Question

I need to update a table and set a column (enabled) to 1 with the following rules

1. There must be 1 (and never 0 or > 1) record with enabled set to 1 per Employee_Guid.
2. It should choose the record that has deleted set to 0 (there should only be one or no records with deleted set to 0 per Employee_Guid) to set enabled to 1.
3. If all records have deleted set to 1 it should use the record with the most recent Create_Date_Time
4. If there is a tie with the create date time any record of the tieing group can be set to 1 but only one of them can be used.

If there are more than 1 record with deleted set to 0 I would like a error thrown as that is a illegal state.

Here is what I have so far

if(exists(select count(employee_guid) from Employees_M where deleted = 0 group by employee_guid having count(employee_guid) > 1))
    RAISERROR('More than one record has deleted set to 0')
Update Employees_M set [ENABLE] = 0

select * into #t from employees_m where deleted = 0
insert into #t select * from employees_m where employee_guid not in (select employee_guid from #t) 
                                         and --fulfill rules 3 and 4

Update Employees_M set [ENABLE] = 1
where pk_guid in (select pk_guid from #t)

Here is the table structure

PK_Guid (primary key, uniuque, uniuqueidenitfier, not null)
Employee_Guid (uniuqueidenitfier, not null)    
Deleted (bit, not null)
Enable (bit, not null)
Create_Date_Time (datetime defaults to getdate(), not null)

This is not just a “Show me teh codez” question. I want to learn how to do it correctly, so links or examples that are similar but not solving the problem would be appreciated too.

Answer 1

It’s often possible to use row_number in this scenario. Its order by clause allows you to assign priorities. In your case, deleted asc and Create_Date desc would seem to capture the requirements.

Here’s an example of how to use row_number in an update query:

update  emp
set     enabled = case when rn = 1 then 1 else 0 end
from    (
        select  row_number() over (partition by employee_guid 
                                   order by deleted, Create_Date desc) as rn
        ,       *
        from    @employees
        ) emp

Full example at SE Data.