Updating multiple fields in access

Posted by / 28-Mar-2020 16:58

Updating multiple fields in access

Also, let’s say we want to make updates only for people in the company “Widget Co”.

“Bob Jones” appears in both companies, so we need to be careful to identify the company, but we don’t want to have to specify it repeatedly for each row.

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.

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.

“value_columns” specifies the columns which will be updated (using SET). The second element provides the value to be SET in the column specified by “value_columns”.

In this case, will spot that the target values are all 1200, and will effect the desired changes using a single UPDATE statement as described above.

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”.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.The ‘set’ parameter takes a Hash Ref whose keys are the columns to be updated.Let us start with a simple table: 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'; UPDATE staff SET salary = 1200 WHERE name = ' Bob'; UPDATE staff SET salary = 1250 WHERE name = ' Jane'; UPDATE staff SET salary = 1200 WHERE name = ' Frank'; UPDATE staff SET salary = 1250 WHERE name = ' Susan'; UPDATE staff SET salary = 1200 WHERE name = ' John'; We are no longer setting all the salary fields to the same value, so we can’t collapse it into a single statement.But we can group the updates according to the value being set, and then do one UPDATE statement per distinct value.

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OK, that’s great, we have some theoretical approaches for reducing the number of queries, now what?

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