Sql updating multiple rows

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So, given a list of updates to apply we could effect them using the following steps: So in the example above we can reduce five statements to four. But now the number of statements is no longer directly dependent on the number of rows requiring updates.Even if we wanted to update a thousand rows with different values, we could still do it with four statements.OK, that’s great, we have some theoretical approaches for reducing the number of queries, now what?

So, given a list of updates to apply we could effect them using the following steps: So in the example above we can reduce five statements to four. But now the number of statements is no longer directly dependent on the number of rows requiring updates.Even if we wanted to update a thousand rows with different values, we could still do it with four statements.OK, that’s great, we have some theoretical approaches for reducing the number of queries, now what?

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.

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”.

Similarly, let’s say we want to update two fields, salary and bonus.

If we want to update more than one column, whereas previously we specified the new value using a scalar, when there is more than one column to update, we use an Array Ref.

The ‘where’ parameter can be used to specify a global condition, e.g. Another thing which we may want to do is update a column with the same value for all rows being updated. Rather than have to specify the same value over and over again, we can use the ‘set’ parameter.

Sql updating multiple rows

The ‘set’ parameter takes a Hash Ref whose keys are the columns to be updated.So is structured so that approaches which are generic across different SQL databases are expressed in a base class, and approaches which only work for specific SQL databases are expressed in a subclass.An object of the relevant class is instantiated when the call is made, and control then passed to the implementation relevant to the database in use.But in many cases this only provides a modest improvement as each UPDATE operation still requires a round-trip communication with the database server.In the case where the application server and database server are on different hosts, the round-trip will involve network latency as well.So we could think in terms of creating a re-usable module which would implement that logic.This is the intention of UPDATE staff SET salary = 1200 WHERE name = ' Bob'; UPDATE staff SET salary = 1200 WHERE name = ' Jane'; UPDATE staff SET salary = 1200 WHERE name = ' Frank'; UPDATE staff SET salary = 1200 WHERE name = ' Susan'; UPDATE staff SET salary = 1200 WHERE name = ' John'; “key_columns” specifies the columns which will be used to identify rows which need to be updated (using WHERE). The first element provides the value of the column (specified by “key_columns”) to identify the row to be updated.A requirement arises in many systems to update multiple SQL database rows.For small numbers of rows requiring updates, it can be adequate to use an UPDATE statement for each row that requires an update.In this case there are two distinct values, so we can do it in two UPDATE statements: So we can reduce the number of queries from five to two. But the extent to which this helps us reduces as the proportion of distinct SET values goes up.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.

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