A self join is a type of join operation in SQL where a table is joined to itself. This is different from joining two separate tables. Instead, you're essentially creating a virtual copy of the table to compare rows within the same table. This is particularly useful when you need to find relationships between different rows of the same table, such as finding hierarchical relationships (e.g., reporting structures in an organization), or identifying pairs of items with a specific relationship (e.g., finding customers who purchased the same product). The key is identifying a common column that links rows in the self-joined table. The syntax is similar to a regular join, but you're joining the table to itself. Crucially, you need to give different aliases to the table to distinguish between the two copies of the table in the join condition. This is essential for avoiding ambiguity in the query.