Identify the Join Type from SQL Consider the SQL statement: SELECT CUSTOMER_T.CUSTOMER_ID, ORDER_T.CUSTOMER_ID, NAME, ORDER_ID FROM CUSTOMER_T, ORDER_T WHERE CUSTOMER_T.CUSTOMER_ID = ORDER_T.CUSTOMER_ID Which join type best describes this query?

Introduction:
Determining the join type from SQL syntax is a key reading skill. The classic comma-join with a WHERE clause equating keys is an inner equi-join, even though it does not use the explicit JOIN ... ON syntax introduced in later SQL standards.

Given Data / Assumptions:

• Two tables: CUSTOMER_T and ORDER_T.
• Join condition: CUSTOMER_T.CUSTOMER_ID = ORDER_T.CUSTOMER_ID.
• Traditional comma-separated FROM clause is used with a WHERE predicate.

Concept / Approach:
An equi-join is any join where the join predicate is equality on one or more columns. The query uses a WHERE clause with equality on CUSTOMER_ID, producing only matching rows from both tables, which is the definition of an inner equi-join. A natural join would match identically named columns implicitly and is not shown here. An outer join would include unmatched rows, which this query does not do. A Cartesian join has no join predicate and returns the product of rows from both tables.

Step-by-Step Solution:
Observe the equality predicate on CUSTOMER_ID in the WHERE clause.Conclude that only rows with equal keys from both tables are returned.Classify this as an inner equi-join due to the equality condition.Exclude outer join or Cartesian join because their behaviors do not match this predicate.

Verification / Alternative check:
Rewrite using explicit syntax: SELECT ... FROM CUSTOMER_T INNER JOIN ORDER_T ON CUSTOMER_T.CUSTOMER_ID = ORDER_T.CUSTOMER_ID; The result is the same, confirming the equi-join classification.

Why Other Options Are Wrong:

• Natural join is implicit on identically named columns and is not explicitly used here.
• Outer join would retain non-matching rows, which the WHERE equality eliminates.
• Cartesian join lacks a join predicate entirely.
• Cross apply is a vendor-specific lateral join construct, not applicable here.

Common Pitfalls:
Assuming that comma-join syntax always means a Cartesian product; once you add an equality predicate in WHERE, it becomes an inner equi-join.

Final Answer:
Equi-join