Oracle COMPOSE Function: A Simple Guide

The COMPOSE function in Oracle SQL is a specialized Unicode function. Its job is to take a base character plus a "combining mark" (like an accent or an umlaut) and merge them into a single, pre-composed character.

This is the opposite of the DECOMPOSE function. It's an advanced function used for standardizing and normalizing Unicode text.

What is the COMPOSE Function in Oracle?

The COMPOSE(char) function takes a string containing a base character followed by a combining mark and returns the single, combined character.

For example, in Unicode, the character ö can be represented in two ways:

1. As a single, pre-composed character (ö).
2. As two characters: a normal o followed by a "combining umlaut" mark (\0308).

COMPOSE is the function that converts the second form into the first.

COMPOSE Function Syntax

The syntax for COMPOSE is very simple:

COMPOSE(char)

Let's break that down:

• char: The string or column containing the characters to be combined. This should be in a Unicode character set (like NCHAR or NVARCHAR2) for the function to work.

Oracle COMPOSE Function Examples

Here are two practical examples of how to use COMPOSE.

Example 1: Composing an 'o' and an Umlaut using COMPOSE

This example shows the most common use. We will provide the letter 'o' followed by the Unicode combining mark for an umlaut (\0308) and COMPOSE will merge them.

Query:

-- We use UNISTR to provide the Unicode code for the umlaut mark
SELECT 
  COMPOSE('o' || UNISTR('\0308')) AS "Composed_Character"
FROM DUAL;

Result: (The output is a single ö character)

Composed_Character
------------------
ö

Example 2: Composing an 'a' and an Acute Accent using COMPOSE

Similarly, let's create the character á (a with an acute accent). We do this by combining a normal 'a' with the combining acute accent mark (\0301).

Query:

SELECT 
  COMPOSE('a' || UNISTR('\0301')) AS "Composed_Character"
FROM DUAL;

Result: (The output is a single á character)

Composed_Character
------------------
á