I am confused with the order of execution of this query, please explain me this.
I am confused with when the join is applied, function is called, a new column is added with the Case and when the serial number is added. Please explain the order of execution of all this.
select Row_number() OVER(ORDER BY (SELECT 1)) AS 'Serial Number',
EP.FirstName,Ep.LastName,[dbo].[GetBookingRoleName](ES.UserId,EP.BookingRole) as RoleName,
(select top 1 convert(varchar(10),eventDate,103)from [3rdi_EventDates] where EventId=13) as EventDate,
(CASE [dbo].[GetBookingRoleName](ES.UserId,EP.BookingRole)
WHEN '90 Day Client' THEN 'DC'
WHEN 'Association Client' THEN 'DC'
WHEN 'Autism Whisperer' THEN 'DC'
WHEN 'CampII' THEN 'AD'
WHEN 'Captain' THEN 'AD'
WHEN 'Chiropractic Assistant' THEN 'AD'
WHEN 'Coaches' THEN 'AD'
END) as Category from [3rdi_EventParticipants] as EP
inner join [3rdi_EventSignup] as ES on EP.SignUpId = ES.SignUpId
where EP.EventId = 13
and userid in (
select distinct userid from userroles
--where roleid not in(6,7,61,64) and roleid not in(1,2))
where roleid not in(19, 20, 21, 22) and roleid not in(1,2))
This is the function which is called from the above query.
CREATE function [dbo].[GetBookingRoleName]
(
#UserId as integer,
#BookingId as integer
)
RETURNS varchar(20)
as
begin
declare #RoleName varchar(20)
if #BookingId = -1
Select Top 1 #RoleName=R.RoleName From UserRoles UR inner join Roles R on UR.RoleId=R.RoleId Where UR.UserId=#UserId and R.RoleId not in(1,2)
else
Select #RoleName= RoleName From Roles where RoleId = #BookingId
return #RoleName
end
Queries are generally processed in the follow order (SQL Server). I have no idea if other RDBMS's do it this way.
FROM [MyTable]
ON [MyCondition]
JOIN [MyJoinedTable]
WHERE [...]
GROUP BY [...]
HAVING [...]
SELECT [...]
ORDER BY [...]
SQL is a declarative language. The result of a query must be what you would get if you evaluated as follows (from Microsoft):
Logical Processing Order of the SELECT statement
The following steps show the logical
processing order, or binding order,
for a SELECT statement. This order
determines when the objects defined in
one step are made available to the
clauses in subsequent steps. For
example, if the query processor can
bind to (access) the tables or views
defined in the FROM clause, these
objects and their columns are made
available to all subsequent steps.
Conversely, because the SELECT clause
is step 8, any column aliases or
derived columns defined in that clause
cannot be referenced by preceding
clauses. However, they can be
referenced by subsequent clauses such
as the ORDER BY clause. Note that the
actual physical execution of the
statement is determined by the query
processor and the order may vary from
this list.
FROM
ON
JOIN
WHERE
GROUP BY
WITH CUBE or WITH ROLLUP
HAVING
SELECT
DISTINCT
ORDER BY
TOP
The optimizer is free to choose any order it feels appropriate to produce the best execution time. Given any SQL query, is basically impossible to anybody to pretend it knows the execution order. If you add detailed information about the schema involved (exact tables and indexes definition) and the estimated cardinalities (size of data and selectivity of keys) then one can take a guess at the probable execution order.
Ultimately, the only correct 'order' is the one described ion the actual execution plan. See Displaying Execution Plans by Using SQL Server Profiler Event Classes and Displaying Graphical Execution Plans (SQL Server Management Studio).
A completely different thing though is how do queries, subqueries and expressions project themselves into 'validity'. For instance if you have an aliased expression in the SELECT projection list, can you use the alias in the WHERE clause? Like this:
SELECT a+b as c
FROM t
WHERE c=...;
Is the use of c alias valid in the where clause? The answer is NO. Queries form a syntax tree, and a lower branch of the tree cannot be reference something defined higher in the tree. This is not necessarily an order of 'execution', is more of a syntax parsing issue. It is equivalent to writing this code in C#:
void Select (int a, int b)
{
if (c = ...) then {...}
int c = a+b;
}
Just as in C# this code won't compile because the variable c is used before is defined, the SELECT above won't compile properly because the alias c is referenced lower in the tree than is actually defined.
Unfortunately, unlike the well known rules of C/C# language parsing, the SQL rules of how the query tree is built are somehow esoteric. There is a brief mention of them in Single SQL Statement Processing but a detailed discussion of how they are created, and what order is valid and what not, I don't know of any source. I'm not saying there aren't good sources, I'm sure some of the good SQL books out there cover this topic.
Note that the syntax tree order does not match the visual order of the SQL text. For example the ORDER BY clause is usually the last in the SQL text, but as a syntax tree it sits above everything else (it sorts the output of the SELECT, so it sits above the SELECTed columns so to speak) and as such is is valid to reference the c alias:
SELECT a+b as c
FROM t
ORDER BY c;
SQL query is not imperative but declarative, so you have no idea which the statement is executed first, but since SQL is evaluated by SQL query engines, most of the SQL engines follows similar process to obtain the results. You may have to understand how the query engine works internally to understand some SQL execution behavior.
Julia Evens has a great post explaining this, it is worth to check it out:
https://jvns.ca/blog/2019/10/03/sql-queries-don-t-start-with-select/
SQL is a declarative language, meaning that it tells the SQL engine what to do, not how. This is in contrast to an imperative language such as C, in which how to do something is clearly laid out.
This means that not all statements will execute as expected. Of particular note are boolean expressions, which may not evaluate from left-to-right as written. For example, the following code is not guaranteed to execute without a divide by zero error:
SELECT 'null' WHERE 1 = 1 OR 1 / 0 = 0
The reason for this is the query optimizer chooses the best (most efficient) way to execute a statement. This means that, for example, a value may be loaded and filtered before a transforming predicate is applied, causing an error. See the second link above for an example
See: here and here.
"Order of execution" is probably a bad mental model for SQL queries. Its hard to actually write a single query that would actually depend on order of execution (this is a good thing). Instead you should think of all join and where clauses happening simultaneously (almost like a template)
That said you could run display the Execution Plans which should give you insight into it.
However since its's not clear why you want to know the order of execution, I'm guessing your trying to get a mental model for this query so you can fix it in some way. This is how I would "translate" your query, although I've done well with this kind of analysis there's some grey area with how precise it is.
FROM AND WHERE CLAUSE
Give me all the Event Participants rows. from [3rdi_EventParticipants
Also give me all the Event Signup rows that match the Event Participants rows on SignUpID inner join 3rdi_EventSignup] as ES on EP.SignUpId = ES.SignUpId
But Only for Event 13 EP.EventId = 13
And only if the user id has a record in the user roles table where the role id is not in 1,2,19,20,21,22
userid in (
select distinct userid from userroles
--where roleid not in(6,7,61,64) and roleid not in(1,2))
where roleid not in(19, 20, 21, 22) and roleid not in(1,2))
SELECT CLAUSE
For each of the rows give me a unique ID
Row_number() OVER(ORDER BY (SELECT 1)) AS 'Serial Number',
The participants First Name EP.FirstName
The participants Last Name Ep.LastName
The Booking Role name GetBookingRoleName
Go look in the Event Dates and find out what the first eventDate where the EventId = 13 that you find
(select top 1 convert(varchar(10),eventDate,103)from [3rdi_EventDates] where EventId=13) as EventDate
Finally translate the GetBookingRoleName in Category. I don't have a table for this so I'll map it manually (CASE [dbo].[GetBookingRoleName](ES.UserId,EP.BookingRole)
WHEN '90 Day Client' THEN 'DC'
WHEN 'Association Client' THEN 'DC'
WHEN 'Autism Whisperer' THEN 'DC'
WHEN 'CampII' THEN 'AD'
WHEN 'Captain' THEN 'AD'
WHEN 'Chiropractic Assistant' THEN 'AD'
WHEN 'Coaches' THEN 'AD'
END) as Category
So a couple of notes here. You're not ordering by anything when you select TOP. You should probably have na order by there. You could also just as easily put that in your from clause e.g.
from [3rdi_EventParticipants] as EP
inner join [3rdi_EventSignup] as ES on EP.SignUpId = ES.SignUpId,
(select top 1 convert(varchar(10),eventDate,103)
from [3rdi_EventDates] where EventId=13
Order by eventDate) dates
There is a logical order to evaluation of the query text, but the database engine can choose what order execute the query components based upon what is most optimal. The logical text parsing ordering is listed below. That is, for example, why you can't use an alias from SELECT clause in a WHERE clause. As far as the query parsing process is concerned, the alias doesn't exist yet.
FROM
ON
OUTER
WHERE
GROUP BY
CUBE | ROLLUP
HAVING
SELECT
DISTINCT
ORDER BY
TOP
See the Microsoft documentation (see "Logical Processing Order of the SELECT statement") for more information on this.
Simplified order for T-SQL -> SELECT statement:
1) FROM
2) Cartesian product
3) ON
4) Outer rows
5) WHERE
6) GROUP BY
7) HAVING
8) SELECT
9) Evaluation phase in SELECT
10) DISTINCT
11) ORDER BY
12) TOP
as I had done so far - same order was applicable in SQLite.
Source => SELECT (Transact-SQL)
... of course there are (rare) exceptions.
Related
declare #StartDate date = '08/01/2021',
#EndDate Date = '08/04/2021';
with cte_FacilityReportingDates as
(
select distinct Facility, REPORTING_DATE
from table1 a
where REPORTING_DATE between #StartDate and #EndDate
),
cte_facility as
(
select distinct Facility
from table1 a
),
cte_ReportingDates as
(
select distinct a.REPORTING_DATE
from table1 a
where a.REPORTING_DATE between #StartDate and #EndDate
),
cte_Combine as
(
select *
from cte_facility f
cross join cte_ReportingDates d
)
select t1.FACILITY, t1.REPORTING_DATE from cte_Combine t1 where not exists (select 1 from cte_FacilityReportingDates t2 where t1.FACILITY = t2.FACILITY and t2.REPORTING_DATE between StartDate and EndDate and t2.FACILITY is null group by t1.facility, t1.REPORTING_DATE)
I've got it down to the last 50 of the race (Hat Tip to the Olympics) but can't get over the finish line. I know it is simply something I've overlooked but I'm racking my brain! I need to show the facilities and dates that are NOT in the result from cte_ReportingDates.
With proper formatting, you will encourage others to help. You removed the efforts that someone else made in formatting your code when you edited it. That was quite discouraging honestly.
When formatted properly, you can clearly see where each CTE is defined and better understand what each does. Seems you overdid your use of DISTINCT - don't just throw it into code in hopes it "fixes" something. The first cte (cte_FacilityReportingDates) does not really need DISTINCT if used to test for existence. TBH that particular CTE it is a bit overkill since the logic can easily be incorporated within the EXISTS clause below - but that is a style choice.
<with ... all your CTEs from original query ...>
select comb.FACILITY, comb.REPORTING_DATE
from cte_Combine comb
where not exists (select * from cte_FacilityReportingDates as trn
where comb.FACILITY = trn.FACILITY
and comb.REPORTING_DATE = trn.REPORTING_DATE)
order by ...;
There is no reason to apply a GROUP BY clause to the final query since it is nothing by a unique set of <FACILITY, REPORT_DATE>. Any time you use/see such a clause with no aggregates, that should be a concern that the writer has lost the path.
Also notice the ORDER BY clause. If the order of rows matters, then the query that generates the resultset must have one. Usually it does matter.
I also used better table aliases. Cryptic ones are not not helpful to the reader; develop good habits. I have no idea what the CTE named cte_FacilityReportingDates (which selects from "table1" - another crap name with equally crap alias "a") so I just made up something.
The last issue I'll highlight is the rather important assumption you made. Your logic assumes that every facility exists within table1. That is not usually a safe assumption for some sort of "activity" table (which is my guess as to what that table represents). The same applies to dates. For dates you can generate the set of all dates between two boundaries easily - I'll leave that adjustment to you if needed. You cannot do with for facility - you might (likely do or should) need another table for that.
I need some help in writing a SQL Server stored procedure. All data group by Train_B_N.
my table data
Expected result :
expecting output
with CTE as
(
select Train_B_N, Duration,Date,Trainer,Train_code,Training_Program
from Train_M
group by Train_B_N
)
select
*
from Train_M as m
join CTE as c on c.Train_B_N = m.Train_B_N
whats wrong with my query?
The GROUP BY smashes the table together, so having columns that are not GROUPED combine would cause problems with the data.
select Train_B_N, Duration,Date,Trainer,Train_code,Training_Program
from Train_M
group by Train_B_N
By ANSI standard, the GROUP BY must include all columns that are in the SELECT statement which are not in an aggregate function. No exceptions.
WITH CTE AS (SELECT TRAIN_B_N, MAX(DATE) AS Last_Date
FROM TRAIN_M
GROUP BY TRAIN_B_N)
SELECT A.Train_B_N, Duration, Date,Trainer,Train_code,Training_Program
FROM TRAIN_M AS A
INNER JOIN CTE ON CTE.Train_B_N = A.Train_B_N
AND CTE.Last_Date = A.Date
This example would return the last training program, trainer, train_code used by that ID.
This is accomplished from MAX(DATE) aggregate function, which kept the greatest (latest) DATE in the table. And since the GROUP BY smashed the rows to their distinct groupings, the JOIN only returns a subset of the table's results.
Keep in mind that SQL will return #table_rows X #Matching_rows, and if your #Matching_rows cardinality is greater than one, you will get extra rows.
Look up GROUP BY - MSDN. I suggest you read everything outside the syntax examples initially and obsorb what the purpose of the clause is.
Also, next time, try googling your problem like this: 'GROUP BY, SQL' or insert the error code given by your IDE (SSMS or otherwise). You need to understand why things work...and SO is here to help, not be your google search engine. ;)
Hope you find this begins your interest in learning all about SQL. :D
I have a custom CLR aggregate function. This function concats strings within a group. Now the question is, can I make this function process the data in some specific order or will it always be some random order the DB found suitable? I understand that for most mathematical aggregate functions (MIN, MAX, AVG etc.) it makes no difference in which order the data is processed, but let's say I want to concat strings alphabetically within a group is there something I can do to achieve this result?
Note that it has to be an aggregate function (don't get mislead by the examples below) and that altering the existing CLR function is out of question (all it does is a basic string concat and nothing more).
I tested adding ORDER BY to the SELECT that contains the GROUP BY, but it produced no meaningful results.
SELECT
user.Id, dbo.concat(cat.Name)
FROM
Users user
JOIN Cats cat ON (cat.Owner = user.Id)
GROUP BY user.Id
ORDER BY user.Id, MAX(cat.Name) --kind of meaningless really
I also tried to ORDER BY the table that contains the data which I want to concat before doing a JOIN, but the result was the same.
SELECT
user.Id, dbo.concat(cat.Name)
FROM
Users user
JOIN (SELECT TOP 100 PERCENT /*hack*/ c.* FROM Cats c ORDER BY c.Name) cat ON (cat.Owner = user.Id)
GROUP BY user.Id
Ordering data in a subquery and then doing a GROUP BY didn't work either.
SELECT
t1.Id, dbo.concat(t1.Name)
FROM
(
SELECT TOP 100 PERCENT /*hack*/
user.Id, cat.Name
FROM
Users user
JOIN Cats cat ON (cat.Owner = user.Id)
ORDER BY user.Id, cat.Name
) t1
GROUP BY t1.Id
I was kind of expecting that neither of those will work, but at least now no one can say I haven't tried anything.
P.S. Yes, I have reasons not to use FOR XML PATH. If what I'm asking here cannot be done, I'll live with it.
Based on information from Damien_The_Unbeliever, Vladimir Baranov, Microsoft pages and from few other users (see comments to the question), I can deduce that:
Ordering rows for aggregate function cannot be done directly in the database; However there are hints that this is\might have been a planned feature (see here and here); If MS ever implements this, some existing CLR aggregate functions might start acting weird (as by default those aggregate functions are flagged to be dependent on order)
Ordering has to be implemented directly in the CLR function; It can be a little tricky due to how CLR aggregate functions are being run, but it can be done
Unfortunately I don't have a piece of code to present here, as I didn't had time to alter my CLR function (and doing unordered concat was good enough in my case).
You can include a function in the order by clause.
try it with this dummy date returner:
create function testDate ()
returns datetime
as
begin
declare #returnDate datetime
select #returnDate = CURRENT_TIMESTAMP
return #returnDate
end
run the function with any table (replace SomeTable with a real table) and order by it:
select dbo.testDate (),
*
from SomeTable
order by dbo.testDate () desc
#jahu
EDIT: I thought you wanted to order by a user defined function. Perhaps I am misunderstanding the question. You can order a query by an aggregate function like this:
select CustomerID,
avg(OrderID)
from Orders
group by CustomerID
order by avg(OrderID) desc
The table above has OrderID as a unique column and there can be multiple CustomerID records
I have been fighting with this all weekend and am out of ideas. In order to have pages in my search results on my website, I need to return a subset of rows from a SQL Server 2005 Express database (i.e. start at row 20 and give me the next 20 records). In MySQL you would use the "LIMIT" keyword to choose which row to start at and how many rows to return.
In SQL Server I found ROW_NUMBER()/OVER, but when I try to use it it says "Over not supported". I am thinking this is because I am using SQL Server 2005 Express (free version). Can anyone verify if this is true or if there is some other reason an OVER clause would not be supported?
Then I found the old school version similar to:
SELECT TOP X * FROM TABLE WHERE ID NOT IN (SELECT TOP Y ID FROM TABLE ORDER BY ID) ORDER BY ID where X=number per page and Y=which record to start on.
However, my queries are a lot more complex with many outer joins and sometimes ordering by something other than what is in the main table. For example, if someone chooses to order by how many videos a user has posted, the query might need to look like this:
SELECT TOP 50 iUserID, iVideoCount FROM MyTable LEFT OUTER JOIN (SELECT count(iVideoID) AS iVideoCount, iUserID FROM VideoTable GROUP BY iUserID) as TempVidTable ON MyTable.iUserID = TempVidTable.iUserID WHERE iUserID NOT IN (SELECT TOP 100 iUserID, iVideoCount FROM MyTable LEFT OUTER JOIN (SELECT count(iVideoID) AS iVideoCount, iUserID FROM VideoTable GROUP BY iUserID) as TempVidTable ON MyTable.iUserID = TempVidTable.iUserID ORDER BY iVideoCount) ORDER BY iVideoCount
The issue is in the subquery SELECT line: TOP 100 iUserID, iVideoCount
To use the "NOT IN" clause it seems I can only have 1 column in the subquery ("SELECT TOP 100 iUserID FROM ..."). But when I don't include iVideoCount in that subquery SELECT statement then the ORDER BY iVideoCount in the subquery doesn't order correctly so my subquery is ordered differently than my parent query, making this whole thing useless. There are about 5 more tables linked in with outer joins that can play a part in the ordering.
I am at a loss! The two above methods are the only two ways I can find to get SQL Server to return a subset of rows. I am about ready to return the whole result and loop through each record in PHP but only display the ones I want. That is such an inefficient way to things it is really my last resort.
Any ideas on how I can make SQL Server mimic MySQL's LIMIT clause in the above scenario?
Unfortunately, although SQL Server 2005 Row_Number() can be used for paging and with SQL Server 2012 data paging support is enhanced with Order By Offset and Fetch Next, in case you can not use any of these solutions you require to first
create a temp table with identity column.
then insert data into temp table with ORDER BY clause
Use the temp table Identity column value just like the ROW_NUMBER() value
I hope it helps,
I have a SQL query that uses both standard WHERE clauses and full text index CONTAINS clauses. The query is built dynamically from code and includes a variable number of WHERE and CONTAINS clauses.
In order for the query to be fast, it is very important that the full text index be searched before the rest of the criteria are applied.
However, SQL Server chooses to process the WHERE clauses before the CONTAINS clauses and that causes tables scans and the query is very slow.
I'm able to rewrite this using two queries and a temporary table. When I do so, the query executes 10 times faster. But I don't want to do that in the code that creates the query because it is too complex.
Is there an a way to force SQL Server to process the CONTAINS before anything else? I can't force a plan (USE PLAN) because the query is built dynamically and varies a lot.
Note: I have the same problem on SQL Server 2005 and SQL Server 2008.
You can signal your intent to the optimiser like this
SELECT
*
FROM
(
SELECT *
FROM
WHERE
CONTAINS
) T1
WHERE
(normal conditions)
However, SQL is declarative: you say what you want, not how to do it. So the optimiser may decide to ignore the nesting above.
You can force the derived table with CONTAINS to be materialised before the classic WHERE clause is applied. I won't guarantee performance.
SELECT
*
FROM
(
SELECT TOP 2000000000
*
FROM
....
WHERE
CONTAINS
ORDER BY
SomeID
) T1
WHERE
(normal conditions)
Try doing it with 2 queries without temp tables:
SELECT *
FROM table
WHERE id IN (
SELECT id
FROM table
WHERE contains_criterias
)
AND further_where_classes
As I noted above, this is NOT as clean a way to "materialize" the derived table as the TOP clause that #gbn proposed, but a loop join hint forces an order of evaluation, and has worked for me in the past (admittedly usually with two different tables involved). There are a couple of problems though:
The query is ugly
you still don't get any guarantees that the other WHERE parameters don't get evaluated until after the join (I'll be interested to see what you get)
Here it is though, given that you asked:
SELECT OriginalTable.XXX
FROM (
SELECT XXX
FROM OriginalTable
WHERE
CONTAINS XXX
) AS ContainsCheck
INNER LOOP JOIN OriginalTable
ON ContainsCheck.PrimaryKeyColumns = OriginalTable.PrimaryKeyColumns
AND OriginalTable.OtherWhereConditions = OtherValues