I am new to sql server, transitioning from mysql.
I have a complicated case statement that I would like to group on 6 whens and an else. Likely to get larger. To be able to run it, I need to copy the statement into the group by each time there is a modification. In mySql I would just group by the column number. Is there any work around for this? Making the code very ugly.
Is there going to be a performance penalty in creating a sub query for my case, then just grouping on the result field. Seems like trying to make the code more elegant will cause the query to use more resources.
Thanks
Below is a field I am grouping on. As I make a modification to the field for more edge cases, then I need to change code in up to 3 places. Makes for some very ugly code, and I need no extra help doing that myself.
dz_code = case
when isnull(dz.dz_code,'N/A') in ('GAB', 'MAB', 'N/A') and dc.howdidyouhear = 'Television' then 'Television'
when isnull(dz.dz_code,'N/A') in ('GAB', 'MAB', 'N/A') and dc.howdidyouhear in ('Other', 'N/A') then 'Other'
WHEN dz.dz_code = 'irs,irs' THEN 'irs'
when dz.dz_code like '%SDE%' THEN 'SDE'
when dz.dz_code like 'referral,' then REPLACE(dz.dz_code, 'referral','')
when charindex(',',dz.dz_code) = 4 then left(dz.dz_code,3)
else
dz.dz_code
END,
Maybe you can wrap the query in a subquery and use the alias in the select and the group by. It looks a little bulky in this example, but if you've got more complex case switches, or more than one of them, then this solution will probably much smaller and more readable.
select
CaseField
from
(select
case when 1 = 2 then
3
else 4 end as CaseField
from
YourTable t) c
group by
CaseField
Related
I'm stuck in an (apparently) extremely trivial task that I can't make work , and I really feel no chance than to ask for advice.
I used to deal with PHP/MySQL more than 10 years ago and I might be quite rusty now that I'm dealing with an SQLite DB using Qt5.
Basically I'm selecting some records while wanting to make some math operations on the fetched columns. I recall (and re-read some documentation and examples) that the keyword "AS" is going to conveniently rename (alias) a value.
So for example I have this query, where "X" is an integer number that I render into this big Qt string before executing it with a QSqlQuery. This query lets me select all the electronic components used in a Project and calculate how many of them to order (rounding to the nearest multiple of 5) and the total price per component.
SELECT Inventory.id, UsedItems.pid, UsedItems.RefDes, Inventory.name, Inventory.category,
Inventory.type, Inventory.package, Inventory.value, Inventory.manufacturer,
Inventory.price, UsedItems.qty_used as used_qty,
UsedItems.qty_used*X AS To_Order,
ROUND((UsedItems.qty_used*X/5)+0.5)*5*CAST((X > 0) AS INT) AS Nearest5,
Inventory.price*Nearest5 AS TotPrice
FROM Inventory
LEFT JOIN UsedItems ON Inventory.id=UsedItems.cid
WHERE UsedItems.pid='1'
ORDER BY RefDes, value ASC
So, for example, I aliased UsedItems.qty_used as used_qty. At first I tried to use it in the next field, multiplying it by X, writing "used_qty*X AS To_Order" ... Query failed. Well, no worries, I had just put the original tab.field name and it worked.
Going further, I have a complex calculation and I want to use its result on the next field, but the same issue popped out: if I alias "ROUND(...)" AS Nearest5, and then try to use this value by multiplying it in the next field, the query will fail.
Please note: the query WORKS, but ONLY if I don't use aliases in the following fields, namely if I don't use the alias Nearest5 in the TotPrice field. I just want to avoid re-writing the whole ROUND(...) thing for the TotPrice field.
What am I missing/doing wrong? Either SQLite does not support aliases on the same query or I am using a wrong syntax and I am just too stuck/confused to see the mistake (which I'm sure it has to be really stupid).
Column aliases defined in a SELECT cannot be used:
For other expressions in the same SELECT.
For filtering in the WHERE.
For conditions in the FROM clause.
Many databases also restrict their use in GROUP BY and HAVING.
All databases support them in ORDER BY.
This is how SQL works. The issue is two things:
The logic order of processing clauses in the query (i.e. how they are compiled). This affects the scoping of parameters.
The order of processing expressions in the SELECT. This is indeterminate. There is no requirement for the ordering of parameters.
For a simple example, what should x refer to in this example?
select x as a, y as x
from t
where x = 2;
By not allowing duplicates, SQL engines do not have to make a choice. The value is always t.x.
You can try with nested queries.
A SELECT query can be nested in another SELECT query within the FROM clause;
multiple queries can be nested, for example by following the following pattern:
SELECT *,[your last Expression] AS LastExp From (SELECT *,[your Middle Expression] AS MidExp FROM (SELECT *,[your first Expression] AS FirstExp FROM yourTables));
Obviously, respecting the order that the expressions of the innermost select query can be used by subsequent select queries:
the first expressions can be used by all other queries, but the other intermediate expressions can only be used by queries that are further upstream.
For your case, your query may be:
SELECT *, PRC*Nearest5 AS TotPrice FROM (SELECT *, ROUND((UsedItems.qty_used*X/5)+0.5)*5*CAST((X > 0) AS INT) AS Nearest5 FROM (SELECT Inventory.id, UsedItems.pid, UsedItems.RefDes, Inventory.name, Inventory.category, Inventory.type, Inventory.package, Inventory.value, Inventory.manufacturer, Inventory.price AS PRC, UsedItems.qty_used*X AS To_Order FROM Inventory LEFT JOIN UsedItems ON Inventory.id=UsedItems.cid WHERE UsedItems.pid='1' ORDER BY RefDes, value ASC))
I have a simple select that is running very slow and have narrowed it down to one particular where statement.
I am not sure if you need to see the whole query, or maybe will be able to help me understand why the case is affecting performance so much. I feel like I found the problem, but can't seem to resolve it. I've worked with case statement before, and have never ran into such huge performance issues.
For this particular example. the declaration is as follows: Declare #lastInvOnly as int = 0
the problem where statement follow and runs for about 20 seconds:
AND ird.inventorydate = CASE WHEN #lastinvonly=0 THEN
-- get the last reported inventory in respect to the specified parameter
(SELECT MAX(ird2.inventorydate)
FROM irdate ird2
WHERE ird2.ris =r.ris AND
ird2.generateddata!='g' AND
ird2.inventorydate <= #inventorydate)
END
Removing the case makes it run in 1 second which is a HUGE difference. I can't understand why.
AND ird.inventorydate =
(SELECT MAX(ird2.inventorydate)
FROM irdate ird2
WHERE ird2.ris = r.ris AND
ird2.generateddata! = 'g' AND
ird2.inventorydate <= #inventorydate)
It should almost certainly be a derived table and you should join to it instead. Sub-selects tend to have poor performance and when used conditionally, even worse. Try this instead:
INNER JOIN (
select
ris
,max(inventorydate) AS [MaxInvDate]
from irdate
where s and generateddata!='g'
and inventorydate <= #inventorydate
GROUP BY ris
) AS MaxInvDate ON MaxInvDate.ris=r.ris
and ird.inventorydate=MaxInvDate.MaxInvDate
and #lastinvonly=0
I'm not 100% positive this logically works with the whole query as your question only provides a small part.
I can't tell for sure without seeing an execution plan but the branch in your filter is likely the cause of the performance problems. Theoretically, the optimizer can take the version without the case and apply an optimization that transforms the subquery in your filter into a join; when the case statement is added this optimization is no longer possible and the subquery is executed for every row. One can refactor the code to help the optimizer out, something like this should work:
outer apply (
select max(ird2.inventorydate) as maxinventorydate
from irdate ird2
where ird2.ris = r.ris
and ird2.generateddata <> 'g'
and ird2.inventorydate <= #inventorydate
and #lastinvonly = 0
) as ird2
where ird.inventorydate = ird2.maxinventorydate
May be this is silly but can we write a case inside another case's WHEN?
Below code working for me but I am not sure if this is correct.
SELECT
(SUM(CASE
WHEN (
CASE
WHEN r.status < b.status
THEN r.status
ELSE b.status
END
) = '4'
THEN 1
ELSE 0
END)
) AS WORKED
FROM
tbl1 r, tbl2 b
All the examples on nested cases are like CASE inside a THEN so I am not sure if this a good practice. Is there a better way to get the same results?
Yes you can. MSDN also informs us that in SQL SERVER, you can only have a maximum of 10 CASE expressions embedded into each other. Oddly enough, a search for ORACLE up negative about this potential limitation. Probably important to note.
Of course, you can also just use more WHEN (up to 255 in ORACLE) statements, too, but that only works if you do not need to nest your logic (such as comparing two different columns values)
Sources:
https://msdn.microsoft.com/en-us/library/ms181765.aspx
http://www.techonthenet.com/oracle/functions/case.php
I am on a work term from school. I am not very comfortable using SQL, I am trying to get a hold of it....
My supervisor gave me a task for a user in which I need to take row data and make columns. We used the Crosstab Wizard and automagically created the SQL to get what we needed.
Basically, we have a table like this:
ReqNumber Year FilledFlag(is a checkbox) FilledBy
1 2012 (notchecked) ITSchoolBoy
1 2012 (checked) GradStudent
1 2012 (notchecked) HighSchooler
2 etc, etc.
What the user would like is to have a listing of all of the req numbers and what is checked
Our automatic pivot code gives us all of the FilledBy options (there are 9 in total) as column headings, and groups it all by reqnumber.
How can you do this without the pivot? I would like to wrap my head around this. Nearest I can find is something like:
SELECT
SUM(IIF(FilledBy = 'ITSchoolboy',1,0) as ITSchoolboy,
SUM(IIF(FilledBy = 'GradStudent',1,0) as GradStudent, etc.
FROM myTable
Could anyone help explain this to me? Point me in the direction of a guide? I've been searching for the better part of a day now, and even though I am a student, I don't think this will be smiled upon for too long. But I would really like to know!
I think your boss' suggestion could work if you GROUP BY ReqNumber.
SELECT
ReqNumber,
SUM(IIF(FilledBy = 'ITSchoolboy',1,0) as ITSchoolboy,
SUM(IIF(FilledBy = 'GradStudent',1,0) as GradStudent,
etc.
FROM myTable
GROUP BY ReqNumber;
A different approach would be to JOIN multiple subqueries. This example pulls in 2 of your categories. If you need to extend it to 9 categories, you would have a whole lot of joining going on.
SELECT
itsb.ReqNumber,
itsb.ITSchoolboy,
grad.GradStudent
FROM
(
SELECT
ReqNumber,
FilledFlag AS ITSchoolboy
FROM myTable
WHERE FilledBy = "ITSchoolboy"
) AS itsb
INNER JOIN
(
SELECT
ReqNumber,
FilledFlag AS GradStudent
FROM myTable
WHERE FilledBy = "GradStudent"
) AS grad
ON itsb.ReqNumber = grad.ReqNumber
Please notice I'm not suggesting you should use this approach. However, since you asked about alternatives to your pivot approach (which works) ... this is one. Stay tuned in case someone else offers a simpler alternative. :-)
This could be a simple question, but it's one I've never seen answered before. Is there a way to use an if statement's condition as its value? This would be really useful in cases where lots of calculation is done to determine if a certain condition is met and if it is, that calculation is the result.
As an example:
if ( [intense calculation] > 0, [same intense calculation], 0)
I'm interested particularly with regards to SQL, as I'm working on a report in Access right now and so can't store the result of the intense calculation in a variable.
Not sure if such a concept exists in the MS Access report world, but how about:
MAX([intense calculation], 0)
The obvious benefit of such an approach is that the calculation would only need to be done once.
One approach in most forms of SQL would be to move the main query to a sub-query, with the intense calculation column aliased and tested in the new outer query - like so:
select v.*,
case intense_calc > 0 then intense_calc else 0 end as positive_calc
from (select [intense calculation] as intense_calc,
[other columns]
from ...) as v