in postgresql i have a boolean field that allows Null values.
i'd like to query for the items that are not 'true'
filter( tablename.is_deleted != True )
creates this sql:
is_deleted != True
however this is incorrect and doesn't match the resultset i want. it
needs to read :
is_deleted IS NOT True
I was hoping that i could do TableName.column_name.not_( True ) , but
that didn't work.
in postgresql ( and they claim the sql standard), !=/== and "IS
NOT"/"IS" are different types of comparisons.
http://www.postgresql.org/docs/8.2/static/functions-comparison.html
"""
Do not write expression = NULL because NULL is not "equal to" NULL.
(The null value represents an unknown value, and it is not known
whether two unknown values are equal.) This behavior conforms to the
SQL standard.
The ordinary comparison operators yield null (signifying "unknown")
when either input is null. Another way to do comparisons is with the
IS [ NOT ] DISTINCT FROM construct:
expression IS DISTINCT FROM expression
expression IS NOT DISTINCT FROM expression
For non-null inputs, IS DISTINCT FROM is the same as the <> operator.
However, when both inputs are null it will return false, and when just
one input is null it will return true. Similarly, IS NOT DISTINCT FROM
is identical to = for non-null inputs, but it returns true when both
inputs are null, and false when only one input is null. Thus, these
constructs effectively act as though null were a normal data value,
rather than "unknown".
Boolean values can also be tested using the constructs
expression IS TRUE
expression IS NOT TRUE
expression IS FALSE
expression IS NOT FALSE
expression IS UNKNOWN
expression IS NOT UNKNOWN
These will always return true or false, never a null value, even when
the operand is null. A null input is treated as the logical value
"unknown". Notice that IS UNKNOWN and IS NOT UNKNOWN are effectively
the same as IS NULL and IS NOT NULL, respectively, except that the
input expression must be of Boolean type.
""""
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