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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 empty strings to be interpreted as 0.

I can convert empty strings into null values:

# select nullif('','');
 nullif 
--------

(1 row)

And I can convert null values into a 0:

# select coalesce(NULL,0);
 coalesce 
----------
        0
(1 row)

And I can convert strings into numbers:

# select cast('3' as float);
 float8 
--------
      3
(1 row)

But when I try to combine these techniques, I get errors:

# select cast( nullif( coalesce('',0), '') as float);
ERROR:  invalid input syntax for integer: ""
LINE 1: select cast( nullif( coalesce('',0), '') as float);

# select coalesce(nullif('3',''),4) as hi;
ERROR:  COALESCE types text and integer cannot be matched
LINE 1: select coalesce(nullif('3',''),4) as hi;

What am I doing wrong?

Answers

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)