I have a SQL query in which I am calculating a function cal(a,b) and performing subtraction like below
case
when a=1 and b=0 then case when (c -cal(a,b)) >0 then cal(a,b) else c end
end
In such a scenario, I have to call cal(a,b) twice which is costly.
Is there any way in oracle SQL such that I can store the value of this function
and it is pre-written SQL query so I can not use PL/SQL.
Depending on how you use your mentionned statement, you can have a table that stores such result, a new column that stores such result, in both cases, the values are either calculated "on demand", or calculated using a trigger. It may also be a materialized view that store such result of cal(a, b), but again, it depends how easy it is to obtain a and b, process and store, but also retrieve the result.
That being said, Oracle caching system is well-done and I'm not sure why Oracle would process cal(a, b) a second time if it was called with the same parameter in the same statement.
Use a subquery:
select t.*,
(case when a = 1 and b = 10and c > cal_11
then cal_11
end)
from (select t.*,
(case when a = 1 and b = 0 then cal(a, b) end) as cal11
from t
) t;
Related
SELECT
SP.SITE,
SYS.COMPANY,
SYS.ADDRESS,
SP.CUSTOMER,
SP.STATUS,
DATEDIFF(MONTH,SP.MEMBERSINCE, SP.EXPIRES) AS MONTH_COUNT
CASE WHEN(MONTH_COUNT = 0 THEN MONTH_COUNT = DATEDIFF(DAY,SP.MEMBERSINCE, SP.EXPIRES) AS DAY_COUNT)
ELSE NULL
END
FROM SALEPASSES AS SP
INNER JOIN SYSTEM AS SYS ON SYS.SITE = SP.SITE
WHERE STATUS IN (7,27,29);
I am still trying to understand SQL. Is this the right order to have everything? I'm assuming my datediff() is unable to work because it's inside case when. What I am trying to do, is get the day count if month_count is less than 1 (meaning it's less than one month and we need to count the days between the dates instead). I need month_count to run first to see if doing the day_count would even be necessary. Please give me feedback, I'm new and trying to learn!
Case is an expression, it returns a value, it looks like you should be doing this:
DAY_COUNT =
CASE WHEN DATEDIFF(MONTH,SP.MEMBERSINCE, SP.EXPIRES) = 0
THEN DATEDIFF(DAY,SP.MEMBERSINCE, SP.EXPIRES))
ELSE NULL END
You shouldn't actually need else null as NULL is the default.
Note also you [usually] cannot refer to a derived column in the same select
It appears that what you are trying to do is define the MonthCount column's value, and then reuse that value in another column's definition. (The Don't Repeat Yourself principle.)
In most dialects of SQL, you can't do that. Including MS SQL Server.
That's because SQL is a "declarative" language. This means that SQL Server is free to calculate the column values in any order that it likes. In turn, that means you're not allowed to do anything that would rely on one column being calculated before another.
There are two basic ways around that...
First, use CTEs or sub-queries to create two different "scopes", allowing you to define MonthCount before DayCount, and so reuse the value without retyping the definition.
SELECT
*,
CASE WHEN MonthCount = 0 THEN foo ELSE NULL END AS DayCount
FROM
(
SELECT
*,
bar AS MonthCount
FROM
x
)
AS derive_month
The second main way is to somehow derive the value Before the SELECT block is evaluated. In this case, using APPLY to 'join' a single value on to each input row...
SELECT
x.*,
MonthCount,
CASE WHEN MonthCount = 0 THEN foo ELSE NULL END AS DayCount
FROM
x
CROSS APPLY
(
SELECT
bar AS MonthCount
)
AS derive_month
Simplified example:
In hive, I have a table t with two columns:
Name, Value
Bob, 2
Betty, 4
Robb, 3
I want to do a case when that uses the total of the Value column:
Select
Name
, CASE
When value>0.5*sum(value) over () THEN ‘0’
When value>0.9*sum(value) over () THEN ‘1’
ELSE ‘2’
END as var
From table
I don’t like the fact that sum(value) over () is computed twice. Is there a way to compute this only once. Added twist, I want to do this in one query, so without declaring user variables.
I was thinking of scalar queries:
With total as
(Select sum(value) from table)
Select
Name
, CASE
When value>0.5*(select * from total) THEN ‘0’
When value>0.9*(select * from total)THEN ‘1’
ELSE ‘2’
END as var
From table;
But this doesn’t work.
TLDR: Is there a way to simplify the first query without user variables ?
Don't worry about that. Let the optimizer worry about it. But, you can use a subquery or CTE if you don't want to repeat the expression:
select Name,
(case when value > 0.5 * total then '0'
when value > 0.9 * total then '1'
else '2'
end) as var
From (select t.*, sum(value) over () as total
from table t
) t;
Cross join a subquery that fetches the sum to the table:
Select
t.Name
, CASE
When t.value>0.9*tt.value THEN '1'
When t.value>0.5*tt.value THEN '0'
ELSE '2'
END as var
From table t cross join (select sum(value) value from table) tt
and change the order of the WHEN clauses in the CASE expression because as they are, the 2nd case will never succeed.
Since I/O is the major factor the slows down Hive queries, we should strive to reduce the num of stages to get better performance.
So it's better not to use a sub-query or CTE here.
Try this SQL with a global window clause:
select
name,
case
when value > 0.5*sum(value) over w then '0'
when value > 0.9*sum(value) over w then '1'
else '2'
end as var
from my_table
window w as (ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)
In this case window clause is the recommended way to reduce repetition of code.
Both the windowing and the sum aggregation will be computed only once. You can run explain select..., confirming that only ONE meaningful MR stage will be launched.
Edit:
1. A simple select clause on a subquery is not sth to worry about. It can be pushed down to the last phase of the subquery, so as to avoid additional MR stage.
2. Two identical aggregations residing in the same query block will only be evaluated once. So don’t worry about potential repeated calculation.
On the same row I have dulpicate data.
I have columns aCust, bCust, aPart, bPart, aSM, bSM, aSales, bSales.
I want to combine the Cust together, parts together, and SM together while keeping Sales separate. Some rows have data in both a and b, some a's are null and some b's are null. How do I combine this? If there is data in both a and b, it is always identical (except for sales).
Try this query. It looks weird but will do the job. You didn't specify what RDBMS you're using (Oracle, MySQL, SQL Server, etc.). That's why I didn't use anything like ISNULL.
select
case when aCust is null then bCust else bCust end as Cust,
case when aPart is null then bPart else bPart end as Part,
case when aSM is null then bSM else bSM end as SM,
aSales, bSales
from
tbl
You can do this with coalesce():
select coalesce(aCust, bCust) as Cust,
coalesce(aPart, bPart) as Part,
coalesce(aSM, bSM) as SM,
aSales, bSales
from table;
This will choose the first non-NULL value to return for each field.
I am writing a search routine with a ranking algorithm and would like to get this in one pass.
My Ideal query would be something like this....
select *, (select top 1 wordposition
from wordpositions
where recordid=items.pk_itemid and wordid=79588 and nextwordid=64502
) as WordPos,
case when WordPos<11 then 1 else case WordPos<50 then 2 else case WordPos<100 then 3 else 4 end end end end as rank
from items
Is it possible to use WordPos in a case right there? It's generating an error on me , Invalid column name 'WordPos'.
I know I can redo the subquery for each case but I think it would actually re-run the case wouldn't it?
For example:
select *, case when (select top 1 wordposition from wordpositions where recordid=items.pk_itemid and wordid=79588 and nextwordid=64502)<11 then 1 else case (select top 1 wordposition from wordpositions where recordid=items.pk_itemid and wordid=79588 and nextwordid=64502)<50 then 2 else case (select top 1 wordposition from wordpositions where recordid=items.pk_itemid and wordid=79588 and nextwordid=64502)<100 then 3 else 4 end end end end as rank from items
That works....but is it really re-running the identical query each time?
It's hard to tell from the tests as the first time it runs it's slow but subsequent runs are quick....it's caching...so would that mean that the first time it ran it for the first row, the subsequent three times it would get the result from cache?
Just curious what the best way to do this would be...
Thank you!
Ryan
You can do this using a subquery. I will stick with your SQL Server syntax, even though the question is tagged mysql:
select i.*,
(case when WordPos < 11 then 1
when WordPos < 50 then 2
when WordPos < 100 then 3
else 4
end) as rank
from (select i.*,
(select top 1 wpwordposition
from wordpositions wp
where recordid=i.pk_itemid and wordid=79588 and nextwordid=64502
) as WordPos
from items i
) i;
This also simplifies the case statement. You do not need nested case statements to handle multiple conditions, just multiple where clauses.
No. Identifiers introduced in the output clause (the fact that it comes from a sub-query is irrelevant) cannot be used within the same SELECT statement.
Here are some solutions:
Rewrite the query using a JOIN1, This will eliminate the issue entirely and fits well with RA.
Wrap the entire SELECT with the sub-query within another SELECT with the case. The outer select can access identifiers introduced by the inner SELECT's output clause.
Use a CTE (if SQL Server). This is similar to #2 in that it allows an identifier to be introduced.
While "re-writing" the sub-query for each case is very messy it should still result in an equivalent plan - but view the query profile! - as the results of the query are non-volatile. As such the equivalent sub-queries can be safely moved by the query planner which should move the sub-query/sub-queries to a JOIN to avoid any "re-running" in the first place.
1 Here is a conversion to use a JOIN, which is my preferred method. (I find that if a query can't be written in terms of a JOIN "easily" then it might be asking for the wrong thing or otherwise be showing issues with schema design.)
select
wp.wordposition as WordPos,
case wp.wordposition .. as Rank
from items i
left join wordpositions wp
on wp.recordid = i.pk_itemid
where wp.wordid = 79588
and wp.nextwordid = 64502
I've made assumptions about the multiplicity here (i.e. that wordid is unique) which should be verified. If this multiplicity is not valid and not correctable otherwise (and you're indeed using SQL Server), then I'd recommend using ROW_NUMBER() and a CTE.
I need to calculate the net total of a column-- sounds simple. The problem is that some of the values should be negative, as are marked in a separate column. For example, the table below would yield a result of (4+3-5+2-2 = 2). I've tried doing this with subqueries in the select clause, but it seems unnecessarily complex and difficult to expand when I start adding in analysis for other parts of my table. Any help is much appreciated!
Sign Value
Pos 4
Pos 3
Neg 5
Pos 2
Neg 2
Using a CASE statement should work in most versions of sql:
SELECT SUM( CASE
WHEN t.Sign = 'Pos' THEN t.Value
ELSE t.Value * -1
END
) AS Total
FROM YourTable AS t
Try this:
SELECT SUM(IF(sign = 'Pos', Value, Value * (-1))) as total FROM table
I am adding rows from a single field in a table based on values from another field in the same table using oracle 11g as database and sql developer as user interface.
This works:
SELECT COUNTRY_ID, SUM(
CASE
WHEN ACCOUNT IN 'PTBI' THEN AMOUNT
WHEN ACCOUNT IN 'MLS_ENT' THEN AMOUNT
WHEN ACCOUNT IN 'VAL_ALLOW' THEN AMOUNT
WHEN ACCOUNT IN 'RSC_DEV' THEN AMOUNT * -1
END) AS TI
FROM SAMP_TAX_F4
GROUP BY COUNTRY_ID;
select a= sum(Value) where Sign like 'pos'
select b = sum(Value) where Signe like 'neg'
select total = a-b
this is abit sql-agnostic, since you didnt say which db you are using, but it should be easy to adapat it to any db out there.