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Case Statement Tricks

,

The CASE statement is a very flexible tool. Here are just a few of the tricks

you can work with it.

Item translation

One of the simplest things you can do is to derive a columns contents based

on it’s contents, or the contents of another data item.

SELECT  pub_name,

        CASE WHEN state is NULL or state = ''

            THEN 'Not supplied'

            ELSE state

        END

FROM publishers

SELECT CASE WHEN datepart (dw, getdate()) In (1, 7)

            THEN 'Weekend overtime rates apply :-)'

            ELSE 'Standard rates apply'

END

Titling subtotals

You can use CUBE and ROLLUP to produce totals and subtotals within a result

set, but because SQL Server places NULLS for non-totaled columns in the total

rows, they do not always look very pretty. In this example we use the CASE

statement to place emphasis on the subtotal lines.

SELECT   CASE GROUPING(stor_name)

                   

WHEN 1 then '[State total]'

                   

ELSE stor_name

               

End as store,

               

state,

               

sum(qty)

               

FROM stores s

JOIN      sales sl on s.stor_id = sl.stor_id

GROUP  BY stor_name, state

WITH     CUBE

HAVING GROUPING(state) = 0

ORDER   BY state, GROUPING(stor_name)

Simplifying Output

This batch lists of sales by store, and gives a general indication of the

stores performance against the average quantity of sales per store.

declare @storeAvg int

select @storeAvg = avg(storeTotal)

from (

    select stor_id,

    sum(qty) as storeTotal

    from sales

    group by stor_id

) as derived

select  stor_name,

        storeTotal,

case    when storeTotal > @storeAvg * 1.1

            then 'Above average'

        when storeTotal < @storeAvg * 0.9

            then 'Below average'

        else 'About average'

end as storePerformance

from (

        select  stor_id,

               

sum(qty) as storeTotal

               

from     sales

               

group    by stor_id

) as derivedStores

join stores

on stores.stor_id = derivedStores.stor_id

This query batch starts by calculating the average of total sales per store,

which is stored in the variable @storeAvg, then that average is used to rate the

stores sales performance.

Grouping data into ranges

This example calculates the number of orders in certain size ranges. It also

shows off the power of SQL as a set-based language: to code this as in a

procedural language like VB would not be able to do this in a single statement.

SELECT    SUM (CASE when qty <= 5 then 1 else 0 end) 

            as '1_to_5',

          SUM (CASE when qty between 6 and 10 then 1 else 0 end) 

            as '6_to_10',

          SUM (CASE when qty between 11 and 20 then 1 else 0 end) 

            as '11_to_20',

          SUM (CASE when qty > 20 then 1 else 0 end) 

            as '20_plus'

FROM sales

Complex Sequencing

CASE statements can be used in an ‘ORDER BY clause to order non-sequential

data. This example sorts stores by name, but puts all the stores in California

at the top of the list.

SELECT   stor_name, state

FROM     stores

ORDER    BY CASE state

             WHEN 'CA' then 1

             ELSE 2

         END,

         stor_name

Further reading

This article discusses how to increase performance of complex

UPDATE statements by using CASE.

This SQL 2000 article contains a section that shows how to use the CASE

statement in a User

Defined Function to enforce complex declarative constraints

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

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