Using COALESCE in MySQL - mysql

Question

I'm just getting used to MySQL, I've come from a SQL Server background...
This SQL query builds an address how it should in SQL Server, how can I adapt it to use within MySQL. When I run it in SQL Server it displays all the data within each field, when run in MySQL it just shows me the first field.
Why would this be, what should I do different in MySQL?
SELECT COALESCE(House, '') + ' ' + COALESCE(StreetName, '') + ' ' + COALESCE(TownCity, '') + ' ' + COALESCE(Postcode, '') AS Display
FROM MyTable
WHERE Postcode LIKE '%A1 2AB%'

Answer 1

Use the concat() function:
SELECT concat(COALESCE(House, ''), ' ', COALESCE(StreetName, ''), ' ',
COALESCE(TownCity, ''), ' ', COALESCE(Postcode, '')
) AS Display
FROM MyTable
WHERE Postcode LIKE '%A1 2AB%';
You can also do this with concat_ws(). This eliminates the need for all the spaces:
SELECT concat_ws(COALESCE(House, ''), COALESCE(StreetName, ''),
COALESCE(TownCity, ''), COALESCE(Postcode, '')
) AS Display
FROM MyTable
WHERE Postcode LIKE '%A1 2AB%';
What happens in MySQL is that the + does just what you expect: it adds numbers. A string that contains a number is converted to a number automatically, with silent errors for strings that have no numbers. In practice, this means that a string that starts with a non-digit (and non-decimal point) is converted to a 0.
So, house, which presumably usually numeric, is converted to a number just fine. All the other strings are converted to numbers but become zero and the house number is not changed. You would have gotten much different results if your post codes were American-style zip codes (which are typically numeric).
EDIT:
As #fthiella points out, the coalesce() is not necessary for concat_ws(). The two statements would do different things, because NULLs in the original query result in repeated separators. NULLs in the concat_ws() version would have only a single separator (which might be desirable).
However, I would tend to keep the coalesce() anyway. The behavior of concat() and concat_ws() varies in this regard. concat() returns NULL if any of its arguments is NULL. concat_ws() skips NULL arguments after the initial separator. Who can remember that distinction? It sounds like a recipe for confusion in production code. So, I would also use coalesce() even though it is optional.