Introduction

One of the keys to database performance if keeping your

transactions as short as possible. In this article we will look at a couple of

tricks using the CASE statement to perform multiple updates on a table in a

single operation.

Multiple updates to a single column

This example uses the pubs database to adjust book prices for

a sale by different amounts according to different criteria. In the example I am

going to knock 25% off all business books from any publisher, and 10% off any

non-business books from a particular publisher. You might be tempted to wrap two

separate update statements into one transaction like this:

begin tran

    update titles set …..

    update titles set …..

commit tran

The down side of this technique is that it will read through

the table twice, once for each update. If we code our update like the example

below, then the table will only need to be read once. For large tables, this can

save us a lot of disk IO, especially if the query requires a table scan over a

long table

update titles

    set price =

    case

    when type = "business"

        then price * 0.75

    when pub_id = "0736"

        then price * 0.9

    end

where pub_id = "0736" OR

type = "business"

Note that there is a definite "top-down" priority

involved in the CASE statement. For business books from publisher 0736 the

"business" discount will apply because this is the first condition in

the list to be fulfilled. However, we will not give a further 10% publisher

discount, even though the criteria for the second "when" clause is

satisfied, because the CASE statement only evaluates criteria until it finds the

first one that fits.

Multi-column updates

We can use the CASE statement to update multiple columns in a

table, even using separate update criteria for each column. This example updates

the publishers table to set the state column to "--" for non-USA

companies, and changes the city for one particular publisher, all in one table

read operation.

update publishers

    set

    state = case

    when country <> "USA"

        then "--"

    else state

    end,

    city = case

    when pub_id = "9999"

        then "LYON"

    else city

    end

where country <> "USA" OR

pub_id = "9999"

The same format will work for updates across three or more

rows with different update criteria.

You may come across fewer opportunities to use this second

technique efficiently. This query will almost invariably result in a table scan

because we are selecting on multiple columns that are unlikely to all be in a

covering index. If each column is updated only a small number of times, and is

indexed, it may still be more efficient to do separate updates.

A good place to use this technique might be in cleaning up

multiple columns in a long interface file from another system.

Because we are using two separate case statements, one for

each test criteria/update, each case statement will be evaluated for every row,

and updates applied where required. Therefore if more than one column in the row

requires an update, they will all be updated.

Two things are particularly important to remember in this

example:

• The else [column] clause

  is required for each case statement used, otherwise you will end up nulling-out

  data you do not want to.

• The where clause at the

  end must be used to restrict the update to rows that require at least one

  column updating, otherwise every column in the table will be updated,

  increasing both execution time and pressure on the transaction log.

About the author

Neil Boyle is an independent SQL Server consultant working out of London,

England. Neil's free SQL Server guide is available on-line at http://www.impetus-sql.co.uk