Best way to check for "empty or null value" - sql

Question

What is best way to check if value is null or empty string in Postgres sql statements?
Value can be long expression so it is preferable that it is written only once in check.
Currently I'm using:
coalesce( trim(stringexpression),'')=''
But it looks a bit ugly.
stringexpression may be char(n) column or expression containing char(n) columns with trailing spaces.
What is best way?

Answer 1

The expression stringexpression = '' yields:
TRUE   .. for '' (or for any string consisting of only spaces with the data type char(n))
NULL   .. for NULL
FALSE .. for anything else
So to check for: "stringexpression is either NULL or empty":
(stringexpression = '') IS NOT FALSE
Or the reverse approach (may be easier to read):
(stringexpression <> '') IS NOT TRUE
Works for any character type including char(n). The manual about comparison operators.
Or use your original expression without trim(), which is costly noise for char(n) (see below), or incorrect for other character types: strings consisting of only spaces would pass as empty string.
coalesce(stringexpression, '') = ''
But the expressions at the top are faster.
Asserting the opposite is even simpler: "stringexpression is neither NULL nor empty":
stringexpression <> ''
About char(n)
This is about the data type char(n), short for: character(n). (char / character are short for char(1) / character(1).) Its use is discouraged in Postgres:
In most situations text or character varying should be used instead.
Do not confuse char(n) with other, useful, character types varchar(n), varchar, text or "char" (with double-quotes).
In char(n) an empty string is not different from any other string consisting of only spaces. All of these are folded to n spaces in char(n) per definition of the type. It follows logically that the above expressions work for char(n) as well - just as much as these (which wouldn't work for other character types):
coalesce(stringexpression, ' ') = ' '
coalesce(stringexpression, '') = ' '
Demo
Empty string equals any string of spaces when cast to char(n):
SELECT ''::char(5) = ''::char(5) AS eq1
, ''::char(5) = ' '::char(5) AS eq2
, ''::char(5) = ' '::char(5) AS eq3;
Result:
eq1 | eq2 | eq3
----+-----+----
t | t | t
Test for "null or empty string" with char(n):
SELECT stringexpression
, stringexpression = '' AS base_test
, (stringexpression = '') IS NOT FALSE AS test1
, (stringexpression <> '') IS NOT TRUE AS test2
, coalesce(stringexpression, '') = '' AS coalesce1
, coalesce(stringexpression, ' ') = ' ' AS coalesce2
, coalesce(stringexpression, '') = ' ' AS coalesce3
FROM (
VALUES
('foo'::char(5))
, ('')
, (' ') -- not different from '' in char(n)
, (NULL)
) sub(stringexpression);
Result:
stringexpression | base_test | test1 | test2 | coalesce1 | coalesce2 | coalesce3
------------------+-----------+-------+-------+-----------+-----------+-----------
foo | f | f | f | f | f | f
| t | t | t | t | t | t
| t | t | t | t | t | t
null | null | t | t | t | t | t
Test for "null or empty string" with text:
SELECT stringexpression
, stringexpression = '' AS base_test
, (stringexpression = '') IS NOT FALSE AS test1
, (stringexpression <> '') IS NOT TRUE AS test2
, coalesce(stringexpression, '') = '' AS coalesce1
, coalesce(stringexpression, ' ') = ' ' AS coalesce2
, coalesce(stringexpression, '') = ' ' AS coalesce3
FROM (
VALUES
('foo'::text)
, ('')
, (' ') -- different from '' in a sane character types
, (NULL)
) sub(stringexpression);
Result:
stringexpression | base_test | test1 | test2 | coalesce1 | coalesce2 | coalesce3
------------------+-----------+-------+-------+-----------+-----------+-----------
foo | f | f | f | f | f | f
| t | t | t | t | f | f
| f | f | f | f | f | f
null | null | t | t | t | t | f
db<>fiddle here
Old sqlfiddle
Related:
Any downsides of using data type "text" for storing strings?

Answer 2

To check for null and empty:
coalesce(string, '') = ''
To check for null, empty and spaces (trim the string)
coalesce(TRIM(string), '') = ''

Answer 3

Checking for the length of the string also works and is compact:
where length(stringexpression) > 0;

Answer 4

A lot of the answers are the shortest way, not the necessarily the best way if the column has lots of nulls. Breaking the checks up allows the optimizer to evaluate the check faster as it doesn't have to do work on the other condition.
(stringexpression IS NOT NULL AND trim(stringexpression) != '')
The string comparison doesn't need to be evaluated since the first condition is false.

Answer 5

another way is
nullif(trim(stringExpression),'') is not null

Answer 6

If there may be empty trailing spaces, probably there isn't better solution. COALESCE is just for problems like yours.

Answer 7

Something that I saw people using is stringexpression > ''. This may be not the fastest one, but happens to be one of the shortest.
Tried it on MS SQL as well as on PostgreSQL.

Answer 8

In ran into a kind of similar case, were I had to do this . My Table definition look like :
id(bigint)|name (character varying)|results(character varying)
1 | "Peters"| [{"jk1":"jv1"},{"jk1":"jv2"}]
2 | "Russel"| null
To filter out the results column with null or empty in it , what worked was :
SELECT * FROM tablename where results NOT IN ('null','{}');
This returned all rows which are not null on results.
I'm not sure how to fix this query to return the same all rows which are not null on results.
SELECT * FROM tablename where results is not null;
--- hmm what am I missing,casting ? any inputs?

Answer 9

found this post looking for a solution to 'don't show me data that is '' (blank or single space char) or null'. in my case, we only want to show the user records with these values populated. i hope this response helps another looking for the same. the answers above didn't work in my case.
our app is running rails with postgres. looking at how rails builds the query for .where.not(company_website: [nil, '']) in our app, which works just fine, i can see the resulting sql statement in console.
WHERE NOT ((contacts.company_website = '' OR contacts.company_website IS NULL))
i added this bit and it works as intended.

Answer 10

I like answer by yglodt, but calculating exact length may be expensive for big sets and big strings, so I go with:
coalesce(trim('a') > '','f')

Answer 11

My preffered way to compare nullable fields is:
NULLIF(nullablefield, :ParameterValue) IS NULL AND NULLIF(:ParameterValue, nullablefield) IS NULL . This is cumbersome but is of universal use while Coalesce is impossible in some cases.
The second and inverse use of NULLIF is because "NULLIF(nullablefield, :ParameterValue) IS NULL" will always return "true" if the first parameter is null.

Answer 12

If database having large number of records then null check can take more time
you can use null check in different ways like :
1) where columnname is null
2) where not exists()
3) WHERE (case when columnname is null then true end)