Record Linkage: Oracle SQL DECODE

Tuesday, February 27, 2007

Oracle SQL DECODE

The DECODE function in Oracle SQL is used for if-then-else statements. A very handy tool for powerful queries.

The syntax for the DECODE function is as follows:

DECODE(value, if1, then1, if2, then2, if3, then3, etc., else)

Here is a good example of this from the devx.com website:

Consider the following example in which the Viewable column can have values of 1, 2, and 3:

SELECT FirstName, Viewable
FROM employee;

Results:

FIRSTNAME Viewable
===========================
John 1
Tim 2
Julie 2
Stacy 1
Rahul 3
Leena 4
Amy 1
Bill 3
Teri 3

Now, we can use Decode to display different things in a report based on the values in Viewable.

SELECT Firstname,
Decode(Viewable, 1,'VISIBLE', 2,'INVISIBLE',
3,'UNKNOWN', 'OTHER')
FROM employee;

Results:

FIRSTNAME Viewable
===========================
John VISIBLE
Tim INVISIBLE
Julie INVISIBLE
Stacy VISIBLE
Rahul UNKNOWN
Leena OTHER
Amy VISIBLE
Bill UNKNOWN
Teri UNKNOWN

Decode checks the column values and interprets the provided values in pairs. This is how it works:

Switch viewable:

Case 1:
Result = VISIBLE
Case 2:
Result = INVISIBLE
Case 3:
Result = UNKNOWN
Default:
Result = OTHER
End Case

OR
====================

If Viewable = 1 Then
Result = VISIBLE
Elsif Viewable = 2 Then
Result = INVISIBLE
Elsif Viewable = 3 Then
Result = UNKNOWN
Else
Result = OTHER
End If

One can use Decode in Updates as well. For example, instead of writing 50 different updates to change state names, we can do it in one Update using Decode as follows:

Update employee
set homestate = decode(homestate,
'AL', 'Alabama',
'AK', 'Alaska',
'AS', 'American Samoa',
'AZ', 'Arizona',
. . . . . . . .
. . . . . . . .
'WA', 'Washington',
'WV', 'West Virginia',
'WI', 'Wisconsin',
'WY', 'Wyoming' , homestate)
where homecountry = 'UNITED STATES'

This will replace state abbreviations with state names. And it leaves the abbreviations alone in case it doesn't match any of the values, such as AL, AK, etc.