Running Boolean Expressions in Postgres

Question

I am trying to demonstrate the difference between how ruby and and postgres treat null equality.

For example, in ruby, I run nil == nil and get true. I want to try to run the same type of query in postgres and show that null = null is false, but not sure how to structure it.

Alternatively show that null = null is true and null != null is also true.

This is not Postgresql feature, but SQL standard one: there are lot of SO answers about NULL = NULL. — Pavel Mikhailyuk
– Pavel Mikhailyuk, Commented Apr 5, 2019 at 14:34

JGH · Accepted Answer · 2019-04-05 11:30:30Z

3

select null = null;
 ?column?
----------

(1 row)

It is not false, nor true, but undefined.

answered Apr 5, 2019 at 11:30

JGH

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1 Comment

Amadan Over a year ago

Or, more understandably, (NULL = NULL) IS NULL.

Anepicpastry · Accepted Answer · 2019-04-05 12:43:15Z

1

Almost any evaluation against a null in postgres will return a null (short of some functions like concat()). To return a boolean when evaluating a null you have to use IS NULL

This will return True: SELECT NULL IS NULL

This will return False: SELECT NULL IS NOT NULL

answered Apr 5, 2019 at 12:43

Anepicpastry

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