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

Question

I have a Postgres table with a string column carrying numeric values. I need to convert these strings to numbers for math, but I need both NULL values as well as em

Answer 1

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)