Sql updating multiple rows with different values
Sql updating multiple rows with different values - teyana taylor who is she dating
Eventually every SET value is different, for example: UPDATE staff SET salary = 1125 WHERE name = ' Bob'; UPDATE staff SET salary = 1200 WHERE name = ' Jane'; UPDATE staff SET salary = 1100 WHERE name = ' Frank'; UPDATE staff SET salary = 1175 WHERE name = ' Susan'; UPDATE staff SET salary = 1150 WHERE name = ' John'; Since every SET clause is now different, we can’t coalesce any queries using the approaches mentioned before, so we still have one UPDATE statement for every row requiring changes. What if we had a database table which contained a representation of the updates we wanted to make, e.g.
If there is no database-specific subclass for the database in use, then will just use the base class which implements approaches that should work for any SQL database.
The code required to implement the above logic is sufficiently fiddly that we would probably not want to have to repeat it.
So we could think in terms of creating a re-usable module which would implement that logic.
At the time of writing, the only database-specific subclass is for Postgre SQL. Let’s expand the original table a bit: “name” has now been split into “first_name” and “last_name”.
So if the caller has a Postgre SQL database, and calls with data to represent our third example (where the target values are all unique), then the Postgre SQL-specific subclass will effect the updates using the table / UPDATE … To match on names we now need to match on two columns.
So whereas previously we specified the matching column using a scalar, when there is more than one column to match, we use an Array Ref.
Similarly, let’s say we want to update two fields, salary and bonus.For small numbers of rows requiring updates, it can be adequate to use an UPDATE statement for each row that requires an update.But if there are a large number of rows that require an update, then the overhead of issuing large numbers of UPDATE statements can result in the operation as a whole taking a long time to complete.The traditional advice for improving performance for multiple UPDATE statements is to “prepare” the required query once, and then “execute” the prepared query once for each row requiring an update.But in many cases this only provides a modest improvement as each UPDATE operation still requires a round-trip communication with the database server.This is in fact entirely possible in many database systems.