Does SQL Server 2012 Lazy Join - sql

I am working on an application that allows users to dynamically choose what they see. The list of columns and tables those belong to is huge. I'm working on building a dynamic query but I don't want to have to store all the join logic if I don't have too.
An example is I have a query that has LEFT JOINS for 10 tables but I only select and filter by one table. Is SQL Server 2012 Smart enough to not join on the other tables that are not needed?
I know Oracle's DBMS works like this but my company uses SQL Server 2012 so I'm stuck with that.
Example Query where the last 4 LEFT JOINS don't need to be performed:
SELECT [IncidentIncidentNumberModValue].[IncidentNumber]
,IncidentNumberModifierNotValue.[Value] AS IncidentNumberNotValue
,[Incident].[IncidentDate]
,[Incident].[ResponseNumber]
,[Incident].[PatientCareReportNumber]
,[IncidentType].[Value] AS IncidentType
,[SceneStreetAddressModValue].[StreetAddress2] AS IncidentAddress
,IncidentStreetAddressNotValue.[Value] AS IncidentAddressNotValue
,COUNT(*) OVER() AS TotalRecordCount
FROM [EmsEvent].[Incident]
LEFT JOIN [Resource].[IncidentType] ON [EmsEvent].[Incident].[IncidentType] = [Resource].[IncidentType].[IncidentTypeID]
LEFT JOIN [EmsEvent].[IncidentIncidentNumberModValue] ON [EmsEvent].[Incident].[IncidentID] = [EmsEvent].[IncidentIncidentNumberModValue].[IncidentID]
LEFT JOIN [GlobalResource].[IncidentNumberModifier] ON [EmsEvent].[IncidentIncidentNumberModValue].[NotValue] = [GlobalResource].[IncidentNumberModifier].[DataElementID]
LEFT JOIN [GlobalResource].[NotValue] IncidentNumberModifierNotValue ON [GlobalResource].[IncidentNumberModifier].[NotValueID] = IncidentNumberModifierNotValue.[NotValueID]
LEFT JOIN [EmsEvent].[Scene] ON [EmsEvent].[Scene].[IncidentID] = [EmsEvent].[Incident].[IncidentID]
LEFT JOIN [EmsEvent].[SceneStreetAddressModValue] ON [EmsEvent].[Scene].[SceneID] = [EmsEvent].[SceneStreetAddressModValue].[SceneID]
LEFT JOIN [GlobalResource].[IncidentStreetAddressModifier] ON [EmsEvent].[SceneStreetAddressModValue].[NotValue] = [GlobalResource].[IncidentStreetAddressModifier].[DataElementID]
LEFT JOIN [GlobalResource].[NotValue] IncidentStreetAddressNotValue ON [GlobalResource].[IncidentStreetAddressModifier].[NotValueID] = IncidentStreetAddressNotValue.[NotValueID]
LEFT JOIN [EmsEvent].[CustomConfig] on [EmsEvent].[Incident].[IncidentID] = [EmsEvent].[CustomConfig].[IncidentID]
LEFT JOIN [EmsEvent].[IncidentDisasterType] on [EmsEvent].[Incident].[IncidentID] = [EmsEvent].[IncidentDisasterType].[IncidentID]
LEFT JOIN [EmsEvent].[Response] on [EmsEvent].[Scene].[SceneID] = [EmsEvent].[Response].[SceneID]
LEFT JOIN [EmsEvent].[CrewMember] on [EmsEvent].[Response].[ResponseID] = [EmsEvent].[CrewMember].[ResponseInformationID]
WHERE [EmsEvent].[Incident].[DeletedStatus] = 0
AND [EmsEvent].[Incident].AgencyID = '607CEE05-276D-49A1-B6BF-FD606EFC2377'
ORDER BY [EmsEvent].[IncidentIncidentNumberModValue].[IncidentNumber] ASC
OFFSET 0 ROWS
FETCH NEXT 25 ROWS ONLY
OPTION(recompile)

It depends on what column in the left outer joined table you are using in the join.
If the column is a primary key or has a unique constraint the table will be excluded from the query plan if it is not referenced elsewhere in the query.
It does not look like your last four tables are joined on their respective primary key.
With tables like this:
create table Parent
(
ParentID int primary key
)
create table Child
(
ChildID int primary key,
ParentID int references Parent(ParentID)
)
This query will not use the table Parent.
select C.*
from Child as C
left outer join Parent as P
on P.ParentID = C.ParentID
But this query have to use the left outer joined Child table because it might add rows to the result if there are more than one hit on the foreign key.
select P.*
from Parent as P
left outer join Child as C
on P.ParentID = C.ParentID

Related

Is it possible to have multiple joins between two tables in stored procedure?

I have two tables, "Booking" and "City". CityName field is primary key in City table and I have used it as foreign key for two columns "SourceCity" and "DestinationCity" in Booking table. I want to create a stored procedure to select all existing data from the Booking table for creating a view list, for which I have written the following.
SELECT [dbo].[Booking].[BookingID],
[dbo].[Booking].[CustomerName],
[dbo].[City].[CityName],
[dbo].[City].[CityName],
[dbo].[Booking].[StartingDate],
[dbo].[Booking].[EndingDate],
[dbo].[Car].[LicensePlateNumber],
[dbo].[Driver].[DriverName],
[dbo].[Booking].[AdvanceTaken],
[dbo].[Booking].[PendingPayment],
[dbo].[Booking].[TotalRent],
[dbo].[Booking].[BookingDate],
[dbo].[Booking].[IDProof]
FROM [dbo].[Booking]
**LEFT OUTER JOIN [dbo].[City]
ON [dbo].[Booking].[SourceCity] = [dbo].[City].[CityName]
AND [dbo].[Booking].[DestinationCity] = [dbo].[City].[CityName]**
LEFT OUTER JOIN [dbo].[Driver]
ON [dbo].[Driver].[DriverID] = [dbo].[Booking].[DriverAllotted]
LEFT OUTER JOIN [dbo].[Car]
ON [dbo].[Car].[CarID] = [dbo].[Booking].[CarAllotted]
ORDER BY [dbo].[Booking].[BookingID]
I am not sure if it is possible to do the following
LEFT OUTER JOIN [dbo].[City]
ON [dbo].[Booking].[SourceCity] = [dbo].[City].[CityName]
AND [dbo].[Booking].[DestinationCity] = [dbo].[City].[CityName]
I guess you need a different JOIN
FROM [dbo].[Booking] as booking
LEFT OUTER JOIN [dbo].[City] as source_city
ON booking.[SourceCity] = source_city.[CityName]
LEFT OUTER JOIN [dbo].[City] as destination_city
ON booking.[DestinationCity] = destination_city.[CityName]
....
Yes it is possible, you just need to use a different table alias. Beyond referencing the same table twice, table aliases can make your code look a lot cleaner, e.g.
SELECT b.CustomerName,
sc.CityName AS SourceCity,
dc.CityName AS DestinationCity,
b.StartingDate,
b.EndingDate,
c.LicensePlateNumber,
d.DriverName,
b.AdvanceTaken,
b.PendingPayment,
b.TotalRent,
b.BookingDate,
b.IDProof
FROM dbo.Booking AS b
LEFT OUTER JOIN dbo.City AS sc
ON sc.CityName= b.SourceCity
LEFT OUTER JOIN dbo.City AS dc -- Different Alias here
ON dc.CityName = b.DestinationCity
LEFT OUTER JOIN dbo.Driver AS d
ON d.DriverID = b.DriverAllotted
LEFT OUTER JOIN dbo.Car AS c
ON c.CarID = b.CarAllotted
ORDER BY
b.BookingID;
I appreciate that cleaner is somewhat subjective, but I would be astonished if anyone found this harder to read than your original query

joining to a table that might not exist

So I have joined a table to itself in order to find the most recently added note of a certain type (;NoteTypesID=20625;). BUT when there is NOTHING of that type, it completely excludes the entire record's results from my dataset, instead of giving me nulls for the fields I am trying to pull from the ;PROPOSALNOTEPAD; table.
---select ... from ... (including the prop table)
left join PROPOSALNOTEPAD PN_SolicitationStrategy on prop.ID = PN_SolicitationStrategy.ParentId
join ( select parentID, max(DateAdded) as MaxDateAdded FROM PROPOSAL_NOTEPAD where NoteTypeID = 20625 group by ParentId) as PN_max_SolicitationStrategy
ON PN_SolicitationStrategy.ParentId = PN_max_SolicitationStrategy.ParentId AND PN_SolicitationStrategy.DateAdded = PN_max_SolicitationStrategy.MaxDateAdded
I need to be able to test if this is null: ;select parentID FROM PROPOSAL_NOTEPAD where NoteTypeID = 20625;. And then do my joins based on that result. How do I do that? How do I join based on a condition?
Use LEFT OUTER JOIN:
left join PROPOSALNOTEPAD PN_SolicitationStrategy on prop.ID = PN_SolicitationStrategy.ParentId
LEFT OUTER JOIN ( select parentID, max(DateAdded) as MaxDateAdded FROM PROPOSAL_NOTEPAD where NoteTypeID = 20625 group by ParentId) as PN_max_SolicitationStrategy
ON PN_SolicitationStrategy.ParentId = PN_max_SolicitationStrategy.ParentId AND PN_SolicitationStrategy.DateAdded = PN_max_SolicitationStrategy.MaxDateAdded
Inner Join always brings the common records from both table. In your case, there is no matching record from 1st table and PN_max_SolicitationStrategy table. That's why you are getting no records.
Using LEFT OUTER JOIN will bring you all the records from 1st table and matching records from the 2nd (PN_max_SolicitationStrategy) table.
Here is simple tutorial to learn about JOINs: https://www.w3schools.com/sql/sql_join_left.asp

Joining multiple tables to one table in sql

I have a rather complex (well for me) sql query happening and I am having trouble with some concepts.
I have the following sql on a webpage that i am building
SELECT
[dbo].[Enrolment].[_identity], [dbo].[Enrolment].CommencementDate,
[dbo].[Enrolment].CompletionDate, [dbo].[Enrolment].enrolmentDate,
[dbo].[Course].name coursename, [dbo].[Course].Identifier as QUALcode,
[dbo].[Person].givenName, [dbo].[Person].Surname,[dbo].[Employer].name as empname,
[dbo].[Employer].Address1,[dbo].[Employer].Suburb,[dbo].[Employer].Phone,
[dbo].[Employer].PostCode,[dbo].[EnrolmentStatus].name as enrolname,
[dbo].[Student].identifier,[dbo].[Student].person,[dbo].[Contact].person as CONTACTid
FROM
(((([dbo].[Enrolment]
LEFT JOIN
[dbo].[Course] ON [dbo].[Enrolment].course = [dbo].[Course].[_identity])
LEFT JOIN
[dbo].[Employer] ON [dbo].[Enrolment].employer = [dbo].[Employer].[_identity])
LEFT JOIN
[dbo].[EnrolmentStatus] ON [dbo].[Enrolment].status = [dbo].[EnrolmentStatus].[_identity])
LEFT JOIN
[dbo].[Student] ON [dbo].[Enrolment].student = [dbo].[Student].[_identity])
LEFT JOIN
[dbo].[Person] ON [dbo].[Student].person = [dbo].[Person].[_identity]
LEFT JOIN
[dbo].[Contact] ON [dbo].[Employer].[_identity] = [dbo].[Contact].employer
WHERE
(([dbo].[EnrolmentStatus].name) = 'training'
OR
([dbo].[EnrolmentStatus].name) = 'enrolled')
This is working fine but what I would like to do is join to the [dbo].[Person] table again but this time joining from another table so the code I effectively need to patch into the above statement is
LEFT JOIN
[dbo].[Trainer] ON [dbo].[Enrolment].Trainer = [dbo].[Trainer].[_identity])
LEFT JOIN
[dbo].[Person] ON [dbo].[Trainer].person = [dbo].[Person].[_identity]
I then need to be able to get from the person table the name of the student and the name of the trainer, so I need 2 records from the person table for every record from the Enrolment table, the fields I need from the person table are the same for both trainer and student in that I am trying to get the given name and surname for both.
Any help or pointers would be most appreciated.
You have to just use replace your from clause with this. You have to just first use the Trainer table join, then Person table, then use the AND keyword to use multiple mapping with single table
FROM (((([dbo].[Enrolment]
LEFT JOIN [dbo].[Course] ON [dbo].[Enrolment].course = [dbo].[Course].[_identity])
LEFT JOIN [dbo].[Employer] ON [dbo].[Enrolment].employer = [dbo].[Employer].[_identity])
LEFT JOIN [dbo].[EnrolmentStatus] ON [dbo].[Enrolment].status = [dbo].[EnrolmentStatus].[_identity])
LEFT JOIN [dbo].[Student] ON [dbo].[Enrolment].student = [dbo].[Student].[_identity])
LEFT JOIN [dbo].[Trainer] ON [dbo].[Enrolment].Trainer = [dbo].[Trainer].[_identity])
LEFT JOIN [dbo].[Person] ON [dbo].[Student].person = [dbo].[Person].[_identity]
AND [dbo].[Trainer].person = [dbo].[Person].[_identity]
LEFT JOIN [dbo].[Contact] ON [dbo].[Employer].[_identity] = [dbo].[Contact].employer
Use aliasing like this..
LEFT JOIN [dbo].[Trainer] ON [dbo].[Enrolment].Trainer = [dbo].[Trainer].[_identity])
LEFT JOIN [dbo].[Person] AS p ON [dbo].[Trainer].person = p.[_identity]
If I get your question right - what you are trying to do is to join the same table twice in your SQL. You have one table Person which has both student and trainer information and you want to see their details side by side in your result set. So you need to join Person once with Student and another time with Trainer
To do this - you will have to join Person table together. Give your tables an alias like the other answers have suggested. Then your FROM clause can look like this -
FROM (((([dbo].[Enrolment]
LEFT JOIN [dbo].[Course] ON [dbo].[Enrolment].course = [dbo].[Course].[_identity])
LEFT JOIN [dbo].[Employer] ON [dbo].[Enrolment].employer = [dbo].[Employer].[_identity])
LEFT JOIN [dbo].[EnrolmentStatus] ON [dbo].[Enrolment].status = [dbo].[EnrolmentStatus].[_identity])
LEFT JOIN [dbo].[Student] ON [dbo].[Enrolment].student = [dbo].[Student].[_identity])
LEFT JOIN [dbo].[Person] P1 ON [dbo].[Student].person = P1.[_identity]
LEFT JOIN [dbo].[Contact] ON [dbo].[Employer].[_identity] = [dbo].[Contact].employer
LEFT JOIN [dbo].[Trainer] ON [dbo].[Enrolment].Trainer = [dbo].[Trainer].[_identity])
LEFT JOIN [dbo].[Person] P2 ON [dbo].[Trainer].person = P2.[_identity]
....
....
Here P1 and P2 are two aliases for [Person]

When would you INNER JOIN a LEFT JOINed table /

I came across below code today.
SELECT StaffGroup.*
FROM StaffGroup
LEFT OUTER JOIN StaffByGroup
ON StaffByGroup.StaffGroupId = StaffGroup.StaffGroupId
INNER JOIN StaffMember
ON StaffMember.StaffMemberId = StaffByGroup.StaffMemberId
WHERE StaffByGroup.StaffGroupId IS NULL
The main table StaffGroup is being LEFT JOINed with StaffByGroup table and then StaffByGroup table is being INNER JOINed with StaffMember table.
I thought the INNER JOIN is trying to filter out the records which exist in StaffGroup and StaffByGroup but do not exist in StaffMember.
But this is not how it is working. The query does not return any records.
Am I missing something in understanding the logic of the query ? Have you ever used INNER JOIN with a table which has been used with LEFT JOIN in earlier part of the query ?
Actually you are missing one concept:
The main table StaffGroup is being LEFT Joined with StaffByGroup table and then this creates a virtual table say VT1 with all records from StaffGroup and matching records from StaffByGroup based on your match/filter condition in ON predicate.Then not StaffByGroup table but this VT1 is being INNER Joined with StaffMember table based on match/filter condition in ON predicate.
So basically the inner join is trying to filter out those records from StaffGroup and hence StaffByGroup which do not have a StaffMemberId.
Adding your where condition adds a final filter like from the final virtual table created by all the above joins remove all such records which don't have a StaffGroupId which in turn might be removing all rows collected in VT1 as all of them will be having some value for StaffGroupId.
To get all records from StaffGroup which have no StaffGroupId along with details from StaffMember for all such records you can add condition in ON predicate as:
SELECT StaffGroup.*
FROM StaffGroup
LEFT OUTER JOIN StaffByGroup
ON StaffByGroup.StaffGroupId = StaffGroup.StaffGroupId and StaffByGroup.StaffGroupId IS NULL
INNER JOIN StaffMember
ON StaffMember.StaffMemberId = StaffByGroup.StaffMemberId
This query looks fundamentally flawed - I guess what was originally intended is
SELECT StaffGroup.*
FROM StaffGroup
LEFT OUTER JOIN
(SELECT * FROM StaffByGroup
INNER JOIN StaffMember
ON StaffMember.StaffMemberId = StaffByGroup.StaffMemberId) StaffByGroup
ON StaffByGroup.StaffGroupId = StaffGroup.StaffGroupId
WHERE StaffByGroup.StaffGroupId IS NULL
which returns all groups from StaffGroup that dont' have existing staffmembers assigned to them (the INNER JOIN with StaffMember filters out those rows from StaffByGroup that don't have a matching row in StaffMember - probably because there exists no foreign key between them)
You are getting 0 records because of your where clause
where StaffByGroup.StaffGroupId is null
The left join links all the records from tbl A which are contained in tbl B and since you have specified StaffGROUPID as your key and then looked for Nulls values in your key, its 100% clear that you will end up with no results

Joining multiples columns from more 5 tables

I am trying to join setid(like a foreign key) which exist in all the tables in the query and also I am trying to join lecid which only exist in lec table and in parktable table as well as joining weekid in week and parktime table. I am also trying to join the roomid in rooms table and parktable table. All together setid is like a foreign key in all those tables. I am looking for a setid which is 48596.
I have tried:
select t.slotid, r.number1, t.weekid, t.duration, p.name as DEPName,
a.name FROM parktime t
JOIN rooms k ON t.setid = k.setid
JOIN week r ON t.setid = r.setid
JOIN structure w ON t.setid = w.setid
FULL OUTER JOIN LEC p
ON
t.LECID = p.LECID
FULL OUTER JOIN week r
ON t.weekid = r.weekid
FULL OUTER JOIN structure w
ON
r.number1 = w.number1
FULL OUTER JOIN rooms k
on
k.roomid = t.roomid
WHERE t.setid = '48596'
The problem is that this query takes too long to run and at the end of it, it doesn't come back with the result. TEMP error.
Is they a problem with the way I am joining it?
don't use join twice, as you have here:
FULL OUTER JOIN JOIN rooms k
If speed is your issue, you may want to put indexes on the cols that you are using to join and in the where clause: setid, number1, etc.