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Posted by: Tara Kirkland
Under most circumstances, SQL updates are performed using direct references to a particular table (UPDATE books SET books.title = 'The Hobbit' WHERE books.id = 1). Yet, on occasion, it may prove beneficial to alter the contents of a table indirectly, by using a subset of data obtained from secondary query statement.
Performing an UPDATE using a secondary SELECT statement can be accomplished in one of two ways, primarily depending upon which version of SQL Server you are using. We’ll briefly explore both options so you can find what works best for you.
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For all SQL Server installations, the most basic method of performing this action is to use an INNER JOIN, whereby values in the columns of two different tables are compared to one another.
UPDATE
books
SET
books.primary_author = authors.name
FROM
books
INNER JOIN
authors
ON
books.author_id = authors.id
WHERE
books.title = 'The Hobbit'In the above example, we’re UPDATING the books.primary_author field to match the authors.name for ‘The Hobbit’ by JOINING both tables in the query to their respective, matching values of authors.id and books.author_id.
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For SQL Server 2008 and newer, Microsoft introduced the exceptionally useful MERGE operation which is similar to the above INNER JOIN method, but MERGE attempts to perform both an UPDATE and an INSERT command together. This effectively synchronizes the two tables based on the query performed, updating and inserting records as necessary for the two to match.
MERGE INTO
books
USING
authors
ON
books.author_id = authors.id
WHEN MATCHED THEN
UPDATE SET
books.primary_author = authors.name
WHEN NOT MATCHED THEN
INSERT
(books.author_id, books.primary_author)
VALUES
(authors.id, authors.name)The full query when using MERGE is certainly a bit more complex then that of a basic INNER JOIN, but once you grasp how the operation functions, you’ll quickly understand how powerful this capability can truly be.
The first few lines are rather self-explanatory:
MERGE INTO
books
USING
authors
ON
books.author_id = authors.idWe want to MERGE INTO (UPDATE/INSERT) the books table by using the secondary authors table, and we’re matching the two based on the same books.author_id = authors.id comparison.
Where the MERGE command differs is in the branching logic that follows.
WHEN MATCHED THEN
UPDATE SET
books.primary_author = authors.nameHere we’re asking SQL to perform an action only when records MATCHED – when an existing record is found. In that case, we perform a standard UPDATE just as we did before, setting the books.primary_author field to equal the authors.name field.
Finally, if the query discovers a matching comparative record that doesn’t exist, we instead perform an INSERT.
WHEN NOT MATCHED THEN
INSERT
(books.author_id, books.primary_author)
VALUES
(authors.id, authors.name)Here we’re simply asking SQL to INSERT a new record into the books table and passing along the values for the author_id and primary_author fields, grabbed from the associated authors table record.
The end result of our MERGE statement is that for every author in the authors table, we verify whether a corresponding book exists in books. If a record is found, we ensure books.primary_author is set using UPDATE, and where no match is found, we add a new record to books.
With that, you should have a solid understanding of two different methods that can be used to UPDATE records in SQL by using secondary, comparative SELECT statements.