Magnus Hagander wrote:
Our documentation for COPY
(http://www.postgresql.org/docs/8.4/static/sql-copy.html) has the
following to say:
"
The CSV format has no standard way to distinguish a NULL value from
an empty string. PostgreSQL's COPY handles this by quoting. A NULL is
output as the NULL string and is not quoted, while a data value
matching the NULL string is quoted. Therefore, using the default
settings, a NULL is written as an unquoted empty string, while an
empty string is written with double quotes (""). Reading values
follows similar rules. You can use FORCE NOT NULL to prevent NULL
input comparisons for specific columns.
"
Shouldn't that be:
"A NULL is output as the NULL string and is not quoted, while a data
value matching the empty string is quoted"?
If not, then what really is the difference between a NULL and a NULL string?
No, it shouldn't. Let's say NULL is represented as "foo". Then a null
between delimiters will be written as
delimiter foo delimiter
while the string "foo" will be
delimiter quotechar foo quotechar delimiter
and an empty non-null string will be
delimiter delimiter
unless you have FORCE QUOTE on for it, in which case it will be
delimiter quotechar quotechar delimiter
We had quite a bit of debate on the shape of CSV output at the time it
was done (during 8.0), and that's what we came up with. It has the
useful property that we can round-trip the data, i.e. we can read back
the data we output without losing information about nulls, no matter
what the NULL string is, something we have always been resistant to
changing.
If you think we could explain it better, by all means have a go at it.
But your proposed change isn't accurate. Here is an illustration of the
above:
andrew=# copy (values (1, 'foo', 2),(3,null,4),(5,'',6 ) ) to
stdout null 'foo' csv header;
column1,column2,column3
1,"foo",2
3,foo,4
5,,6
andrew=# copy (values (1, 'foo', 2),(3,null,4),(5,'',6 ) ) to
stdout null 'foo' csv header force quote column2;
column1,column2,column3
1,"foo",2
3,foo,4
5,"",6
HTH
cheers
andrew
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