How can I concat columns as null-safe? - mysql

Question

Here is my code:
SELECT COALESCE(CONCAT(u.user_fname, ' ', u.user_lname), 'unknown') name
FROM users u
WHERE id = 10;
The result will be unknown when either user_fname or user_lname is null. That's not what I want, I want to select unknown only when both user_fname and user_lname are null.
Otherwise, I want to get the value of not-null column. How can I do that?

Answer 1

Use concat_ws():
SELECT CONCAT_WS(' ', u.user_fname, u.user_lname) name
FROM users u
WHERE id = 10;
This has the nice benefit that if either of the names are null, you don't get a spurious space in the result.
It gets a little tricky if you want to convert NULLs to "unknown". This should do the trick:
SELECT COALESCE(NULLIF(CONCAT_WS(' ', u.user_fname, u.user_lname) , ''), 'unknown') as name
FROM users u
WHERE id = 10;

Answer 2

You could use:
SELECT COALESCE(
TRIM(CONCAT(COALESCE(u.user_fname,''), ' ', COALESCE(u.user_lname,''))),
'unknown') name
FROM users u
WHERE id = 10;

Answer 3

I always like the isnull function in sql server (or nvl in oracle):
SELECT isnull(u.user_fname + ' ','') + isnull(u.user_lname, '') name
FROM users u
WHERE id=10
but then to switch include the Nulls, I would use a case:
SELECT
CASE WHEN u.user_fname IS NULL AND u.user_lname is NULL THEN 'unknown'
ELSE isnull(u.user_fname + ' ','') + isnull(u.user_lname, '') END name
FROM users u
WHERE id=10
yes, it's a little longer than the other answers, but easier to read and perhaps more flexible in the future in case you have other conditions.
Not really a performance hit either way, so it's down to personal preference.