HASH JOIN Concept (Detailed)

HASH JOIN Concept (Detailed) is a SQL statement in the JOIN Types category. Hash join is the most common join algorithm for large, unsorted tables joined on equality conditions. The syntax is -- Hash join: build a hash table on the smaller input, probe with the larger input.. It returns join algorithm explanation. A typical example: -- Phase 1: Build -- Read the smaller table (e.g., departments with 50 rows) -- Create a hash table in memory using the join key (department.id) -- Phase 2: Probe -- Read the larger table (e.g., employees with 10000 rows) -- For each employee, hash the dept_id and look up in hash table -- If match found, output the combined row -- Cost: O(n + m) -- linear in both… A close relative is INNER JOIN, which returns only rows where there is a match in BOTH tables. The most common JOIN type.