Ms Access Update One Table From Another Sql Interview
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. Using INNER JOINSFor all SQL Server installations, the most basic method of performing this action is to use an INNER JOIN, whereby values in the columns of are compared to one another. UPDATE books SET books. Primaryauthor = authors. Name FROM books INNER JOIN authors ON books.
Authorid = authors. Id WHERE books. Title = 'The Hobbit'In the above example, we’re UPDATING the books.primaryauthor 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.authorid. Using MERGE to UPDATE and INSERT SimultaneouslyFor SQL Server 2008 and newer, Microsoft introduced the exceptionally useful 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. Authorid = authors.
Ms Access Update Query Multiple Tables
Id WHEN MATCHED THEN UPDATE SET books. Primaryauthor = authors. Name WHEN NOT MATCHED THEN INSERT ( books. Authorid, books. Primaryauthor ) VALUES ( 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. WHEN NOT MATCHED THEN INSERT ( books.
Authorid, books. Primaryauthor ) VALUES ( authors. Name )Here we’re simply asking SQL to INSERT a new record into the books table and passing along the values for the authorid and primaryauthor 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.primaryauthor 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.