oracle decode function

Oracle/PLSQL: DECODE Function

The Oracle/PLSQL DECODE function has the functionality of an IF-THEN-ELSE statement.

Oracle DECODE Syntax

The syntax for the Oracle/PLSQL DECODE function is:

DECODE( expression , search , result [, search , result]... [, default] )

Parameters or Arguments

expression is the value to compare.
search is the value that is compared against expression.
result is the value returned, if expression is equal to search.
default is optional. If no matches are found, the DECODE function will return default. If default is omitted, then the DECODE function will return null (if no matches are found).

Applies To

The DECODE function can be used in the following versions of Oracle/PLSQL:
Oracle 12c, Oracle 11g, Oracle 10g, Oracle 9i

Oracle DECODE Example

The DECODE function can be used in Oracle/PLSQL.
You could use the DECODE function in a SQL statement as follows:

SELECT supplier_name,
DECODE(supplier_id, 10000, 'IBM',
10001, 'Microsoft',
10002, 'Hewlett Packard',
'Gateway') result
FROM suppliers;

The above DECODE statement is equivalent to the following IF-THEN-ELSE statement:

IF supplier_id = 10000 THEN
result := 'IBM';

ELSIF supplier_id = 10001 THEN
result := 'Microsoft';

ELSIF supplier_id = 10002 THEN
result := 'Hewlett Packard';

ELSE
result := 'Gateway';

END IF;

The DECODE function will compare each supplier_id value, one by one.

Frequently Asked Questions (DECODE Function)

Question: One of our viewers wanted to know how to use the DECODE function to compare two dates (ie: date1 and date2), where if date1 > date2, the DECODE function should return date2. Otherwise, the DECODE function should return date1.
Answer: To accomplish this, use the DECODE function as follows:

DECODE((date1 - date2) - abs(date1 - date2), 0, date2, date1)

The formula below would equal 0, if date1 is greater than date2:

(date1 - date2) - abs(date1 - date2)

Helpful Tip #1: One of our viewers suggested combining the SIGN function with the DECODE function as follows:
The date example above could be modified as follows:

DECODE(SIGN(date1-date2), 1, date2, date1)

The SIGN/DECODE combination is also helpful for numeric comparisons e.g. Sales Bonuses

DECODE(SIGN(actual-target), -1, 'NO Bonus for you', 0,'Just made it', 1, 'Congrats, you are a winner')

Helpful Tip #2: One of our viewers suggested using the LEAST function (instead of the DECODE function) as follows:
The date example above could be modified as follows:

LEAST(date1, date2)

Question: I would like to know if it's possible to use the DECODE function for ranges of numbers, ie 1-10 = 'category 1', 11-20 = 'category 2', rather than having to individually decode each number.
Answer: Unfortunately, you can not use the DECODE function for ranges of numbers. However, you can try to create a formula that will evaluate to one number for a given range, and another number for the next range, and so on.
For example:

SELECT supplier_id,
DECODE(TRUNC ((supplier_id - 1) / 10), 0, 'category 1',
1, 'category 2',
2, 'category 3',
'unknown') result
FROM suppliers;

In this example, based on the formula:

TRUNC ((supplier_id - 1) / 10

The formula will evaluate to 0, if the supplier_id is between 1 and 10.
The formula will evaluate to 1, if the supplier_id is between 11 and 20.
The formula will evaluate to 2, if the supplier_id is between 21 and 30.
and so on...
Question: I need to write a DECODE statement that will return the following:

If yrs_of_service < 1 then return 0.04
If yrs_of_service >= 1 and < 5 then return 0.04
If yrs_of_service > 5 then return 0.06

How can I do this?
Answer: You will need to create a formula that will evaluate to a single number for each one of your ranges.
For example:

SELECT emp_name,
DECODE(TRUNC (( yrs_of_service + 3) / 4), 0, 0.04,
1, 0.04,
0.06) as perc_value
FROM employees;

Question: Is there a limit to the number of arguments that you can have in one DECODE statement? I'm getting an error, "ORA-00939: too many arguments for function".
Answer: Yes, the maximum number of components that you can have in a DECODE function is 255. This includes the expression, search, and result arguments.
原文出处：http://www.techonthenet.com/oracle/functions/decode.php
ps：小弟翻译水平有限，就不献丑了，我也是大部分看得明白的，相信大家也会明白的。