Question

Consider a real-life business situation and discuss under what circumstances you would consider using the MERGE statement. In addition to detail explanation, provide an SQL example to merge at least three tables. Your example should be unique (do not use textbook and/or internet examples).

Answer 1

Solution: The Merge statement or the Merge procedure in the world of databases is actually a very strong procedure that is required when the programmer needs to modify an existing table on the basis of the values that are stored in another table. If done properly, it can produce the effects of insert, update as well as delete in a single operation. In order to understand its working properly, let us first understand that it works upon the source table to finally bring changes to the destination table on the basis of the data that is contained within the source table.

Actually, these two tables are compared using a merge condition which is most of the time a primary key, that can be used to match the rows. This kind of operation is best suited when you are actually working on a database containing a number of tables and the database is large then you may have to synchronize the changes among the tables that you are working upon and hence you can simply apply the Merge operations over these tables. Please refer to the figure given down below to understand the Merge operation and the SQL code is also included to give you a better understanding.

Screenshot:

Code:

MERGE targetTable USING sourceTable
ON merge_condition
WHEN MATCHED
THEN update_records;
WHEN NOT MATCHED
THEN insert_records;
WHEN NOT MATCHED BY SOURCE
THEN DELETE;

Note: It is a generic program structure that would work in all situations.

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