NVL vs. COALESCE in Oracle: When Should You Choose Which?

Difference analysis of NVL and COALESCE functions in Oracle database

Both the NVL and COALESCE functions return non-null values, but there are subtle but important differences between them.

Modernization and Standardization

COALESCE is a more modern function in Oracle Database that complies with the ANSI-92 SQL standard. In contrast, NVL is an Oracle-specific function that predates the standard.

Synonymity of two parameters

NVL and COALESCE behave like synonyms for two values. However, this equivalence does not hold for more than two parameters.

Evaluation Behavior

The key difference is their evaluation behavior. NVL always evaluates its two arguments regardless of whether the first argument is empty. COALESCE, on the other hand, usually stops evaluating when it encounters the first non-null argument. This difference can significantly affect performance in some cases.

The following example illustrates this contrast vividly. Consider the following query:

<code class="language-sql">SELECT SUM(val)
FROM (
    SELECT NVL(1, LENGTH(RAWTOHEX(SYS_GUID()))) AS val
    FROM dual
    CONNECT BY level < 10000
)</code>

Here, NVL computes two parameters for each row even if 1 is non-empty for all rows. This results in quite long query execution times.

Instead, use the following query with COALESCE:

<code class="language-sql">SELECT SUM(val)
FROM (
    SELECT COALESCE(1, LENGTH(RAWTOHEX(SYS_GUID()))) AS val
    FROM dual
    CONNECT BY level < 10000
)</code>

will recognize that 1 is non-null and avoid computing the second argument, significantly reducing execution time.

To summarize, while NVL and COALESCE look similar for two values, the optimized evaluation behavior of COALESCE makes it the preferred choice when dealing with multiple potentially null values.

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