What are the data merging strategies from a database developer’s perspective?

Data merging (MERGE) in databases is an operation that allows performing INSERT, UPDATE, or DELETE on the target table based on data from the source. There are several merging strategies that define the logic of comparison and data modification.

Main strategies from a database developer's perspective:

1. Key-Based Matching:

   • The most common strategy.
   • Comparison of source and target rows is done based on one or more key columns.
   • If keys match (WHEN MATCHED), an UPDATE or DELETE is performed on the target table.
   • If keys are not found in the target table (WHEN NOT MATCHED BY TARGET), an INSERT from the source table is performed.
   • There may be a condition for rows missing in the source but present in the target (WHEN NOT MATCHED BY SOURCE) to perform DELETE in the target table (not supported by all DBMS).

   Example syntax (for SQL Server):

   -- Merging data from SourceTable into TargetTable
   MERGE TargetTable AS TGT
   USING SourceTable AS SRC
   ON TGT.PrimaryKeyColumn = SRC.PrimaryKeyColumn -- Key matching
   WHEN MATCHED THEN -- Row found in both tables
       UPDATE SET TGT.Column = SRC.Column; -- Update
   WHEN NOT MATCHED BY TARGET THEN -- Row exists in SourceTable but not in TargetTable
       INSERT (PrimaryKeyColumn, Column) VALUES (SRC.PrimaryKeyColumn, SRC.Column); -- Insert
   -- WHEN NOT MATCHED BY SOURCE THEN -- Row exists in TargetTable but not in SourceTable (not always supported)
   --    DELETE; -- Delete
   

2. Checksum-Based Matching:

   • Compares a checksum (e.g., SHA-256, MD5) or hash of the entire set of row columns, excluding keys.
   • If keys match but checksums differ, data has changed, and an UPDATE is performed.
   • Efficient when there are many columns, as it avoids row-by-row comparison of all data.
   • Requires additional checksum calculation, which can be overhead.

3. Timestamp/Version-Based Matching:

   • Uses a special column (timestamp, version, modification date) to determine if a row has changed since the last merge.
   • When keys match and a newer timestamp exists in the source, an UPDATE is performed.
   • Requires the presence and updating of the relevant column in both tables.

Each strategy has its advantages and disadvantages, and the choice depends on specific requirements for performance, data volume, merge logic complexity, and DBMS capabilities. Key-based matching is most commonly used due to its simplicity and universality.