I am confuse about what is the value of NULL in SQL. NULL I think is empty.
So will it contain any garbage value or unknown value or like 0 in integer or blank in character?
In simple worlds you can say that Null is not a data value, but a marker for an unknown value.
So any mathematical operations performed on NULL will result in NULL. For example,
10 + NULL = NULL
Similarly if you do string concatenation with string you get:-
'String ' || NULL || 'Concatenation' -- Result is NULL
So you can say that Null means either "not applicable" or "don't know": it is not the same as zero (0) or any other default value, but more importantly, null is treated quite differently from other values in SQL, because it literally has no value.
An example to explain what it means when we say that NULL means UNKNOWN VALUE:
StudentName TestResult
X 78
A 89
B 67
C NULL
So you can see that the student C got NULL marks in the test. So what
does that mean?
Now one can say that the student does not sit in the test or it may be that the student's data is not avaialable. But it definitely does not mean that the student got 0(as if you assign 0 to the student then it would mean that the student appeared in the test and got zero) marks in the test. All we can say that the data for the student is UNKNOWN or NULL
A field with a NULL value is a field with no value. It is very important to understand that a NULL value is different than a zero value or a field that contains spaces.
If a column in a table is optional, we can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value.
NULL values are treated differently from other values.
NULL is used as a placeholder for unknown or inapplicable values.
Read more about this here.
NULL is a keyword used to represent unknown/missing data.
Say you have an optional column in your table. You can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value.
Check this for more details.
It is to indicate that a data value does not exist in the database.NULL is also an SQL reserved keyword used to identify the Null special marker.
NULL means “a missing unknown value” and it is treated somewhat differently from other values.
A field with a NULL value is a field with no value. It is very important to understand that a NULL value is different than a zero value or a field that contains spaces.
10 * NULL -- Result is NULL
Null (SQL)
SQL NULL
Working with NULL Values
Null is not empty actually but it is considered as uknown value, from dumentation
Conceptually, NULL means “a missing unknown value” and it is treated somewhat differently from other values.
You can ready more here
Related
When filtering a column:
Ex.
Column
DontShow
null
null
using df.filter(~col('Column).contains('DontShow))
Expected result.
Column
null
null
However, this returns nothing. I assume it treats null values differently. Seeing how this works in where I specify it could also be null: df.filter(~(col('Column).contains('DontShow)) | col('Column').isNull())
Out of curiosity why is this? One would assume it would return every row not containing the condition regardless if its null or not.
Use isNull
df.where(col('Column').isNull()).show()
This question already has answers here:
postgresql NOT ILIKE clause does not include null string values
(2 answers)
Closed 7 months ago.
select * from test;
select * from test where name not in ('amtf');
Why?
As others have said, the problem here is, that you're comparing against a null value, so it returns nothing, because it considers it as false, and I'll go even further that even if you say where name <> 'admf' it wont work, and even if you add more rows it will ignore the null row, and it's not just in PostgreSQL, it doesn't work in SQL-Server or MySQL either.
As you can see in these db<>fiddles SQL-Server, MySQL, and PostgreSQL.
And the reason why it doesn't work is, because you're saying name should not equal a specific value. First name needs to be a value it should not be equal to a value, but when name is null it doesn't have a value, and even more for a side note null itself is not equal null.
The way to solve it is to convert it to a empty string by using COALESCE(name,'') or in SQL-Server you can also use isnull(name,''), and then compare it, or you can add or name is null which will return you all rows, including null, where name <> 'some value'.
Well the condition is right and the response for that as soo .
select * from test where name not in ('amtf');
your query is saying : give me all the records that the name Column is not in ('amtf').
you have 2 column's on is amtf and the other is null.
amtf will no be brought because of the condition and the other column is null -> no name set
select field from table where field = 'value'
select field from table where field in ('value')
The reason I'm asking is that the second version allow me to use the same syntax for null values, while in the first version I need to change the condition to 'where field is null'...
When you are comparing a field to a null like field_name=NULL you are comparing to a known data type from a field say varchar to not only an unknown value but also an unknown data type as well, that is, for NULL values. When comparison like field_name=NULL again implies therefore a checking of data type for both and thus the two could not be compared even if the value of the field is actually NULL thus it will always result to false. However, using the IS NULL you are only comparing for the value itself without the implied comparison for data type thus it could result either to false or true depending on the actual value of the field.
See reference here regarding the issue of NULL in computer science and here in relation to the similarity to your question.
Now, for the IN clause (i.e. IN(NULL)) I don't know what RDBMS you are using because when I tried it with MS SQL and MySQL it results to nothing.
See MS SQL example and MySQL example.
There is no difference in your example. The second, slightly longer, query is not usually used for a single value, it is usally seen for multiple values, such as
select field from table where field in ('value1', 'value2')
yes there is difference in both this queries. In first statment you can insert only 1 value in where clause "where field = 'value'" but in second statement in where field you can insert many values using IN clause "where field in (value1,value2..)"
Examples:
1) select field from table where field ='value1';
2) select field from table where field in ('value1', 'value2')
To check null values
SELECT field
FROM tbl_name
WHERE
(field IN ('value1', 'value2', 'value3') OR field IS NULL)
I've a Status column in a table which has 3 values - 'N/A' , 'Single' ,'Multiple'. Some rows have a NULL value for the Status column.
I need to pull up all the rows in which Status is not null and is not 'N/A'. Basically, I need all the rows whose status is "Single" or "Multiple".
I've been just reading up about NULL actually being equivalent to 'UNKNOWN'.
If I say
SELECT *
FROM t_userstatus
WHERE status <> 'N/A'
I get the results (All rows containing "Single" or "Multiple" only).
What I would like to know is that , does the above WHERE clause always exclude the rows having NULL values?Is that the expected behaviour?
What causes this to exclude null rows even though I've not explicitly specified it?
In my query,do I have to explicitly say status IS NOT NULL ?
I am relatively new to programming, any help is appreciated.
It is not normal behavior as N/A (Single & Multiple as well) is a string that is not related to NULL. Even though the NULL is evaluated to unknown and may not return, you should explicitly use IS NOT NULL.
SELECT [column_list] FROM t_userstatus
WHERE status IS NOT NULL AND status <> 'N/A'
I would also recommend you get in the habit of specifying a column list.
SQL uses three-valued logic: true, false, and unknown. Any comparison to null results in unknown.
So null <> 'N/A' evaluates to unknown. Since unknown is not true, that means the row gets excluded.
Yes, that is the normal behavior. A NULL value has no value, so it does not satisfy a WHERE condition. If you want to also include null values, you need to specify IS NOT NULL as well.
As others have said, null is generally not comparable. So, a status != 'N/A' comes back as false when status is null.
But what others haven't mentioned yet is that that is controlled through the Ansi_nulls setting. By default it is on and nulls are not comparable (as you state in the question, the principal behind this is that they are unknown so it is false that they are not necessarily N/A). But you can use
SET ANSI_NULLS OFF
To change this behavior in which case a null will be equal to a null and not equal to anything else. There is more details on MSDN here.
A bit non-obviously, SQL has three valued logic (true/false/unknown). Any direct comparison with NULL will result in the value unknown which in the WHERE clause is considered non-true. Your condition;
WHERE status <> 'N/A'
will in other words never match a null since NULL is never "not equal" to anything.
The only ways to match a NULL using a comparison is using the special null operations IS NULL or IS NOT NULL.
As a side note, life is naturally never quite that simple.
Allowing NULL values on a column is normally done to allow the absense of a value to be represented. When using NVARCHAR there is aldready a possibility to have an empty string, without setting the column to NULL. In most cases I cannot see a semantical difference between an NVARCHAR with an empty string and a NULL value for such a column.
Setting the column as NOT NULL saves me from having to deal with the possibility of NULL values in the code and it feels better to not have to different representations of "no value" (NULL or an empty string).
Will I run into any other problems by setting my NVARCHAR columns to NOT NULL. Performance? Storage size? Anything I've overlooked on the usage of the values in the client code?
A NULL indicates that the value in the column is missing/inapplicable. A blank string is different because that is an actual value. A NULL technically has no data type where as a blank string, in this case, is nvarchar.
I cant see any issues with having default values rather than NULL values.
In fact, it would probably be beneficial as you wouldn't have to worry about catering for NULL values in any of your queries
e.g
Select Sum(TotalPrice) as 'TotalPrice' From myTable Where CountOfItems > 10
much easier than
Select Sum(IsNull(TotalPrice,0)) as 'TotalPrice' From myTable Where IsNull(CountOfItems,0) > 10
You should use default constraints in your DDL to ensure that no rogue NULL's appear in your data.
The concept of the NULL value is a common source of confusion. NULL is not the same as an empty string, or a value of zero.
Conceptually, NULL means "a missing unknown value" and it is treated somewhat differently from other values. For example, to test for NULL, you cannot use the arithmetic comparison operators such as =, <, or <>.
If you have columns that may contain "a missing unknown value", you have to set them to accept NULLs. On the other hand, an empty string simply means that the value is known, but is empty.
For example: If a "Middle Name" field in a "Users" table is set to NULL, it should mean that the middle name of that user is not known. The user might or might not have a middle name. However, if the "Middle Name" field is set to an empty string, this should mean that the user is known to have no middle name.
If anything I'd say that it'll be easier to use the table if you don't allow NULLs, since you won't have to check for NULLs everywhere in code, so I'd only set a column to allow NULLs if I need to handle unknown rather than empty values.
What ho1 said. But you'd be ill-advised to define a column NOT NULL and then have a special value for 'Unknown'.