How to manage NULL values in numeric fields returned by cursor in Select stament, to manage efficienly aritmetic operations ?
Don't use cursors.
If you must (really?), you can use the ISNULL function:
SELECT ISNULL(fieldname, 0)
will give you a "0" (zero) instead of NULL.
ISNULL(value, replacement)
will replace value with replacement if value IS NULL
http://msdn.microsoft.com/en-us/library/ms184325.aspx
Assuming you cannot avoid a cursor in the first place, I don't understand why a NULL would be handled much differently in a variable than you would in a query - there is INSULL, COALESCE, CASE WHEN etc.
One interesting thing:
DECLARE #v as int -- initialized to NULL
{ -- loop through a cursor
FETCH NEXT INTO #v
}
You won't be able to necessarily distinguish an uninitialized #v from when the last row's #v setting was NULL.
Related
I've encountered an interesting SQL server behaviour while trying to generate random values in T-sql using RAND and CHOOSE functions.
My goal was to try to return one of two given values using RAND() as rng. Pretty easy right?
For those of you who don't know it, CHOOSE function accepts in an index number(int) along with a collection of values and returns a value at specified index. Pretty straightforward.
At first attempt my SQL looked like this:
select choose(ceiling((rand()*2)) ,'a','b')
To my surprise, this expression returned one of three values: null, 'a' or 'b'. Since I didn't expect the null value i started digging. RAND() function returns a float in range from 0(included) to 1 (excluded). Since I'm multiplying it by 2, it should return values anywhere in range from 0(included) to 2 (excluded). Therefore after use of CEILING function final value should be one of: 0,1,2. After realising that i extended the value list by 'c' to check whether that'd be perhaps returned. I also checked the docs page of CEILING and learnt that:
Return values have the same type as numeric_expression.
I assumed the CEILINGfunction returned int, but in this case would mean that the value is implicitly cast to int before being used in CHOOSE, which sure enough is stated on the docs page:
If the provided index value has a numeric data type other than int,
then the value is implicitly converted to an integer.
Just in case I added an explicit cast. My SQL query looks like this now:
select choose(cast(ceiling((rand()*2)) as int) ,'a','b','c')
However, the result set didn't change. To check which values cause the problem I tried generating the value beforehand and selecting it alongside the CHOOSE result. It looked like this:
declare #int int = cast(ceiling((rand()*2)) as int)
select #int,choose( #int,'a','b','c')
Interestingly enough, now the result set changed to (1,a), (2,b) which was my original goal. After delving deeper in the CHOOSE docs page and some testing i learned that 'null' is returned in one of two cases:
Given index is a null
Given index is out of range
In this case that would mean that index value when generated inside the SELECT statement is either 0 or above 2/3 (I'm assuming that negative numbers are not possible here and CHOOSE function indexes from 1). As I've stated before 0 should be one of possibilities of:
ceiling((rand()*2))
,but for some reason it's never 0 (at least when i tried it 1 million+ times like this)
set nocount on
declare #test table(ceiling_rand int)
declare #counter int = 0
while #counter<1000000
begin
insert into #test
select ceiling((rand()*2))
set #counter=#counter+1
end
select distinct ceiling_rand from #test
Therefore I assume that the value generated in SELECT is greater than 2/3 or NULL. Why would it be like this only when generated in SELECT statement? Perhaps order of resolving CAST, CELING or RAND inside SELECT is different than it would seem? It's true I've only tried it a limited number of times, but at this point the chances of it being a statistical fluctuation are extremely small. Is it somehow a floating-point error? I truly am stumbled and looking forward to any explanation.
TL;DR: When generating a random number inside a SELECT statement result set of possible values is different then when it's generated before the SELECT statement.
Cheers,
NFSU
EDIT: Formatting
You can see what's going on if you look at the execution plan.
SET SHOWPLAN_TEXT ON
GO
SELECT (select choose(ceiling((rand()*2)) ,'a','b'))
Returns
|--Constant Scan(VALUES:((CASE WHEN CONVERT_IMPLICIT(int,ceiling(rand()*(2.0000000000000000e+000)),0)=(1) THEN 'a' ELSE CASE WHEN CONVERT_IMPLICIT(int,ceiling(rand()*(2.0000000000000000e+000)),0)=(2) THEN 'b' ELSE NULL END END)))
The CHOOSE is expanded out to
SELECT CASE
WHEN ceiling(( rand() * 2 )) = 1 THEN 'a'
ELSE
CASE
WHEN ceiling(( rand() * 2 )) = 2 THEN 'b'
ELSE NULL
END
END
and rand() is referenced twice. Each evaluation can return a different result.
You will get the same problem with the below rewrite being expanded out too
SELECT CASE ceiling(( rand() * 2 ))
WHEN 1 THEN 'a'
WHEN 2 THEN 'b'
END
Avoid CASE for this and any of its variants.
One method would be
SELECT JSON_VALUE ( '["a", "b"]' , CONCAT('$[', FLOOR(rand()*2) ,']') )
I'm trying to use the following CASE WHEN in computed column alias, but it shows syntax error.
[Password_Last_Changed] [datetime] AS
SELECT CASE
WHEN ([SUA_History1_Date] IS NOT NULL) then [SUA_History1_Date]
WHEN ([SUA_History1_Date] IS NULL) then [SUA_History2_Date]
WHEN ([SUA_History2_Date] IS NULL) then [SUA_History3_Date]
WHEN ([SUA_History3_Date] IS NULL) then [SUA_History4_Date]
WHEN ([SUA_History4_Date] IS NULL) then [SUA_History5_Date]
ELSE NULL
END
Not sure what went wrong. If there is a better approach for this logic, please let me try it.
Using COALESCE is better option than ISNULL or CASE..WHEN for this problem since the input values for the COALESCE expression can be evaluated multiple times.
You can also use NULLIF to check the conditions in those NULL valued columns.
A NULL value for ISNULL is converted to int datatype whereas for COALESCE, you must provide a data type. ISNULL takes only 2 parameters whereas COALESCE takes a variable number of parameters.
You can use COALESCE:
COALESCE([SUA_History1_Date],[SUA_History2_Date],[SUA_History3_Date],[SUA_History4_Date],[SUA_History5_Date])
Evaluates the arguments in order and returns the current value of the first expression that initially does not evaluate to NULL.
I have a table with details of family members staying in a particular locality. Since these are government data, it has lot mistakes. Like in one column 'houseno', there a 2 values 'Ti 303' and '303' which are same house numbers.
In the end, I want Ti 303 to be updated with '303'. (As these are family members living in same house)
Similarly 'P-101' and 'P/101' are same houseno's and I want it to be converted to either 'P-101' or 'P/101'. I tried difference, substring etc but of now use to me. Please help!
You just need to strip out the characters to compare the content?
CREATE FUNCTION dbo.FN_GetNumberPart (#strMixedString VARCHAR(200))
RETURNS VARCHAR(200)
AS
BEGIN
DECLARE #NumberPart INT
-- Get the next non numeric character position
SET #NumberPart = PATINDEX('%[^0-9]%', #strMixedString)
-- While there are non numeric characters remaining
WHILE #NumberPart > 0
BEGIN
-- Remove the non numeric character from the string
SET #strMixedString = STUFF(#strMixedString, #NumberPart , 1, '' )
-- Get the next non numeric character position
SET #NumberPart = PATINDEX('%[^0-9]%', #strMixedString)
END
-- Spit out the cleansed string
RETURN ISNULL(#strMixedString,0)
END
GO
SELECT dbo.FN_GetNumberPart(HouseNo)
from TblAddresses
You should use the REPLACE command. For the two examples give you could hard code it as follows:
select REPLACE('Ti 303','Ti ','')
select REPLACE('P-101','P-','P/')
You would use REPLACE in your UPDATE command and not as a SELECT obviously.
If you have a list of strings to replace in a column with an update then you could put these into a table. Then use this in your REPLACE command for the string pattern to be replaced.
I understand why null + 1 or (1 + null) returns null: null means "unknown value", and if a value is unknown, its successor is unknown as well. The same is true for most other operations involving null.[*]
However, I don't understand why the following happens:
SELECT SUM(someNotNullableIntegerField) FROM someTable WHERE 1=0
This query returns null. Why? There are no unknown values involved here! The WHERE clause returns zero records, and the sum of an empty set of values is 0.[**] Note that the set is not unknown, it is known to be empty.
I know that I can work around this behaviour by using ISNULL or COALESCE, but I'm trying to understand why this behaviour, which appears counter-intuitive to me, was chosen.
Any insights as to why this makes sense?
[*] with some notable exceptions such as null OR true, where obviously true is the right result since the unknown value simply does not matter.
[**] just like the product of an empty set of values is 1. Mathematically speaking, if I were to extend $(Z, +)$ to $(Z union {null}, +)$, the obvious choice for the identity element would still be 0, not null, since x + 0 = x but x + null = null.
The ANSI-SQL-Standard defines the result of the SUM of an empty set as NULL. Why they did this, I cannot tell, but at least the behavior should be consistent across all database engines.
Reference: http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt on page 126:
b) If AVG, MAX, MIN, or SUM is specified, then
Case:
i) If TXA is empty, then the result is the null value.
TXA is the operative resultset from the selected column.
When you mean empty table you mean a table with only NULL values, That's why we will get NULL as output for aggregate functions. You can consider this as by design for SQL Server.
Example 1
CREATE TABLE testSUMNulls
(
ID TINYINT
)
GO
INSERT INTO testSUMNulls (ID) VALUES (NULL),(NULL),(NULL),(NULL)
SELECT SUM(ID) FROM testSUMNulls
Example 2
CREATE TABLE testSumEmptyTable
(
ID TINYINT
)
GO
SELECT SUM(ID) Sums FROM testSumEmptyTable
In both the examples you will NULL as output..
How can I know if a VARCHAR field's value can be successfully converted to an integer?
I want to do it massively to insert records from one table to another...
IsNumeric() function returns 1 for strings (varchars) which can be converted to a number and 0 for those that cannot..
Check out IsNumeric function
One issue whit IsNumeric() function is that You will get True and if number got decimal separator,
What is totally right, But if someone as I need to check straight to numbers in varchar, without decimal symbols, (I got that when I needed to calculate CHECK digit on barcode) You can use castom
made function like
create FUNCTION [dbo].[checkbarkod]
(
#ean_kod varchar(13)
)
RETURNS bit
AS
begin
declare #duzina int
declare #slovo char(1)
declare #pozicija int
declare #uredu bit
set #duzina=len(#ean_kod)
while #duzina>0
begin
set #slovo=(substring(#ean_kod,#duzina,1))
if (#slovo not in('1','2','3','4','5','6','7','8','9','0'))
begin
set #uredu=convert(bit,0)
break
end
else
begin
set #uredu=convert(bit,1)
set #duzina=#duzina-1
end
end
RETURN #uredu
end