SQL LEFT JOIN return 0 rather than NULL

Question

Hi

I want to join two tables, with the number of records for each type being counted. If there are no records of that type in the left table I want a 0 to be returned, not a null.

How can I do this? Karl

Answer 1

Look at IsNull in SQL Server and Sybase. Use NVL in Oracle.

Answer 2

Use:

ISNULL(count(*), 0)

Answer 3

ISNULL(nullable, value_if_null) for MsSQL, COALESCE(nullable1, nullable2, ..., value_if_null) for MySQL.

Edit: As I'm told, COALESCE works for both, so I'd choose that to replace NULL columns.

Now I think that COUNT()ing NULL values returns 0 in MySQL too, so I agree with Rashmi. Could you show us the query and the wanted result ?

Answer 4

I am not sure if I have understood your exact problem, but in sqlserver on a left join, you will get a count as 0 if your query is something like this:

select t1.id, count(t2.id)
from table1 t1
left outer join table2 t2
on t1.id = t2.id
group by t1.id

Answer 5

COALESCE is more cross-compatible than ISNULL or NVL (it works on MSSQL, Oracle, MySQL, Derby, et al.). But I am not sure about the performance differences.