If I select a column from a MYSQL column, I try to check if it's different from zero (0) by the following constraint:
WHERE my_column_name <> 0
It returns false if it's zero (0) but also if it's NULL. And that's not the purpose.
How do I have to solve this problem?
NULL - safe comparison is done with <=>, you only need to add a NOT() as an alternative:
WHERE NOT(my_column_name <=> 0)
Related
What is the difference between the following 2 queries?
DELETE FROM experience WHERE end IS NOT NULL;
And
DELETE FROM experience WHERE NOT( end=NULL);
First query was accepted as a correct answer but second was not.
NULLs are a bit weird. A NULL is never equal to anything including another NULL. Further, any boolean operation against a NULL returns NULL.
The expression end IS NOT NULL will evaluate false if end is NULL, and true if end is not NULL.
The expression NOT( end=NULL) will actually always evaluate to NULL because (end = NULL) equals NULL and NOT (NULL) also equals NULL. More to the point in a WHERE clause, it will never evaluate true.
NULL values are treated differently from other values.
NULL is used as a placeholder for unknown or inapplicable values.
It is not possible to test for NULL values with comparison operators, such as =, <, or <>
You will have to use the IS NULL and IS NOT NULL operators instead.
Please refer below link for more details.
http://www.w3schools.com/sql/sql_null_values.asp
Comparing a variable to null (i.e. field = NULL) is the same as assigning an unknown value to "field." IS NOT NULL checks to see if the field is null. The second NOT (end=NULL) is assigning a value of "Unknown" to field and then NOTing the result. You should not assign a variable to an unknown value.
You should not use "NOT" alone. It should be followed by "IS". So if you want the second query to work then use - DELETE FROM experience WHERE IS NOT( end=NULL);
An expression with NULL in it can never be evaluated as true except if combined with IS (but see further). As stated in the MySql docs:
You cannot use arithmetic comparison operators such as =, <, or <> to test for NULL.
Because the result of any arithmetic comparison with NULL is also NULL, you cannot obtain any meaningful results from such comparisons.
Exceptions to the Rule
Not all expressions with NULL as argument evaluate to NULL. Apart from NULL IS NULL, also the following will yield a truthy value:
NULL <=> NULL,
COALESCE(NULL, true),
IFNULL(NULL, true),
ISNULL(NULL),
1 IN (1, NULL),
INTERVAL(1, 0, 2, NULL)
CASE 1 WHEN 1 THEN TRUE WHEN NULL THEN false END
The reason that the last three do not yield NULL is that these expressions perform some form of short-circuit evaluation. So if during the evaluation from left-to-right the outcome is clear, the rest of the expression (including the NULL) is not evaluated.
But all of the following will be NULL:
NULL = NULL
CASE NULL WHEN NULL THEN TRUE END,
'test' || NULL,
GREATEST(1, NULL),
1 NOT IN (2, NULL)
Note how in the last two expressions, short-circuiting is not a possibility: all arguments must be evaluated, at least until a NULL is found.
ANSI_NULLS Setting
There is an ANSI_NULLS setting that influences the above behaviour when set to OFF. It is ON by default, and it should in fact not be altered. As stated in the docs:
Important
In a future version of SQL Server, ANSI_NULLS will always be ON and any applications that explicitly set the option to OFF will generate an error. Avoid using this feature in new development work, and plan to modify applications that currently use this feature.
So, I don't believe it is worth discussing this setting further: don't touch it.
I have a table named 'datatablecoulmn' with the following columns.
now i want all rows where the column FkID is NULL.FkID is an integer field
i tried the following queries
SELECT * FROM `datatablecoulmn` WHERE `FkID`=NULL
SELECT * FROM `datatablecoulmn` WHERE `FkID`<1
SELECT * FROM `datatablecoulmn` WHERE `FkID`='null'
All of these returns empty rows .Any help?
In MySQL, NULL is considered as a 'missing, unknown value', as opposed to no value. Any arithmetic comparison with NULL does not return true or false, but returns NULL instead., So, NULL != 'C' returns NULL, as opposed to returning true.
Use IS NULL condition in your query and try like this
SELECT * FROM `datatablecoulmn` WHERE `FkID` IS NULL
For handling NULL values MySQL provides three operators
IS NULL: operator returns true if column value is NULL.
IS NOT NULL: operator returns true if column value is not NULL.
<=>: operator compares values, which (unlike the = operator) is true
even for two NULL values.
You can refer to these links for more
Link 1,Link 2,Link 3
You can't compare with NULL. So you gotta check for YourColumn IS NULL (or maybe YourColumn IS NOT NULL.
NULL is a value like infinity is a number. In other words, not at all. NULL is the absence of certain information.
For the same reason that NaN (not a number) in IEEE754 floating point is not equal to other instances (or even the same instance) of NaN, nothing in SQL is equal to NULL, including NULL.
That's something that may sound strange but, when you think of the purpose of NULL, that of specifying unknown or inappropriate values, it makes sense.
In order to see if a value is NULL, you have to therefore use something like:
where COLUMN_NAME is null
More details on working with NULL in MySQL can be found here.
Use something like:
SELECT * FROM `datatablecoulmn` WHERE `FkID` is NULL
NULL is a placeholder to say there is the absence of a value. Which is why you can only use IS NULL/IS NOT NULL as predicates for such situations and not = or != or <> which is used by values.
Here is another way to exclude the records with FkID is NOT NULL:
SELECT D1.*
FROM datatablecoulmn D1
WHERE NOT EXISTS (SELECT D2.*
FROM datatablecoulmn D2
WHERE D2.`FkID` IS NOT NULL)
I have a column called CODE in a MySQL table which can be NULL. Say I have some rows with CODE='C' which I want to ignore in my select result set. I can have either CODE=NULL or CODE!='C' in my result set.
The following query does not return a row with CODE as NULL:
SELECT * from TABLE where CODE!='C'
But this query works as expected and I know it is the right way to do it.
SELECT * from TABLE where CODE IS NULL OR CODE!='C'
My question is why does having only CODE!='C' does not return rows where CODE=NULL? Definitely 'C' is not NULL. We are comparing no value to a character here. Can someone throw some light as why it doesn't work that way?
In MySQL, NULL is considered as a 'missing, unknown value', as opposed to no value. Take a look at this MySQL Reference on NULL.
Any arithmetic comparison with NULL does not return true or false, but returns NULL instead., So, NULL != 'C' returns NULL, as opposed to returning true.
Any arithmetic comparison with 'NULL' will return false. To check this in SQL:
SELECT IF(NULL=123,'true','false')
To check NULL values we need to use IS NULL & IS NOT NULL operator.
Based on my tests and the documentation here: http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html
You can compare null and get a boolean result using <=>
NOTE: it looks like NOT EQ operator, but it's EQ operator
For example:
select x <=> y;
or
select #x <=> #y;
This also compares string vs null, string vs string, etc.
In SQL, the NULL value is a special value, not comparable with any other one.
The result of a direct comparison with a NULL is always NULL, although (unfortunately) you may find FALSE in some implementation.
To test a null value you should use IS NULL and IS NOT NULL.
SELECT *
FROM `table_name`
WHERE IFNULL(`column_name` != 'C', TRUE)
The specified problem can also appear in joins and the above answers aren't particularly helpful. The way I prefer to do it is by coalescing to otherwise impossible value. For example, this
select foo from bar
inner join baz on bar.x = baz.y
won't work if bar.x and baz.y are both nulls (join won't bring results). The workaround is to use e.g.
select foo from bar
inner join baz on coalesce(bar.x, -1) = coalesce(baz.y, -1)
where -1 is "impossible" value meaning it can never appear in the data set.
select * from user where application_id='1223333344' and name is null;
I use:
SELECT * from TABLE where NOT(CODE <=> 'C')
I'm having a problem where when I try to select the rows that have a NULL for a certain column, it returns an empty set. However, when I look at the table in phpMyAdmin, it says null for most of the rows.
My query looks something like this:
SELECT pid FROM planets WHERE userid = NULL
Empty set every time.
A lot of places said to make sure it's not stored as "NULL" or "null" instead of an actual value, and one said to try looking for just a space (userid = ' ') but none of these have worked. There was a suggestion to not use MyISAM and use innoDB because MyISAM has trouble storing null. I switched the table to innoDB but now I feel like the problem may be that it still isn't actually null because of the way it might convert it. I'd like to do this without having to recreate the table as innoDB or anything else, but if I have to, I can certainly try that.
SQL NULL's special, and you have to do WHERE field IS NULL, as NULL cannot be equal to anything,
including itself (ie: NULL = NULL is always false).
See Rule 3 https://en.wikipedia.org/wiki/Codd%27s_12_rules
SELECT pid FROM planets WHERE userid IS NULL
As all are given answers I want to add little more. I had also faced the same issue.
Why did your query fail? You have,
SELECT pid FROM planets WHERE userid = NULL;
This will not give you the expected result, because from mysql doc
In SQL, the NULL value is never true in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression.
Emphasis mine.
To search for column values that are NULL, you cannot use an expr = NULL test. The following statement returns no rows, because expr = NULL is never true for any expression
Solution
SELECT pid FROM planets WHERE userid IS NULL;
To test for NULL, use the IS NULL and IS NOT NULL operators.
operator IS NULL tests whether a value is NULL.
operator IS NOT NULL tests whether a value is not NULL.
MySQL comparison operators
There's also a <=> operator:
SELECT pid FROM planets WHERE userid <=> NULL
Would work. The nice thing is that <=> can also be used with non-NULL values:
SELECT NULL <=> NULL yields 1.
SELECT 42 <=> 42 yields 1 as well.
See here: https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#operator_equal-to
Info from http://w3schools.com/sql/sql_null_values.asp:
1) NULL values represent missing unknown data.
2) By default, a table column can hold NULL values.
3) NULL values are treated differently from other values
4) It is not possible to compare NULL and 0; they are not equivalent.
5) It is not possible to test for NULL values with comparison
operators, such as =, <, or <>.
6) We will have to use the IS NULL and IS NOT NULL operators instead
So in case of your problem:
SELECT pid FROM planets WHERE userid IS NULL
Had the same issue where query:
SELECT * FROM 'column' WHERE 'column' IS NULL;
returned no values.
Seems to be an issue with MyISAM and the same query on the data in InnoDB returned expected results.
Went with:
SELECT * FROM 'column' WHERE 'column' = ' ';
Returned all expected results.
SELECT pid FROM planets WHERE userid is null;
I had the same issue when converting databases from Access to MySQL (using vb.net to communicate with the database).
I needed to assess if a field (field type varchar(1)) was null.
This statement worked for my scenario:
SELECT * FROM [table name] WHERE [field name] = ''
I have a table with name,age and address.I have totally five rows of data in the table.For some rows the age is left null.I have to display all the data where age is not null.
select * from sample_table where (age !=null);
But no output is displayed and it doesn't give an error also.Please explain this.Thanks.
With NULL you have to use IS or IS NOT. The following query should work:
SELECT * FROM sample_table WHERE (age IS NOT NULL)
The explanation from MySQL
The concept of the NULL value is a common source of confusion for
newcomers to SQL, who often think that
NULL is the same thing as an empty
string ''. This is not the case.
In SQL, the NULL value is never true in comparison to any other value,
even NULL. An expression that contains
NULL always produces a NULL value
unless otherwise indicated in the
documentation for the operators and
If you want to search for column values that are NULL, you cannot use
an expr = NULL test.
To look for NULL values, you must use the IS NULL test.
You have to use IS NULL or IS NOT NULL.
You can not compare directly against NULL because it is not value (no pedants please!)
NULL on Wikipedia
MySQL, Sybase, SQL Server... it's all the same