special marker and keyword in SQL indicating that something has no value
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Why does NULL = NULL evaluate to false in SQL server - Stack Overflow
What is a null in sql? is it the same as a empty cell?
Need some knowledge on NULL and NOT NULL
What is NULL in SQL? - Stack Overflow
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Think of the null as "unknown" in that case (or "does not exist"). In either of those cases, you can't say that they are equal, because you don't know the value of either of them. So, null=null evaluates to not true (false or null, depending on your system), because you don't know the values to say that they ARE equal. This behavior is defined in the ANSI SQL-92 standard.
EDIT: This depends on your ansi_nulls setting. if you have ANSI_NULLS off, this WILL evaluate to true. Run the following code for an example...
set ansi_nulls off
if null = null
print 'true'
else
print 'false'
set ansi_nulls ON
if null = null
print 'true'
else
print 'false'
How old is Frank? I don't know (null).
How old is Shirley? I don't know (null).
Are Frank and Shirley the same age?
Correct answer should be "I don't know" (null), not "no", as Frank and Shirley might be the same age, we simply don't know.
Lets say i have a Excel sheet , col : name and ID . some rows cells in "name" are empty in datasheet.
now sql pulls this data, will the empty cells be filled with a null value by default?
or is null a unique type of character that is only used to overwrite existing values , so when system reads it it thinks its empty?
Where and why exactly a null is used?
What is exactly null and not null? To my understanding Not null we use when its mandatory to insert some value in that field, also when we give check constraint so by default the column will be not null right?
By adding new column through alter method default values are null, so how would I be able to insert values in it and is it right to give not null constraint to that new column while adding through alter method, basically when null and when not null to be used?...
god this is so confusing please help me, ik im asking alot but im really confused
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.
Use the COALESCE function to replace NULL values with an empty string.
SELECT Column1 + COALESCE(Column2, '') AS Result
FROM YourTable
A few posts I have made tagged MSSQL have been renamed to 'SQL' by a moderator. So I am assuming you are using MSSQL
COALESCE will return the FIRST non-null value.
SELECT COALESCE('a', NULL, 'c')
will only return 'a'
If you want Firstname + Lastname, where sometimes one or the other is NULL, use CONCAT. Concat adds the strings together and replaces NULLS with 0 length non-null value.
SELECT CONCAT('a', NULL, 'c')
will return 'ac'
If you want Fn space + middle name space + LN, combine concatinate with CONCAT:
SELECT CONCAT('a' + ' ', NULL + ' ', 'c')
Will return 'a c'.
The space after middlename (null) is eliminated with the + and NULL.
NULL + ' ' is null.
So in cases where Middlename or Firstname is null, you won't get extra unwanted spaces.
An
instatement will be parsed identically tofield=val1 or field=val2 or field=val3. Putting a null in there will boil down tofield=nullwhich won't work.
(Comment by Marc B)
I would do this for clairity
SELECT *
FROM tbl_name
WHERE
(id_field IN ('value1', 'value2', 'value3') OR id_field IS NULL)
Your query fails due to operator precedence. AND binds before OR!
You need a pair of parentheses, which is not a matter of "clarity", but pure logic necessity.
SELECT *
FROM tbl_name
WHERE other_condition = bar
AND another_condition = foo
AND (id_field IN ('value1', 'value2', 'value3') OR id_field IS NULL);The added parentheses prevent AND binding before OR. If there were no other WHERE conditions (no AND) you would not need additional parentheses. The accepted answer is misleading in this respect.