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 what ever 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:
-- 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…
-- 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:
COALESCE(option1, option2, option3, 'No Option Selected')
This is a shortcut for this CASE statement:
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'
CASE is used in more circumstances, but when replacing NULL, COALESCE is a handy shortcut!