This is not really a problem because I solved it. But, I wanted to share something that ate my brain today. So, run this query and check it out:
The query:
select 2 as ID1,0 as ID2 into #TEMP
insert into #TEMP (ID1,ID2)
values (2,1),(2,2),(2,0)
select
case when min(case when ID1=2 and ID2=0 then 0
else 1
end)=0 then 0
else sum(log(ID2))
end
from #TEMP
The fix:
select
case when min(case when ID1=2 and ID2=0 then 0
else 1
end)=0 then 0
else sum(log(case when ID1=2 and ID2<>0 then ID2
else 1
end))
end
from #TEMP
My query was larger and more difficult to debug, but what do you say about the plan that MSSQL is making and the fact that it gets it wrong with this query? How can it be modified to work except my little fix that I showed before? I am guessing that computing scalars before the query would make things slow if the scalars are not easy to compute and we compute for all values.
SQL Server does not perform short circuit evaluation (i.e. should not be relied upon). It's a fairly well known problem.
Ref:
Don’t depend on expression short circuiting in T-SQL (not even with CASE)
Short Circuit
Short-Circuit Evaluation
Is the SQL WHERE clause short-circuit evaluated?
EDIT: I misunderstood the question with my original answer.
I supposed you could add a where clause, as shown below. Side note: this query might benefit from an index on (ID1, ID2).
select
sum(log(convert(float, ID2)))
from #TEMP
where
-- exclude all records with ID1 = 2 if there are any records with ID1 = 2 and ID2 = 0
not exists (
select 1
from #TEMP NoZeros
where
NoZeros.ID1 = #TEMP.ID1
and NoZeros.ID2 = 0
)
Update: Just in case performance is a concern, I got fairly comparable performance from this query after adding the following indexes:
create index ix_TEMP_ID1_ID2 on #TEMP (ID1, ID2)
create index ix_TEMP_ID2 on #TEMP (ID2) include (ID1)
Original Answer
How about modifying your sum function as shown below?
sum(case ID2 when 0 then null else log(convert(float, ID2)) end)
Related
I am developing a data validation framework where I have this requirement of checking that the table fields should have at least one non-null value i.e they shouldn't be completely empty having all values as null.
For a particular column, I can easily check using
select count(distinct column_name) from table_name;
If it's greater than 0 I can tell that the column is not empty. I already have a list of columns. So, I can execute this query in the loop for every column but this would mean a lot of requests and it is not the ideal way.
What is the better way of doing this? I am using Microsoft SQL Server.
I would not recommend using count(distinct) because it incurs overhead for removing duplicate values. You can just use count().
You can construct the query for counts using a query like this:
select count(col1) as col1_cnt, count(col2) as col2_cnt, . . .
from t;
If you have a list of columns you can do this as dynamic SQL. Something like this:
declare #sql nvarchar(max);
select #sql = concat('select ',
string_agg(concat('count(', quotename(s.value), ') as cnt_', s.value),
' from t'
)
from string_split(#list) s;
exec sp_executesql(#sql);
This might not quite work if your columns have special characters in them, but it illustrates the idea.
You should probably use exists since you aren't really needing a count of anything.
You don't indicate how you want to consume the results of multiple counts, however one thing you could do is use concat to return a list of the columns meeting your criteria:
The following sample table has 5 columns, 3 of which have a value on at least 1 row.
create table t (col1 int, col2 int, col3 int, col4 int, col5 int)
insert into t select null,null,null,null,null
insert into t select null,2,null,null,null
insert into t select null,null,null,null,5
insert into t select null,null,null,null,6
insert into t select null,4,null,null,null
insert into t select null,6,7,null,null
You can name the result of each case expression and concatenate, only the columns that have a non-null value are included as concat ignores nulls returned by the case expressions.
select Concat_ws(', ',
case when exists (select * from t where col1 is not null) then 'col1' end,
case when exists (select * from t where col2 is not null) then 'col2' end,
case when exists (select * from t where col3 is not null) then 'col3' end,
case when exists (select * from t where col4 is not null) then 'col4' end,
case when exists (select * from t where col5 is not null) then 'col5' end)
Result:
col2, col3, col5
I asked a similar question about a decade ago. The best way of doing this in my opinion would meet the following criteria.
Combine the requests for multiple columns together so they can all be calculated in a single scan.
If the scan encounters a not null value in every column under consideration allow it to exit early without reading the rest of the table/index as reading subsequent rows won't change the result.
This is quite a difficult combination to get in practice.
The following might give you the desired behaviour
SELECT DISTINCT TOP 2 ColumnWithoutNull
FROM YourTable
CROSS APPLY (VALUES(CASE WHEN b IS NOT NULL THEN 'b' END),
(CASE WHEN c IS NOT NULL THEN 'c' END)) V(ColumnWithoutNull)
WHERE ColumnWithoutNull IS NOT NULL
OPTION ( HASH GROUP, MAXDOP 1, FAST 1)
If it gives you a plan like this
Hash match usually reads all its build input first meaning that no shortcircuiting of the scan will happen. If the optimiser gives you an operator in "flow distinct" mode it won't do this however and the query execution can potentially stop as soon as TOP receives its first two rows signalling that a NOT NULL value has been found in both columns and query execution can stop.
But there is no hint to request the mode for hash aggregate so you are dependent on the whims of the optimiser as to whether you will get this in practice. The various hints I have added to the query above are an attempt to point it in that direction however.
I have a table with a lot of columns (say 200) they are all boolean. I want to know which of those has at least one record set to true. I have come up with the following query which works fine:
SELECT sum(Case When [column1] = 1 Then 1 Else 0 End) as column1,
sum(Case When [column2] = 1 Then 1 Else 0 End) as column2, sum(Case
When [column3] = 1 Then 1 Else 0 End) as column3, FROM [tablename];
It will return the number of rows that are 'true' for a column. However, this is more information than I need and thereby maybe a more expensive query then needed. The query keeps scanning all fields for all records even though that would not be necessary.
I just learned something about CHECKSUM(*) that might be useful. Try the following code:
DECLARE #T TABLE (
b1 bit
,b2 bit
,b3 bit
);
DECLARE #T2 TABLE (
b1 bit
,b2 bit
,b3 bit
,b4 bit
,b5 bit
);
INSERT INTO #T VALUES (0,0,0),(1,1,1);
INSERT INTO #T2 VALUES (0,0,0,0,0),(1,1,1,1,1);
SELECT CHECKSUM(*) FROM #T;
SELECT CHECKSUM(*) FROM #T2;
You will see from the results that no matter how many columns are in a row, if they are all bit columns with a value of 0, the result of CHECKSUM(*) is always 0.
This means that you could use WHERE CHECKSUM(*)<>0 in your query to save the engine the trouble of summing rows where all the values are 0. Might improve performance.
And even if it doesn't, it's a neat thing to know.
EDIT:
You could do an EXISTS() function on each column. I understand that the EXISTS() function stops scanning when it finds a value that exists. If you have more rows than columns, it might be more performant. If you have more columns than rows, then your current query using SUM() on every column is probably the fastest thing you can do.
If you just want to know the rows that have at last one boolean field, you will need to test every of them.
Something like this (maybe):
SELECT ROW.*
FROM TABLE ROW
WHERE ROW.COLUMN_1 = 1
OR ROW.COLUMN_2 = 1
OR ROW.COLUMN_3 = 1
OR ...
OR ROW.COLUMN_N = 1;
If you actually have 200 columns/fields on one table with boolean then something like the following should work.
SELECT CASE WHEN column1 + column2 + column3 + ... + column200 >= 1 THEN 'Something was true for this record' ELSE NULL END AS My_Big_Field_Test
FROM [TableName];
I'm not in front of my machine, but you could also try the bitwise or operator:
SELECT * FROM [table name] WHERE column1 | column2 | column3 = 1
The OR answer from Arthur is the other suggestion I would offer. Try a few different suggestions and look at the query plans. Also take a look at disk reads and CPU usage. (SET STATISTICS IO ON and SET STATISTICS TIME ON).
See whatever method gives the desires results and the best performance...and then let us know :-)
You can use a query of the form
SELECT
CASE WHEN EXISTS (SELECT * FROM [Table] WHERE [Column1] = 1) THEN 0 ELSE 1 END AS 'Column1',
CASE WHEN EXISTS (SELECT * FROM [Table] WHERE [Column2] = 1) THEN 0 ELSE 1 END AS 'Column2',
...
The efficiency of this critically depends on how sparse your table is. If there are columns where every single row has a 0 value, then any query that searches for a 1 value will require a full table scan, unless an index is in place. A really good choice for this scenario (millions of rows and hundreds of columns) is a columnstore index. These are supported from SQL Server 2012 onwards; from SQL Server 2014 onwards they don't cause the table to be read-only (which is a major barrier to their adoption).
With a columnstore index in place, each subquery should require constant time, and so should the query as a whole (in fact, with hundreds of columns, this query gets so big that you might run into trouble with the input buffer and need to split it up into smaller queries). Without indexes, this query can still be effective as long as the table isn't sparse -- if it "quickly" runs into a row with a 1 value, it stops.
Okay, I've done some web digging and cannot seem to find how to make this situation work or even if it is possible.
I am using SQL Server 2008 and have a calculated column where i am trying to figure some values regarding foreign currency exchange rates.
Although one single table might be easier in the long run, I would like to use the correct solution which involves two different tables.
table t1 houses a log of transactions. table t2 houses basic information for a given currency. The two tables can be joined using the currencyCode attribute.
The sticky part comes with currency dependent calculations. Presently the calculations only differ for CR1 CR2 CR3 or CR4 but in the future as more currencies are added, the potential exists for the new currencies to differ. So ideally, a new row would be added to t2 and a bit flag (flagCurrencyCalc) set that specifies the alternate calculation be used or not used.
So the present formula looks something like this:
(case when [currencyCode]='CR1' OR [currencyCode]='CR2' OR [currencyCode]='CR3' OR [currencyCode]='CR4' then (formula1) else (formula2) end)
As you can see, i would have to manually go in and alter the formual by adding another OR statement with the new currencyCode.
Would it be possible to do something along the line of:
(case when t2.flagCurrencyCalc=True for a given currency code in a record found in t1 then (formula1) else (formula2) end)
???
You cannot access another table in a computed column directly (see here).
Here are some options.
You can simplify your formula using in:
(case when currencyCode in ('CR1', 'CR2', 'CR3', 'CR4')
then (formula1)
else (formula2)
end)
You can use a view:
create view vw_tablename as
select t.*,
(case when currencyCode in (select currencyCode from ListOfCurrencies where . . . )
then (formula1)
else (formula2)
end)
from t;
You can define a function to check the currency.
Yes it is possible. If the other table you are proposing is say
CurrencyFlag (curr_code varchar(5), flag bit)
curr_code flag
--------- -----
C1 1
C2 0
...
then your case can have a sub query
SELECT t1.col1, t1.col2,
CASE
WHEN (SELECT flag FROM CurrencyFlag t2 WHERE t2.curr_code = t1.currencyCode) = 1
THEN formula1
ELSE formula2
END , t1.col3
FROM
table1 t1
I have a valid SQL select which returns an empty result, up and until a specific transaction has taken place in the environment.
Is there something available in SQL itself, that will allow me to return a 0 as opposed to an empty dataset? Similar to isNULL('', 0) functionality. Obviously I tried that and it didn't work.
PS. Sadly I don't have access to the database, or the environment, I have an agent installed that is executing these queries so I'm limited to solving this problem with just SQL.
FYI: Take any select and run it where the "condition" is not fulfilled (where LockCookie='777777777' for example.) If that condition is never met, the result is empty. But at some point the query will succeed based on a set of operations/tasks that happen. But I would like to return 0, up until that event has occurred.
You can store your result in a temp table and check ##rowcount.
select ID
into #T
from YourTable
where SomeColumn = #SomeValue
if ##rowcount = 0
select 0 as ID
else
select ID
from #T
drop table #T
If you want this as one query with no temp table you can wrap your query in an outer apply against a dummy table with only one row.
select isnull(T.ID, D.ID) as ID
from (values(0)) as D(ID)
outer apply
(
select ID
from YourTable
where SomeColumn = #SomeValue
) as T
alternet way is from code, you can check count of DataSet.
DsData.Tables[0].Rows.count > 0
make sure that your query matches your conditions
I am wondering whether is possible to assign a value to a casted column in SQL depending on real table values.
For Example:
select *, cast(null as number) as value from table1
where if(table1.id > 10 then value = 1) else value = 0
NOTE: I understand the above example is not completely Oracle, but, it is just a demonstration on what I want to accomplish in Oracle. Also, the above example can be done multiple ways due to its simplicity. My goal here is to verify if it is possible to accomplish the example using casted columns (columns not part of table1) and some sort of if/else.
Thanks,
Y_Y
select table1.*, (case when table1.id > 10 then 1 else 0 end) as value
from table1