Differences between Merge Join and hash join

Hash Join and Merge Join are two different types of join operators in SQL Server, each optimized for different scenarios based on the data characteristics, availability of indexes, and the size of the data. Below are the differences between them.

Sno	Feature	Hash Join	Merge Join
1	Use Case	Best for large, unsorted datasets without indexes	Best for sorted or indexed datasets on join columns
2	Data Requirements	No specific data order required	Both inputs must be sorted on the join key
3	Execution Strategy	Builds a hash table on one input and probes with the other	Sequentially matches rows from two sorted inputs
4	Performance	Efficient for large datasets without indexes	Efficient for large, pre-sorted datasets
5	Index Dependency	No index needed	Requires sorted inputs or indexed join columns
6	Memory Requirement	High; memory needed to store the hash table	Lower than hash join, as it requires sorted inputs only
7	Typical Complexity	O(N + M) for equality joins	O(N + M), but only with sorted inputs
8	Join Type Support	Best for equality joins (e.g., =)	Supports both equality and inequality joins
9	Spill to Disk	May spill to disk if insufficient memory for hash table	Low risk; relies on sequential access
10	Scenarios	Ideal for large, unsorted data, no indexes available	Ideal for sorted or indexed large tables, low memory usage