How to Cast a String to Integer and Have 0 in Case of Error in the Cast with Postgresql

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

I was just wrestling with a similar problem myself, but didn't want the overhead of a function. I came up with the following query:

SELECT myfield::integer FROM mytable WHERE myfield ~ E'^\\d+$';

Postgres shortcuts its conditionals, so you shouldn't get any non-integers hitting your ::integer cast. It also handles NULL values (they won't match the regexp).

If you want zeros instead of not selecting, then a CASE statement should work:

SELECT CASE WHEN myfield~E'^\\d+$' THEN myfield::integer ELSE 0 END FROM mytable;

Cast string to number, interpreting null or empty string as 0

The types of values need to be consistent; coalescing the empty string to a 0 means that you cannot then compare it to null in the nullif. So either of these works:

# create table tests (orig varchar);
CREATE TABLE

# insert into tests (orig) values ('1'), (''), (NULL), ('0');
INSERT 0 4


# select orig, cast(coalesce(nullif(orig,''),'0') as float) as result from tests;
 orig | result 
------+--------
    1 |      1
      |      0
      |      0
    0 |      0
(4 rows)


# select orig, coalesce(cast(nullif(orig,'') as float),0) as result from tests;
 orig | result 
------+--------
 1    |      1
      |      0
      |      0
 0    |      0
(4 rows)

How to cast String into int in PostgreSql

Remove all non-digit characters, then you cast it to an integer:

regexp_replace(salary, '[^0-9]+', '', 'g')::int

But instead of trying to convert the value every time you select it, fix your database design and convert the column to a proper integer. Never store numbers in text columns.

alter table bad_design 
     alter salary type int using regexp_replace(salary, '[^0-9]+', '', 'g')::int;

Change type of varchar field to integer: cannot be cast automatically to type integer

There is no implicit (automatic) cast from text or varchar to integer (i.e. you cannot pass a varchar to a function expecting integer or assign a varchar field to an integer one), so you must specify an explicit cast using ALTER TABLE ... ALTER COLUMN ... TYPE ... USING:

ALTER TABLE the_table ALTER COLUMN col_name TYPE integer USING (col_name::integer);

Note that you may have whitespace in your text fields; in that case, use:

ALTER TABLE the_table ALTER COLUMN col_name TYPE integer USING (trim(col_name)::integer);

to strip white space before converting.

This shoud've been obvious from an error message if the command was run in psql, but it's possible PgAdmin-III isn't showing you the full error. Here's what happens if I test it in psql on PostgreSQL 9.2:

=> CREATE TABLE test( x varchar );
CREATE TABLE
=> insert into test(x) values ('14'), (' 42  ');
INSERT 0 2
=> ALTER TABLE test ALTER COLUMN x TYPE integer;
ERROR:  column "x" cannot be cast automatically to type integer
HINT:  Specify a USING expression to perform the conversion. 
=> ALTER TABLE test ALTER COLUMN x TYPE integer USING (trim(x)::integer);
ALTER TABLE        

Thanks @muistooshort for adding the USING link.

See also this related question; it's about Rails migrations, but the underlying cause is the same and the answer applies.

If the error still occurs, then it may be related not to column values, but indexes over this column or column default values might fail typecast. Indexes need to be dropped before ALTER COLUMN and recreated after. Default values should be changed appropriately.

Casting an array of strings to an array of integers

demo:db<>fiddle

SELECT
    array_agg(elems::int)
FROM unnest(ARRAY['5', '6']) as elems

1. Expand the array into one record per element
2. Reaggregate cast integer values

To ensure the original order, you need to add WITH ORDINALITY, which adds an index to the original array:

SELECT
    array_agg(elems.value::int ORDER BY elems.index)
FROM unnest(ARRAY['5', '6']) WITH ORDINALITY as elems(value, index)

If you have a JSON array instead, the algorithm is the same, only the used functions have different names:

SELECT
    json_agg(elems.value::int ORDER BY elems.index)
FROM json_array_elements_text('["5", "6"]'::json) WITH ORDINALITY as elems(value, index)

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EDIT: According to comment:

this is my query. SELECT data->>'pid' FROM user_data where
data->>'pid' is not null How can I update pid to array of integers ?

demo:db<>fiddle

You have to expand and reaggregate nonetheless:

SELECT 
    json_agg(elems::int)                              -- 2
FROM user_data,
    json_array_elements_text(data -> 'pid') as elems  -- 1
WHERE data->>'pid' IS NOT NULL
GROUP BY id

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