Two tables may be joined:

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BlackTom AI · Accepted Answer

Question restatement: The prompt asks about how two tables may be joined in a database context, and provides four options to choose from. 
Option 1: 'Using only the primary key in one table and foreign key in another.' While joining on keys is a common pattern and joining on a primary key to a foreign key is a typical and efficient choice, this option is not universally required or the only way to join tables. Joins can occur on non-key columns as long as there is a logical relation and matching values, so this statement is too restrictive. 
Option 2: 'Using any column in either table with a similar data type.' This is the best generalization among the options: as long as the columns have compatible data types (and, in practice, compatible semantics and values), you can specify a join condition that relates them (e.g., ON table1.colA = table2.colB). The similarity of data types is a prerequisite for a meaningful join condition, though you still need a valid predicate beyond type compatibility. 
Option 3: 'Only using the Cartesian product formula.' A Cartesian product ( CROSS JOIN ) produces all possible combinations of rows from both tables and is not a join on related data; it is a valid operation but not a join based on relationships, and it is not the general method for joining two tables. Therefore this statement is incorrect as a general rule. 
Option 4: 'Only to columns in another table with exactly the same length and data type.' While having the same data type is important for compatibility, requiring identical length is not a standard or necessary constraint for SQL joins; length may differ (for example, char(n) vs varchar(n)) and still be joinable if the data types are compatible. This makes the statement too strict and inaccurate in practical terms. 
Summary of reasoning: The most accurate general claim among the choices is that a join can be performed using columns from either table that share similar data types, as this enables a valid equality (or other) join condition. The other options impose constraints (strictly key-based, Cartesian product only, or exact length equality) that do not reflect how joins are typically defined or implemented in relational databases.

Two tables may be joined:

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