SQL COALESCE Function and NULL

Knowing when to use the SQL COALESCE function is a lifesaver when you’re dealing with NULL.

As you know, NULL is a tricky concept, and it seem whatever NULL “touches” in an expression, it renders the result NULL. So, when you’re dealing with NULL, how can you break out of the cycle? That is, how can you display another value instead?

This is where SQL COALESCE comes into play. With this function, you can test for NULL, and when present, substitute NULL for another value.

What is COALESCE?

COALESCE is a built-in SQLServer Function. Use COALESCE when you need to replace a NULL with another value. It takes the form:

COALESCE(value1, value2, ..., valuen)

It returns the first non NULL from the value list.

Consider the baseball parks from Lahmans Baseball Database. If we want to display the parkname in place of the parkalias, which may be null, COALESCE works well here:

SELECT COALESCE(parkalias, parkname) as ParkAlias, city, state FROM Parks

-- Answer
SELECT COALESCE(parkalias, parkname) as ParkAlias,
             city,
             state
FROM   Parks

In this example, COALESCE(parkalias, parkname) returns the value found in parkalias if the value is not NULL; however, if it is NULL, then parkname is returned.

What SQL would you write if you wanted to display ‘Unknown’ if the parkalias is NULL? Why not try to write the query… here is a start…

SELECT parkalias, parkname, city, state FROM Parks

-- Answer
SELECT COALESCE(parkalias, 'Unknown') as parkalias,
            parkname,
            city,
            state
FROM  Parks

Comparing SQL COALESCE to CASE

Did you know SQL COALESCE is a shortcut for the CASE statement?

Consider the table survey, which has the columns answerID, option1, option2, option3.

We want to list the answerID and first non NULL option.

From what we have learned, we can use COALESCE as:

SELECT answerID,
       COALESCE(option1, option2, option3, 'No Option Selected')
FROM   survey

This is a shortcut for this CASE statement:

SELECT answerID,
       CASE
          WHEN option1 is not NULL then option1 
          WHEN option2 is not NULL then option2  
          WHEN option3 is not NULL then option3  
          ELSE 'No Option Selected'
       END
FROM   survey 

CASE is used in more circumstances, but when replacing NULL, COALESCE is a handy shortcut!

For the record, SQL Server has many more interesting functions to learn and explore. If you haven’t done so already, check out these string functions to manipulate text data!

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