order_T.CustomerID,
CustomerName, OrderID
FROM Customer_T, Order_T
WHERE Customer_T.CustomerID = Order_T. CustomerID;

Question

The following code is an example of a(n):

SELECT Customer_T.CustomerID, Order_T.CustomerID,
CustomerName, OrderID
FROM Customer_T, Order_T
WHERE Customer_T.CustomerID = Order_T. CustomerID;

BlackTom AI · Accepted Answer

The SQL uses a comma-separated FROM plus a WHERE equality condition, which is the classic implicit form of an INNER JOIN (an equijoin).

1. Relevant concepts
- JOIN types: INNER JOIN returns only rows that match on the join condition; OUTER JOINs (LEFT/RIGHT/FULL) include nonmatching rows with NULLs; NATURAL JOIN automatically matches columns with the same name; a subquery is a nested SELECT used in WHERE/SELECT, not a table-pair join.
- Implicit vs explicit join syntax: "FROM A, B WHERE A.col = B.col" is equivalent to "FROM A INNER JOIN B ON A.col = B.col".

2. Step-by-step reasoning
- FROM Customer_T, Order_T produces the Cartesian product of the two tables.
- WHERE Customer_T.CustomerID = Order_T.CustomerID filters that product to only rows where the CustomerID values are equal.
- The result is exactly the set of row pairs present in both tables that satisfy the equality — the definition of an inner (equijoin) match.

3. Intermediate transformation (showing equivalent explicit syntax)
- Original: SELECT ... FROM Customer_T, Order_T WHERE Customer_T.CustomerID = Order_T.CustomerID;
- Equivalent explicit form: SELECT ... FROM Customer_T INNER JOIN Order_T ON Customer_T.CustomerID = Order_T.CustomerID;

4. Why the correct answer is correct
- Because the WHERE equality condition restricts the Cartesian product to matching CustomerID values, this returns only rows present in both tables — that behavior is INNER JOIN.

5. Why other options are incorrect
- Natural JOIN: would use the NATURAL keyword and implicitly match same-named columns; the given code uses an explicit equality, so it is not NATURAL JOIN.
- Full Outer JOIN: would include nonmatching rows from both tables with NULLs; the WHERE equality prevents any nonmatches.
- Right Outer JOIN: would include nonmatching rows from the right table; the query does not preserve nonmatches.
- Subquery: no nested SELECT is used, so this is not a subquery.

6. Conclusion
Confirming the provided answer: Inner JOIN.

The following code is an example of a(n): SELECT Customer_T.CustomerID, Order_T.CustomerID, CustomerName, OrderID FROM Customer_T, Order_T WHERE Customer_T.CustomerID = Order_T. CustomerID; 单项选择题

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