6 Most Common SQL Join Interview Questions

Question 1

What is a self-join, and when would you use it?

Answer 1

A self-join is a join in which a table is joined with itself. It is useful when you need to compare rows within the same table or find relationships between rows of the same dataset.

When to Use:
* Hierarchical relationships: Finding managers and employees in an organization (e.g., manager_id in the same employees table).
* Comparing rows: Identifying duplicates or closely related data (e.g., finding overlapping date ranges).
* Recursive relationships: Traversing parent-child relationships (e.g., in category trees).

Question 2

What is an anti-join, and when would you use it?

Answer 2

An anti-join is a type of join that returns rows from one table that do not have a matching row in another table. It is often implemented using NOT EXISTS, NOT IN, or LEFT JOIN with a filter.

When to Use:
* Identifying unmatched records, such as:
* Customers who haven’t placed an order.
* Employees who are not assigned to a project.

Question 3

What are the performance considerations of SQL join queries?

Answer 3

Key Considerations:
* Indexes:
* Proper indexing on join keys (e.g., primary and foreign keys) can significantly improve performance.
* Missing or non-optimal indexes can lead to full table scans.

• Join Order:
  
  • The order of tables in a query matters for performance, especially in large datasets.
• Join Types:
  
  • INNER JOIN is generally faster than OUTER JOIN because it doesn’t need to account for unmatched rows.
  • CROSS JOIN can produce large result sets and should be avoided unless necessary.
• Data Skew:
  
  • Uneven data distribution can lead to performance bottlenecks.
• Row Size:
  
  • Avoid selecting unnecessary columns, especially with large SELECT * queries.

Question 4

How do you optimize a slow join query?

Answer 4

• Add Proper Indexes:
  
  • Index the columns used in join conditions.
    Use composite indexes for multi-column joins.
• Filter Early:
  
  • Use WHERE clauses to reduce the number of rows processed before the join.
• Use Smaller Result Sets:
  
  • Create temporary tables or Common Table Expressions (CTEs) to pre-filter data.
• Reevaluate Join Order:
  
  • Place the smaller table or dataset first in the join sequence.

Question 5

How do you join more than two tables?

Answer 5

To join more than two tables, simply chain multiple join operations together in the FROM clause, specifying the join condition for each.

Question 6

Does a join always have to be on two rows sharing the same value (non-equi joins)?

Answer 6

No, joins do not always require rows to share the same value. Non-equi joins use operators other than = in the join condition, such as <, >, BETWEEN, or other custom logic.