62

What are the practical differences between COALESCE() and ISNULL(,'')?

When avoiding NULL values in SQL concatenations, which one is the best to be used?

Thanks!

D. Caan
  • 1,907
  • 6
  • 22
  • 36
  • 12
    `coalesce` is defined by the SQL standard and will work on nearly all DBMS. `isnull()` only works with Microsoft products. –  Sep 16 '13 at 12:57
  • 1
    `ISNULL` takes two parameters, `COALESCE` can take `n` parameters, it depends on the requirement and version of SQL Server as to which is preferred – T I Sep 16 '13 at 12:59
  • 1
    MySQL also has isnull. – Colonel Thirty Two Sep 16 '13 at 13:11
  • 1
    I found the ISNULL vs COALESCE question before asking this one. But I asked the practical differences when concatenating columns (performance, best practices, code behaviour etc), that's why I created a new question. Sorry if it was a duplicated question. – D. Caan Sep 16 '13 at 13:19

4 Answers4

88

Comparing COALESCE and ISNULL

The ISNULL function and the COALESCE expression have a similar purpose but can behave differently.

  1. Because ISNULL is a function, it is evaluated only once. As described above, the input values for the COALESCE expression can be evaluated multiple times.

  2. Data type determination of the resulting expression is different. ISNULL uses the data type of the first parameter, COALESCE follows the CASE expression rules and returns the data type of value with the highest precedence.

  3. The NULLability of the result expression is different for ISNULL and COALESCE. The ISNULL return value is always considered NOT NULLable (assuming the return value is a non-nullable one) whereas COALESCE with non-null parameters is considered to be NULL. So the expressions ISNULL(NULL, 1) and COALESCE(NULL, 1) although equivalent have different nullability values. This makes a difference if you are using these expressions in computed columns, creating key constraints or making the return value of a scalar UDF deterministic so that it can be indexed as shown in the following example.

    USE tempdb; 
GO

-- This statement fails because the PRIMARY KEY cannot accept NULL values
-- and the nullability of the COALESCE expression for col2 
-- evaluates to NULL. 

CREATE TABLE #Demo  (  col1 integer NULL,  col2 AS COALESCE(col1, 0) PRIMARY KEY,  col3 AS ISNULL(col1, 0)  ); 

-- This statement succeeds because the nullability of the 
-- ISNULL function evaluates AS NOT NULL.

CREATE TABLE #Demo  (  col1 integer NULL,  col2 AS COALESCE(col1, 0), col3 AS ISNULL(col1, 0) PRIMARY KEY  );

Validations for ISNULL and COALESCE are also different. For example, a NULL value for ISNULL is converted to int whereas for COALESCE, you must provide a data type. ISNULL takes only 2 parameters whereas COALESCE takes a variable number of parameters.

Source: BOL

Giacomo1968
  • 25,759
  • 11
  • 71
  • 103
SchmitzIT
  • 9,227
  • 9
  • 65
  • 92
  • 8
    +1. What's more, `COALESCE` is just a shortcut for a `CASE` expression, meaning that the optimizer rewrites it to use `CASE` instead – Lamak Sep 16 '13 at 13:01
  • @paxdiablo Books OnLine, the official SQL Server documentation. Sorry, not sure it is still called it, but it used to be. – SchmitzIT Sep 16 '13 at 13:05
  • 1
    Ah, thank you, that makes substantially more sense than my attempts :-) – paxdiablo Sep 16 '13 at 13:06
  • An [example](http://stackoverflow.com/a/26224588/538763) of type handling for (2): – crokusek Oct 06 '14 at 20:46
20

The main difference is, that COALESCE is ANSI-Standard, so you will also find it in other RDBMSs, the other difference is you can give a whole list of values to be checked to COALESCE whereas to ISNULL you can only pass one.

DrCopyPaste
  • 4,023
  • 1
  • 22
  • 57
13

Because ISNULL is a function, it is evaluated only once. As described above, the input values for the COALESCE expression can be evaluated multiple times. COALESCE basically translates to CASE expression and ISNULL is a built-in implemented in the database engine.

MSDN

Suraj Singh
  • 4,041
  • 1
  • 21
  • 36
9

COALESCE() can have multiple inputs and it will evaluate in order until one of them is not null such as COALESCE(Col1, Col2, Col3, 'N/A'). It's recommended to use this by MS instead of ISNULL()

ISNULL() can only have one input, however it's been shown to be slightly faster than COALESCE.

Mike M.
  • 12,343
  • 1
  • 24
  • 28