How do I cast a string to integer and have 0 in case of error in the cast with PostgreSQL?

Question

In PostgreSQL I have a table with a varchar column. The data is supposed to be integers and I need it in integer type in a query. Some values are empty strings. The followin

Answer 1

I had the same sort of need and found this to work well for me (postgres 8.4):

CAST((COALESCE(myfield,'0')) AS INTEGER)

Some test cases to demonstrate:

db=> select CAST((COALESCE(NULL,'0')) AS INTEGER);
 int4
------
    0
(1 row)

db=> select CAST((COALESCE('','0')) AS INTEGER);
 int4
------
    0
(1 row)

db=> select CAST((COALESCE('4','0')) AS INTEGER);
 int4
------
    4
(1 row)

db=> select CAST((COALESCE('bad','0')) AS INTEGER);
ERROR:  invalid input syntax for integer: "bad"

If you need to handle the possibility of the field having non-numeric text (such as "100bad") you can use regexp_replace to strip non-numeric characters before the cast.

CAST(REGEXP_REPLACE(COALESCE(myfield,'0'), '[^0-9]+', '', 'g') AS INTEGER)

Then text/varchar values like "b3ad5" will also give numbers

db=> select CAST(REGEXP_REPLACE(COALESCE('b3ad5','0'), '[^0-9]+', '', 'g') AS INTEGER);
 regexp_replace
----------------
             35
(1 row)

To address Chris Cogdon's concern with the solution not giving 0 for all cases, including a case such as "bad" (no digit characters at all), I made this adjusted statement:

CAST((COALESCE(NULLIF(REGEXP_REPLACE(myfield, '[^0-9]+', '', 'g'), ''), '0')) AS INTEGER);

It works similar to the simpler solutions, except will give 0 when the value to convert is non-digit characters only, such as "bad":

db=> select CAST((COALESCE(NULLIF(REGEXP_REPLACE('no longer bad!', '[^0-9]+', '', 'g'), ''), '0')) AS INTEGER);
     coalesce
----------
        0
(1 row)