Oracle DECODE Function

Summary: in this tutorial, you will learn how to use the Oracle DECODE() function to embed if-then-else logic in SQL queries.

Introduction to Oracle DECODE() function

The Oracle DECODE() function allows you to add the procedural if-then-else logic to the query.

In the following example, the Oracle DECODE() function compares the first argument (1) with the second argument (1). Because they are equal, the function returns the third argument which is the string 'One':

SELECT
  DECODE(1, 1, 'One')
FROM
  dual;

It works like the following if statement

IF 1 = 1 THEN 
    RETURN 'One'; 
END IF

The following example is slightly different from the one above. The query returns a null value because one does not equal two.

SELECT
  DECODE(1, 2, 'One')
FROM
  dual; 

If you want to specify a default value when the first argument is not equal to the second one, you append the default value to the argument list as shown below:

SELECT
  DECODE(1, 2, 'One','Not one')
FROM
  dual; 

It works like the following if-then-else statement:

IF 1 = 2 THEN
    RETURN 'One';
ELSE
    RETURN 'Not one';
END IF;

What if you want to compare the first argument with a list of arguments? See the following example:

SELECT
  DECODE(2, 1, 'One',  2, 'Two')
FROM
  dual;

The result is:

Two

In this example, the function compares the first argument (2) with the second one. If the first argument equals the second one, the function returns the third argument (One). Otherwise, it compares the first argument with the fourth argument (2). If they are equal, the function returns the fifth argument (Two).

It works like the following if-then-elsif statement:

IF 2 = 1 THEN 
     RETURN 'One';
ELSIF 2 = 2 THEN 
    RETURN 'Two';
END IF;

If you want to specify a default value when the function does not find any match, you do it as follows:

SELECT
  DECODE(3, 1, 'One',  2, 'Two', 'Not one or two')
FROM
  dual; 

The query returned:

Not one or two

The query works like the following if-then-elsif-else statement:

IF 3 = 1 THEN 
     RETURN 'One';
ELSIF 3 = 2 THEN 
    RETURN 'Two';
ELSE
    RETURN 'Not one or two';
END IF;

Oracle DECODE() function syntax

The following illustrates the syntax of the Oracle DECODE() function:

DECODE (e , s1, r1[, s2, r2], ...,[,sn,rn] [, d]);

Arguments

e

The first argument e is the value to be searched. The function automatically converts e to the data type of s1 before comparing.

s1, s2, .. sn

The s1, s2, … or sn is an expression to search for. Note that s2, s3, … sn are automatically converted to the data type of s1 before comparing.

r1, r2, .. rn

The r1, r2, …, or rn is the expression to return when e is equal to s.

d

d is an expression to return when e does not equal to any searched value s1, s2, .. sn.

Return value

The DECODE() function returns a value with the data type of the first result (r1, r2, .. rn or d) argument.

Note

You can use expressions for the search (s), the result (r), and default value (d) in the DECODE() function. The DECODE() function evaluates each search value (s1, s2, .., or sn) only before comparing it to the first argument (e), rather than evaluating all search values. In other words, the DECODE() function never evaluates a search (si+1) when a previous search (si) equals e.

Oracle DECODE() function examples

Let’s take some examples of using the DECODE() function to see how it works.

A) Use DECODE() function to make data more meaningful

See the following locations table in the sample database:

The following statements return the country that has at least two locations stored in the database:

SELECT
  country_id,
  COUNT(*)
FROM
  locations
GROUP BY
  country_id
HAVING
  COUNT(*) >= 2
ORDER BY
  country_id;

The country id is quite cryptic. You can use the DECODE() function to make the country data more meaningful as follows:

SELECT
  DECODE(country_id, 'US','United States', 'UK', 'United Kingdom', 'JP','Japan'
  , 'CA', 'Canada', 'CH','Switzerland', 'IT', 'Italy', country_id) country ,
  COUNT(*)
FROM
  locations
GROUP BY
  country_id
HAVING
  COUNT(*) > =2
ORDER BY
  country_id;

B) Oracle DECODE with ORDER BY example

Consider the following employees table:

The following query uses the DECODE() function in the ORDER BY clause to sort the employees result set based on an input argument:

SELECT
  first_name,
  last_name,
  job_title
FROM
  employees
ORDER BY
  DECODE('J', 'F', first_name, 'L', last_name, 'J', job_title);

In this example, we sorted the employee list by job title because we passed the character J as the first argument of the DECODE() function.

Oracle DECODE with GROUP BY example

See the following products table:

The following statement illustrates how to use the DECODE() function in the GROUP BY clause. It returns the number of products whose list prices are higher than, equal to, and less than the average list price.

WITH list_prices AS(
    SELECT
      ROUND(AVG(list_price),2) average
    FROM
      products
)
SELECT
  DECODE( SIGN( (list_price - average ) ), 
  1, '> Average of ' || average , 
  0, 'Average', 
  -1, '< Average of ' || average) list_price,
  COUNT(*)
FROM
  products,
  list_prices
GROUP BY
  DECODE( SIGN( (list_price - average ) ), 
  1, '> Average of ' || average , 
  0, 'Average', 
  -1, '< Average of ' ||average );

Oracle DECODE() function with SUM() example

We will use the products and product_categories tables in this example for the demonstration.

The following example uses the DECODE() function to change the ranges to the binary number and uses the SUM() function to count the number of values with a specified range:

SELECT 
  category_name, 
  SUM(DECODE(GREATEST(list_price, 0), LEAST(list_price, 1000), 1, 0)) "< 1000",
  SUM(DECODE(GREATEST(list_price,1001), LEAST(list_price, 2000), 1, 0)) "1001-2000", 
  SUM(DECODE(GREATEST(list_price,2001), LEAST(list_price,3000), 1, 0)) "2001-3000", 
  SUM(DECODE(GREATEST(list_price,3001), LEAST(list_price,8999), 1, 0)) "3001-8999"
FROM 
  products 
INNER JOIN product_categories USING (category_id)
GROUP BY 
  category_name; 

Here is the output:

Oracle DECODE() function and NULL

NULL cannot be compared to anything even NULL. However, DECODE() function treats two null values are being equal.

The following statement returns the string Equal:

SELECT
  DECODE(NULL,NULL,'Equal','Not equal')
FROM
  dual;

In this tutorial, you have learned how to use the Oracle DECODE() function to add procedure if-then-else logic to SQL queries.