Question

This is a question for a SQL expert. I’m using SQL Server 2008 R2

I have two relevant tables: Labs and LabUsers.

Users are assigned to Labs with no repetitions of an entire groups of any order.

The goal is to insert a @userName (for the example @user = "Paul") into LabUsers fulfilling all the following limitations:

1. No more than @maxUsers in a group (for the example @maxUsers=4)

2. No duplicates of complete groups (full labs). The order of the users in a group is not significant. [edited]

3. If no existing Lab is allowed, create (INSERT) a new lab, then insert the row for @user, given no exceeding @maxLabs (for the example @maxLabs=5).

4. Very important: There are many concurrent same requests from the server in a split of a second, which may interfere one to the other. Therefore, as soon as the command begins to execute, no other queries are allowed to execute until the end of this command.

5. The query should return 0 in cases it cannot meet the above restrictions, and return the LabID of the inserted row.

6. [EDITED] There are several Labs’ zones. The zones are independent. Each zone #labCount is bounded by the @maxLabs. The @maxLabs is equal for all zones, therefore the Total_maxLabs = @maxLabs x #zonesCount . For the example @zone=51 (later on @zone=52, 53 etc.). (The same LabUsers can use zones with no limitations. Zones do not ‘know’ about each other)

7. LabID in LabUsers is a foreign key from Labs.

The example:

Here is the Labs table:

LabID   LabName     LabZone
-----   -------     -------
1       North       51  
2       North East  51
3       South West  51

And the LabUsers is:

LabUserID   LabUserName LabID
---------   ----------- -----
1           Diana       3
2           Julia       2
3           Paula       2
4           Romeo       1
5           Julia       3
6           Rose        2
7           Diana       1
8           Diana       2
9           Julia       1
10          Romeo       3
11          Paul        1

In the example the users are assigned like this:

LabID   LabName     LabZone LabUsers (ordered LTR a>z)
-----   -------     ------- --------
1       North       51      Diana•Julia•Paul•Romeo
2       North East  51      Diana•Julia•Paula•Rose
3       South West  51      Diana•Julia•Romeo

• The insert should not be into LabID=1 or 2 because there are already 4 users in these labs.
• The insert should not take place into LabID=3 due to creating duplicate with LabID=1.

Therefore, because the @maxLabs is not 3 (existing labs), it is necessary to insert a new row into Labs with the value LabZone=@zone=51.

The IDENTITY will set the LabID to 4 for the new row.

Now is the time to insert Paul into LabUsers with LabID just returned from inserting a new lab.

How to solve this problem?

What is the method to use in order to ensure that the command is executed as a whole with no interference?

The script to create the database is:

CREATE DATABASE [Allocation]
GO

USE [Allocation]
GO

CREATE TABLE [dbo].[LabUsers](
    [LabUserID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY CLUSTERED ,
    [LabUserName] [nvarchar](50) NOT NULL,
    [LabID] [int] NOT NULL)
GO

SET IDENTITY_INSERT [dbo].[LabUsers] ON
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (1, N'Diana', 3)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (2, N'Julia', 2)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (3, N'Paula', 2)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (4, N'Romeo', 1)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (5, N'Julia', 3)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (6, N'Rose', 2)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (7, N'Diana', 1)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (8, N'Diana', 2)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (9, N'Julia', 1)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (10, N'Romeo', 3)
INSERT [dbo].[LabUsers] ([LabUserID], [LabUserName], [LabID]) VALUES (11, N'Paul', 1)
SET IDENTITY_INSERT [dbo].[LabUsers] OFF

CREATE TABLE [dbo].[Labs](
    [LabID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY CLUSTERED ,
    [LabName] [nvarchar](50) NULL,
    [LabZone] [int] NOT NULL)
GO

SET IDENTITY_INSERT [dbo].[Labs] ON
INSERT [dbo].[Labs] ([LabID], [LabName], [LabZone]) VALUES (1, N'North', 51)
INSERT [dbo].[Labs] ([LabID], [LabName], [LabZone]) VALUES (2, N'North East', 51)
INSERT [dbo].[Labs] ([LabID], [LabName], [LabZone]) VALUES (3, N'South West', 51)
SET IDENTITY_INSERT [dbo].[Labs] OFF

Answer 1

I piggy backed off dradu’s variables and implemented a similar but different solution. It does make the assumption that a new lab will be 1 more than the maximum available current lab. I also make the assumption a lab does NOT remove users.

The goal of this solution is to see what the end result of insertion of the user would look like and run checks on it to see which end result is valid. The logic is as follows:

1. Get available labs to insert into
   • Check to make sure user not in lab here
   • Check to make sure lab not full here
   • Include new lab possibility here as well
2. Create a list of all lab users per lab sorted alphabetically if the lab will be full after the user is inserted
   • the new possible labs are flagged
3. Compare the flagged lab lists vs the unflagged lab lists and choose the minimum labId that isn’t a duplicate of an existing full lab list
4. Return LabId inserted into or 0 as output

Given the starting data from the original question and execution in the order below:

1. Insert @userName = “Paul”, @labZone = 51
   • Paul gets added to the newly created Lab 4
2. Insert @userName = “Paul”, @labZone = 51
   • Paul gets added to the newly created Lab 5
3. Insert @userName = “Paul”, @labZone = 51
   • No more new labs and no existing labs for Paul to go to so return 0
4. Insert @userName = “Rose”, @labZone = 51
   • Rose gets added to the existing Lab 3
5. Insert @userName = “Rose”, @labZone = 51
   • Rose gets added to the existing Lab 4
6. Insert @userName = “Rose”, @labZone = 51
   • Rose gets added to the existing Lab 5

A tablockx within the transaction on LabUsers should prevent concurrent transactions from causing havoc.

Also, when debugging common table expressions, it helps to replace them with a temporary table so you can look at the results of each step along the way.

BEGIN TRAN

DECLARE @maxUsers INT
DECLARE @maxLabs INT
DECLARE @userName VARCHAR(50)
DECLARE @labZone INT
DECLARE @labID INT

SET @maxUsers = 4
SET @maxLabs = 5

SET @userName = 'Paul'
SET @labZone = 52
SET @labID = NULL

declare @currentLabCount int

-- get current number of labs
select @currentLabCount = count(*)
from Labs l
/*
-- uncomment this if the max labs applies individual lab zones rather than across all lab zones
where LabZone = @labZone
*/  

;with availableLabs as ( -- get available labs to insert into
    -- check existing labs for valid spots
    select
        lu.LabID
    ,   count(*) + 1 as LabUserCount -- need this to see when we're at max users
    from LabUsers lu with (tablockx) -- ensures blocking until this completes (serialization)
      inner join Labs l with (tablockx) -- might as well lock this too
        on l.LabId = lu.LabID
        and l.LabZone = @labZone -- check Lab Zone
    where not exists( -- make sure lab user isn't already in this lab
        select 1
        from LabUsers lu2
        where lu2.LabId = lu.LabId
        and lu2.LabUserName = @userName
    )
    group by lu.LabID
    having count(*) < @maxUsers -- make sure lab isn't full
    union all
    -- create new lab if not at limit
    select
        max(LabId) + 1 as LabId
    ,   1 as LabUserCount
    from Labs -- check all labs
    where @currentLabCount < @maxLabs -- don't bother checking new labs if going to exceed max allowable labs
)
-- only do this check if lab is going to be filled
, dupeCheck as( -- generates a lab user list sorted alphabetically by lab user name per lab
    select
        y.LabId
    ,   max(y.newLabFlag) as newLabFlag -- if existing lab getting new lab user, then 1, if new lab with new lab user, then 1 else 0
    ,   replace(replace(replace(stuff( -- cool way to comma concatenate without looping/recursion taking advantage of "XML path"
            (
                select
                    ',' + x.LabUserName + '' -- lab users
                from (
                    select
                        LabId
                    ,   @userName as LabUserName 
                    from availableLabs -- the new user and his/her potential labs
                    union all
                    select
                        lu.LabId
                    ,   lu.LabUserName
                    from LabUsers lu -- the current lab users and the labs they belong to
                ) x
                where x.LabID = y.LabId -- make sure the LabId's match
                and max(y.LabUserCount) = @maxUsers -- don't generate this list if lab is not full
                order by x.LabUserName -- sorted alphabetically
                for xml path('')
            ), 1, 1, ''
        )
        , '&lt;', '<'), '&gt;', '>'), '&amp;', '&') as LabUserList
    from (
        -- get list of old labs and flag them as such
        select
            lu.LabId
        ,   convert(tinyint,0) as newLabFlag
        ,   count(*) as LabUserCount -- need the current lab user count
        from LabUsers lu
        /*
            -- uncomment this if full labs can be duplicated across lab zones
            inner join Labs l
                on l.LabId = lu.LabId
                and l.LabZone = @labZone
        */
        group by lu.LabId
        union all
        -- get list of potential candidate labs for lab user and flag them as such
        select
            al.LabId
        ,   convert(tinyint,1) as newLabFlag
        ,   al.LabUserCount -- new lab user count if we were to insert the new user
        from availableLabs al
    ) y
    group by y.LabId
)
select
    @labID = min(dc.LabID)
from dupeCheck dc
where dc.newLabFlag = 1
-- make sure the same list of users does not already exist at an existing lab
and not exists(
    select 1
    from dupeCheck dupe
    where dupe.LabUserList = dc.LabUserList
    and dupe.newLabFlag = 0
)

-- insert new lab if doesn't exist
insert into Labs(LabName, LabZone) -- always better to be clearer
select
    'New Lab' as LabName
,   @labZone as LabZone
where @currentLabCount < @maxLabs -- make sure we can't have more than max labs
and not exists(
    select 1
    from Labs
    where LabId = @labId
)

-- insert lab users
insert into LabUsers(LabUserName, LabId)
select
    @userName as LabUserName
,   @labId as LabId
where @labId is not null

-- return labId
select isnull(@labId,0)
commit tran