Postgresql COPY empty string as NULL not work

Question

I have a CSV file with some integer column, now it 's saved as "" (empty string).

I want to COPY them to a table as NULL value.

With JAVA code, I have try these:

String sql = "COPY " + tableName + " FROM STDIN (FORMAT csv,DELIMITER ',',  HEADER true)";
String sql = "COPY " + tableName + " FROM STDIN (FORMAT csv,DELIMITER ',', NULL ''  HEADER true)";

I get: PSQLException: ERROR: invalid input syntax for type numeric: ""

String sql = "COPY " + tableName + " FROM STDIN (FORMAT csv,DELIMITER ',', NULL '\"\"'  HEADER true)";

I get: PSQLException: ERROR: CSV quote character must not appear in the NULL specification

Any one has done this before ?

Answer 1

I assume you are aware that numeric data types have no concept of "empty string" ('') . It's either a number or NULL (or 'NaN' for numeric - but not for integer et al.)

Looks like you exported from a string data type like text and had some actual empty string in there - which are now represented as "" - " being the default QUOTE character in CSV format.

NULL would be represented by nothing, not even quotes. The manual:

NULL

Specifies the string that represents a null value. The default is \N (backslash-N) in text format, and an unquoted empty string in CSV format.

You cannot define "" to generally represent NULL since that already represents an empty string. Would be ambiguous.

To fix, I see two options:

1. Edit the CSV file / stream before feeding to COPY and replace "" with nothing. Might be tricky if you have actual empty string in there as well - or "" escaping literal " inside strings.

2. (What I would do.) Import to an auxiliary temporary table with identical structure except for the integer column converted to text. Then INSERT (or UPSERT?) to the target table from there, converting the integer value properly on the fly:

-- empty temp table with identical structure
CREATE TEMP TABLE tbl_tmp AS TABLE tbl LIMIT 0;

-- ... except for the int / text column
ALTER TABLE tbl_tmp ALTER col_int TYPE text;

COPY tbl_tmp ...;

INSERT INTO tbl  -- identical number and names of columns guaranteed
SELECT col1, col2, NULLIF(col_int, '')::int  -- list all columns in order here
FROM   tbl_tmp;

Temporary tables are dropped at the end of the session automatically. If you run this multiple times in the same session, either just truncate the existing temp table or drop it after each transaction.

Related:

• How to update selected rows with values from a CSV file in Postgres?
• Rails Migrations: tried to change the type of column from string to integer
• postgresql thread safety for temporary tables

Answer 2

Since Postgres 9.4 you now have the ability to use FORCE_NULL. This causes the empty string to be converted into a NULL. Very handy, especially with CSV files (actually this is only allowed when using CSV format).

The syntax is as follow:

COPY table FROM '/path/to/file.csv' 
WITH (FORMAT CSV, DELIMITER ';', FORCE_NULL (columnname));

Further details are explained in the documentation: https://www.postgresql.org/docs/current/sql-copy.html