When comparing boolean values in PostgreSQL we use the comparison operator =.
This is all well but when I want to check against matching NULL values what should I use?
I have used IS NOT DISTINCT FROM when comparing integers that have to match if both are NULL but is it a correct way to go when dealing with boolean values?
I have used IS NOT DISTINCT FROM when comparing integers that have to match if both are NULL but is it a corract way to go when dealing with boolean values?
I see no reason why not. :)
x IS NOT DISTINCT FROM y is basically short-hand for (x = y) OR (x IS NULL AND y IS NULL)
Use is null clause. Like this:
select * from my_table where my_booleae_field is null
Try using coalesce
Like:
where coalesce(your_field, true) = true
Then null values and boolean true will pass the statement
Related
Having a table
CREATE TABLE t (x int)
INSERT INTO t VALUES (null), (0) ,(42)
And query with two placeholders:
SELECT x FROM t WHERE x = ? OR x IS NULL AND ? IS NULL
The logic is the following: if I resolve those placeholders with 42 it returns 42. If I resolve them with null it returns null. In other words, it is a null-including search.
The question is:
Is it possible to rewrite this query (in Postgresql) to have a single placeholder ? instead of two?
Postgres implements null-safe equality through operator IS [NOT] DISTINCT FROM, which is exactly what you ask for.
So:
SELECT x FROM t WHERE x IS NOT DISTINCT FROM ?
The placeholder for parameters in PostgreSQL is $1, $2, etc. Letting you use ? instead is something some drivers implement for convenience, but they do offer less flexibility.
Using the real notation, you can specify one parameter to occur in more than one place:
SELECT x FROM t WHERE x = $1 OR x IS NULL AND $1 IS NULL
This has the advantage over IS NOT DISTINCT FROM in that it can use an index.
Yes. You can use is not distinct from and only use one placeholder:
where x is not distinct from ?
is distinct from/is not distinct from are null-safe comparison operators that treat null as a "real" value for comparison purposes (so null = null for instance).
I have table, in which a column has empty fields.when i try to run simple query like below it is not
returning the data.
select * from table1 where "colunm1" = ''
But below two queries returns data
1,
select * from table1
where coalesce("colunm1", '') = ''
2,
select * from table1
where "colunm1" is null
Can someone tell me the reason?
TIA
You have describe the behavior of a column that is NULL. NULL is not the same as an empty string.
It fails any equality comparison. However, you can use is null or (less preferentially) coalesce().
The only database that treats an empty string like a NULL value, is Oracle.
Logically, for a DBMS, that would be wrong.
Relational DBMSs are all about :
set theory
Boolean algebra
A NULL value is a value that we don't know. A string literal of '' is a string whose value we know. It is a string with no characters, with a length of 0. We don't know of a NULL as a string how long it is, how many and, if any, which, characters it contains.
So it is only logical that:
the comparison '' = '' will evaluate to TRUE
the comparison NULL = NULL will evaluate to FALSE , as any comparison with a NULL value will evaluate to FALSE.
And functions like COALESCE() or NVL(), IFNULL(), ISNULL() will return the first parameter if it does not contain a NULL value. That is consistent.
Except in Oracle
I am currently learning SQL utilizing Codecademy and am curious if there is a difference between using "IS" or "=".
In the current lesson, I wrote this code:
SELECT *
FROM nomnom
WHERE neighborhood IS 'Midtown'
OR neighborhood IS 'Downtown'
OR neighborhood IS 'Chinatown';
Which ran perfectly fine. I always like to look at the answer after to see if there was something I did wrong or could improve on. The answer had this code:
SELECT *
FROM nomnom
WHERE neighborhood = 'Midtown'
OR neighborhood = 'Downtown'
OR neighborhood = 'Chinatown';
Do IS and = function the same?
All that you want to know you can find it here:
The IS and IS NOT operators work like = and != except when one or both
of the operands are NULL. In this case, if both operands are NULL,
then the IS operator evaluates to 1 (true) and the IS NOT operator
evaluates to 0 (false). If one operand is NULL and the other is not,
then the IS operator evaluates to 0 (false) and the IS NOT operator is
1 (true). It is not possible for an IS or IS NOT expression to
evaluate to NULL. Operators IS and IS NOT have the same precedence as
=.
taken from: SQL As Understood By SQLite.
The important part is: ...except when one or both of the operands are NULL... because when using = or != (<>) and 1 (or both) of the operands is NULL then the result is also NULL and this is the difference to IS and IS NOT.
They work the same but "IS" is a keyword in MySQL and is generally used while comparing NULL values. While comparing NULL values "=" does not work.
SELECT * FROM nomnom WHERE neighborhood IS NULL
The above statement would run perfectly fine but
SELECT * FROM nomnom WHERE neighborhood = NULL
would result in an error.
They are the same for these cases, but further down the line you will discover one nifty little value called NULL.
NULL is a pain because... it doesn't exist.
0 = NULL returns FALSE;
Date <> [Column] will not return lines with NULL, only those with a value that is different.
Hell, even NULL = NULL returns false. And NULL <> NULL also returns false. That is why "IS" exists. Because NULL IS NULL will return true.
So as a general rule, use = for values.
Keep "IS" for null.
[Column] IS NULL
or
[Column] IS NOT NULL
And remember to always check if your column is nullable that you need to plan for null values in your WHERE or ON clauses.
I have following table:
And following simple query:
SELECT * FROM dbo.Calendars WHERE calname NOT IN(NULL)
My question is why always NOT IN(NULL) return nothing?
PS:No matter what is your table,if you add NOT IN(NULL) to any column's condition,result is nothing.
Thank you in advance :)
Because value equals NULL is not defined, and will never evaluate to True.
Check This post on bad practices. Don't use IN and NULL, use EXISTS to test for at least one row where something exists.
NULL is treated differently to other values in most databases. For instance, this predicate will never be true:
WHERE foo = NULL
whereas this predicate is true if the value of foo is indeed NULL:
WHERE foo IS NULL
To your problem, if you only wish to check if calname is not null, use the following predicate:
WHERE calname IS NOT NULL
If on the other hand you have a set of values, of which NULL is one of those values (say [1,2,3,4,NULL]), and you still want to use the NOT IN syntax, you have to do it like this:
WHERE calname IS NOT NULL AND calname NOT IN(1, 2, 3, 4)
EDIT: A further way of doing this, if you are constrained to use the NOT IN syntax is to COALESCE the column calname into a value that you definitely know is not stored in that column. For instance, if calname can only take positive integer values, then we can do this instead:
WHERE COALESCE(calname, -1) NOT IN (-1)
Your query evaluates to calname <> null , which results in UNKNOWN.
Change your query to this ,
SELECT * FROM dbo.Calendars WHERE calname IS NOT NULL;
I am trying to generate a hive query which will take multiple numeric column names and check whether it is has numeric values. If the column has numeric values then the output should be (column name,true) else if the field has NULL or some string value the output should be (column name,false)
SELECT distinct (test_nr1,test_nr2) FROM test.abc WHERE (test_nr1,test_nr2) not like '%[^0-9]%';
SELECT distinct test_nr1,test_nr2 from test.abc limit 2;
test_nr1 test_nr2
NULL 81432269
NULL 88868060
the desired output should be :
test_nr1 false
test_nr2 true
Since test_nr1 is a decimal field and it has NULL values, it should output false.
Appreciate valuable suggestions.
You can use cast function. It returns NULL when the value can not not be cast to numeric.
For example:
select case when cast('23ccc' as double) is null then false else true end as IsNumber;
You're trying to use character class pattern matching syntax here, and it doesn't work in every SQL implementation IIRC, however, regexp matching works in most, if not all, SQL implementations.
Considering you're using hive, this should do it:
SELECT ('test_nr1', test_nr1 RLIKE '\d'), ('test_nr2', test_nr2 RLIKE '\d') FROM test.abc;
You should remember that regexp matching is very slow in SQL though.