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Wednesday, May 26, 2010

Using CASE Expressions



CASE expressions are often over-looked but can be
extremely useful to change very complex query requirements into simpler, and sometimes
more efficient SQL statements. The CASE expression enables many forms of conditional
processing to be placed into a SQL statement. By using CASE, more logic can be placed
into SQL statements instead of being expressed in a host language or 4GL program.
Microsoft’s implementation of CASE expressions in
SQL Server is compliant with the ANSI SQL-92 standard.
A Quick Example
Sometimes a quick example is worth several hundred descriptive words, so let’s take
a look at a sample SQL statement using a CASE expression on the title table in the
sample pubs database:

SQL Statement #1

SELECT title, price,
Budget = CASE price
WHEN
price > 20.00 THEN 'Expensive'



WHEN price BETWEEN 10.00 AND 19.99 THEN 'Moderate'


WHEN price < 10.00 THEN 'Inexpensive'


ELSE 'Unknown'

END,
FROM titles
This statement would return results similar to these:
Title
Price Budget

---------------------- ----------- ---------------

Working with Computers 11.95
Moderate
Normal Words About Company
19.99 Moderate
Busy Engineers's D
19.99 Moderate
I Can Duell Execute 2.99 Inexpensive
Silicon Valley 19.99
Moderate
China Microwave 2.99 Inexpensive
But Is It User Friendl
22.95 Expensive
Secrets of Silicon Val 20.00
Moderate
Net Etiquette (null)
Unknown

This output is not comprehensive but shows enough detail
to help describe the effects of the CASE expression.

The CASE expression is exercised on each row returned.
It categorizes the contents of the price column into four different groups: expensive,
moderate,
inexpensive, and
unknown. This is easier and more efficient than writing a SQL statement that uses
UNION to combine the results of the four categories. The following UNION statement
would return equivalent results:



SQL Statement #2
SELECT title, price, 'Expensive'
FROM titles
WHERE price > 20.00

UNION ALL

SELECT title, price, 'Moderate'

FROM titles
WHERE price BETWEEN 10.00 AND 19.99

UNION ALL

SELECT title, price, 'Inexpensive'

FROM titles
WHERE price < 10.00

UNION ALL

SELECT title, price, 'Unknown'

FROM titles
WHERE price IS NULL

go

UNION ALL is used instead of just UNION because no duplicates
need to be removed. Each SELECT in the UNION returns a distinct result set.
You can see where this UNION formulation would be less
efficient than the previous CASE formulation. In the UNION example SQL Server would
have to make four passes through the data—one for each SELECT used. In the CASE
example, one pass through the data is sufficient to return the correct results.
Obviously, the CASE formulation will outperform the UNION formulation.
Another very useful capability of CASE expressions is
to transform a result set from multiple rows into a summary row. Consider the situation
where sales data is stored in a table by month. One row is stored per month with
a table structure that looks like the following:
CREATE TABLE prodsales
(product char(3),
mnth
smallint,
sales
money)



In this table, sales amounts (sales) are stored by month (mnth) and product code
(product). The mnth column stores an integer value ranging from 1 (for January)
to 12 (for December). You can use the following single SQL statement to product
one row per product with 12 totals, one for each month:

SQL Statement #3

SELECT product,
SUM(CASE mnth WHEN 1 THEN
sales ELSE NULL END) AS jan,
SUM(CASE mnth WHEN 2 THEN
sales ELSE NULL END) AS feb,
SUM(CASE mnth WHEN 3 THEN
sales ELSE NULL END) AS mar,
SUM(CASE mnth WHEN 4 THEN
sales ELSE NULL END) AS apr,
SUM(CASE mnth WHEN 5 THEN
sales ELSE NULL END) AS may,
SUM(CASE mnth WHEN 6 THEN
sales ELSE NULL END) AS jun,
SUM(CASE mnth WHEN 7 THEN
sales ELSE NULL END) AS jul,
SUM(CASE mnth WHEN 8 THEN
sales ELSE NULL END) AS aug,
SUM(CASE mnth WHEN 9 THEN
sales ELSE NULL END) AS sep,
SUM(CASE mnth WHEN 10 THEN
sales ELSE NULL END) AS oct,
SUM(CASE mnth WHEN 11 THEN
sales ELSE NULL END) AS nov,
SUM(CASE mnth WHEN 12 THEN
sales ELSE NULL END) AS dec

FROM prodsales

GROUP BY product

This statement generates a row for each product with twelve monthly sales totals.
The CASE expression causes the sales amount to by added to the appropriate bucket
by checking the mnth column. If the month value is for the appropriate “month bucket”,
then the sales amount is added using SUM; if not, then NULL is specified, thereby
avoiding adding anything to the SUM. Using CASE expressions in this manner simplifies
aggregation and reporting. It provides a quick way of transforming normalized data
structures into the more common denormalized formats that most business users are
accustomed to viewing on reports.
This same basic idea can be used to create many types
of summary rows. For example, to produce a summary row by quarter instead of by
month, simply modify the CASE expressions as shown below:
SQL Statement #4
SELECT product,
SUM(CASE WHEN mth BETWEEN
1 AND 3 THEN sales

ELSE NULL END) AS q1,
SUM(CASE WHEN mth BETWEEN
4 AND 6 THEN sales

ELSE NULL END) AS q2,
SUM(CASE WHEN mth BETWEEN
7 AND 9 THEN sales

ELSE NULL END) AS q3,
SUM(CASE WHEN mth BETWEEN
10 AND 12 THEN sales

ELSE NULL END) AS q4

FROM prodsales

GROUP BY product

A More Complicated Example

Using searched CASE expressions and nested subqueries in SELECT statements very
complex processing can be accomplished with a single SQL statement. Consider, once
again, the sample pubs database. The following query checks the royalty percentage
by title and places the percentage into a category based on its value:
SQL Statement #5
SELECT au_lname, au_fname, title, Category =
CASE

WHEN (SELECT AVG(royaltyper) FROM titleauthor ta

WHERE t.title_id = ta.title_id) > 65

THEN 'Very High'

WHEN (SELECT AVG(royaltyper) FROM titleauthor ta

WHERE t.title_id = ta.title_id)

BETWEEN 55 and
64

THEN 'High'

WHEN (SELECT AVG(royaltyper) FROM titleauthor ta

WHERE t.title_id = ta.title_id)

BETWEEN 41 and
54

THEN 'Moderate'

ELSE 'Low'
END
FROM authors a,
titles t,
titleauthor ta
WHERE a.au_id = ta.au_id

AND
ta.title_id = t.title_id
ORDER BY Category, au_lname, au_fname


Within a SELECT statement, the searched CASE expression
allows values to be replaced in the results set based on comparison values. In this
example, the royalty percentage (royaltyper) in the titleauthor table is checked
and a category is specified for each author based on the royalty percentage returned.
Usage and Syntax
CASE expressions can be used in SQL anywhere an expression
can be used. This provides great flexibility because expressions can be used in
a wide number of places. Example of where CASE expressions can be used include in
the SELECT list, WHERE clauses, HAVING clauses, IN lists, DELETE and UPDATE statements,
and inside of built-in functions.
There are two basic formulations that a CASE expression
can take: simple CASE expressions and searched CASE expressions. A simple CASE expression
checks one expression against multiple values. Within a SELECT statement, a simple
CASE expression allows only an equality check; no other comparisons are made. A
simple CASE expression operates by comparing the first expression to the expression
in each WHEN clause for equivalency. If these expressions are equivalent, the expression
in the THEN clause will be returned.
The basic syntax for a simple CASE expressions is
shown below:
CASE
expression
WHEN
expression1 THEN
expression1
[[WHEN
expression2 THEN
expression2] [...]]
[ELSE
expressionN]
END
A searched CASE expression is more feature-laden.
It allows comparison operators, and the use of AND and/or OR between each Boolean
expression. The simple CASE expression checks only for equivalent values and can
not contain Boolean expressions. The basic syntax for a searched CASE expressions
is shown below:
CASE
WHEN
Boolean_expression1 THEN expression1
[[WHEN
Boolean_expression2
THEN
expression2] [...]]
[ELSE
expressionN]
END
You have seen samples
of each type of CASE expression in the SQL examples depicted previously in this
article. SQL statements #1 and #3 are simple CASE expressions; SQL statements #4
and #5 are searched CASE expressions.

Using CASE Expressions When Modifying Data

CASE expressions can also be used with data modification statements. Using CASE
in conjunction with a SQL UPDATE statement enables developers to conditionally modify
large amounts of data using a single SQL statement. Consider the following example:

SQL Statement #6

UPDATE titles
SET price =
CASE
WHEN (price <
5.0 AND ytd_sales > 999.99)

THEN price * 1.25

WHEN (price < 5.0 AND ytd_sales < 1000.00)
THEN price * 1.15

WHEN (price > 4.99 AND ytd_sales > 999.99)
THEN price * 1.2

ELSE price
END
This statement examines book title criteria to determine
whether prices should be modified. The CASE expression uses a combination of current
price and year-to-date sales to specify a price increase percentage. Any criteria
that can be expressed in terms of SQL predicates in CASE expressions can be used
to update rows conditionally.

General Usage Guidelines

All data types used in the THEN clause of CASE expressions
must be compatible data types. If the data type used is not compatible then SQL
Server will return an error because implicit data type conversion is not supported.
Be sure that all possibilities are covered within
the CASE expressions. For example, consider a CASE expression that must be coded
on numeric data that can range from -1 to 100, and then an outlying value of 1000.
Do not omit or overlap any of the possible data values. Furthermore, be aware of
the data type of the values being examined in the CASE expression. Do not leave
gaps in the range of possible values by not using appropriate values. For example,
if the data is decimal (instead of integer, say) do not ignore the portion of the
number to the right of the decimal point. A good example of this is shown in the
previous SQL statement #6. If the value for 999.99 is changed to 999.0 then any
values between 999.01 and 999.99 will be lumped in with the ELSE condition – which
is clearly not the desired intent of this statement.
The bottom line is that CASE expressions are very
powerful, yet often neglected. Favor using CASE expressions under the following
circumstances:



  • when data needs to be converted from
    one type to another for display and no function exists to accomplish the task



  • when a summary row needs to be created
    from detail data



  • when conditional processing needs to be executed
    to determine results



  • when using UNION to “glue together” different subsets
    of a single table
One final usage guideline is to use the COALESCE
function with your CASE expressions when you wish to avoid NULLs. Consider an employee
table that contains three columns for wages: salary, commission, and retainer_fee.
Any single employee can only receive one of these types of wages. So, two of the
columns will contain NULLs, but one will always contain a value. The following statement
uses the COALESCE function to return only the non-NULL value:
SELECT COALESCE(salary, commission, retainer_fee,
0)
FROM employee


The COALESCE function will go through the list of expressions (in this case columns
and constants) and return the first non-NULL value encountered. In this case, the
numeric constant 0 is added at the end of the list to provide a default value if
all of the columns should happen to be NULL.
Summary
CASE expressions bring a vast degree of power and control to SQL Server programmers.
A working knowledge of CASE expressions can make accessing and updating SQL Server
data easier and perhaps, even more efficient. Additionally, CASE expressions enable
more work to be accomplished using a single SQL statement, which should also improve
efficiency and decrease development time. As such, CASE expressions should be a
component of every SQL Server developer’s arsenal of programming techniques.

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