Natural Join (self doubt)

Question

Question: Behavior of Natural Join with Common Attribute in Different Domains

Suppose there are two relations, R and S, and both have a common attribute named 'a'. However, the attribute 'a' belongs to different domains in the two relations (for example, R.a is an integer and S.a is a string, or R.a is a string and S.a is a date).

What will happen if I perform a natural join between R and S in this case?

• Will the query throw an error due to the domain mismatch?

• Or will it return an empty result set?

I came across information suggesting that comparisons between compatible types (like integers and strings) might not throw errors in SQL but could result in an empty set. In contrast, incompatible types (like strings and dates) might raise an error. Can someone clarify how this works, especially in the context of natural joins?

Answer 1

Deeper Explanation:

1. Natural Join Basics:
A natural join automatically matches columns by name and implicitly adds an equality condition between them (e.g., R.a = S.a).

2. Domain/Type Requirements:
When SQL processes the join condition (R.a = S.a), it expects that the data types of R.a and S.a are either:

• exactly the same
• or compatible types that can be safely compared (based on type coercion rules of the database).

3. What Happens When Domains Differ?

Case	Example	Outcome
Strongly incompatible types	string vs. date, integer vs. date	Error (type mismatch)
Possibly compatible types	integer vs. string	Allowed in some systems, but usually no match (empty result)
Compatible types	integer vs. integer	Works normally

4. Why Errors Occur:

• SQL engines need to compare values.
• Comparing values of fundamentally different types (like "abc" and a timestamp) often doesn’t make sense, and SQL refuses to guess how to do it.
• Examples:
  • PostgreSQL: strict will throw an error.
  • MySQL: more permissive might silently cast (e.g., string to 0 in numeric context), which can lead to odd behaviours.
  • SQL Server: somewhere in between sometimes allows implicit conversions, sometimes errors.

5. Typical SQL behaviour:

• PostgreSQL: Natural join will throw an error if the common attribute has incompatible types.
• MySQL: Might allow the natural join but produce an empty result set because implicit type conversion leads to no matches.
• SQL Server: Depends might allow or throw an error.