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

This might be somewhat of a hack, but it got the job done in our case:

(0 || myfield)::integer

Explanation (Tested on Postgres 8.4):

The above mentioned expression yields NULL for NULL-values in myfield and 0 for empty strings (This exact behaviour may or may not fit your use case).

SELECT id, (0 || values)::integer from test_table ORDER BY id

Test data:

CREATE TABLE test_table
(
  id integer NOT NULL,
  description character varying,
  "values" character varying,
  CONSTRAINT id PRIMARY KEY (id)
)

-- Insert Test Data
INSERT INTO test_table VALUES (1, 'null', NULL);
INSERT INTO test_table VALUES (2, 'empty string', '');
INSERT INTO test_table VALUES (3, 'one', '1');

The query will yield the following result:

 ---------------------
 |1|null        |NULL|
 |2|empty string|0   |
 |3|one         |1   |
 ---------------------

Whereas select only values::integer will result in an error message.

Hope this helps.