Postgres incrementing null value in a column

Question

In Postgres 9.3 when a field in a table has a null value the following expression doesn't work:

update table_statatistic set members = members + 1 WHERE user_id = $1; 

However when the field has an integer value then this query increments it by 1 without a problem.

The questions are:

1. Why is this happening.
2. How to fix it.

Answer 1

you need to use coalesce for checking null values

update table_statatistic set members = coalesce(members, 0) + 1 WHERE user_id = $1