SQL Server 2017

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declare @bactisurvey table (
    district_distribution varchar(200)
    ,sample_type varchar(100)
    ,collection_location varchar(100)
    ,date_sample_collected datetime
    ,last_edited_date datetime
    ,chlorine_mgl float
);

insert into @bactisurvey 
values
    -- Unique Entry 1 (district 1, Type A, AUS, 30th Mar)
    ('district 1', 'type A', 'AUS', '2020-03-30', '2020-04-01', 1.2)
    ,('district 1', 'type A', 'AUS', '2020-03-30', '2020-04-01', 2.8)
    ,('district 1', 'type A', 'AUS', '2020-03-30', '2020-04-03', 1.1)
    ,('district 1', 'type A', 'AUS', '2020-03-30', '2020-04-03', 5.2)
    ,('district 1', 'type A', 'AUS', '2020-03-30', '2020-04-05', 1.21)
    ,('district 1', 'type A', 'AUS', '2020-03-30', '2020-04-05', 1.44)
    
    -- Unique Entry 2 (district 2, Type B, AUS, 30th Mar)
    ,('district 2', 'type B', 'AUS', '2020-03-30', '2020-04-01', 1.2)
    ,('district 2', 'type B', 'AUS', '2020-03-30', '2020-04-01', 2.8)
    ,('district 2', 'type B', 'AUS', '2020-03-30', '2020-04-05', 1.11)
    ,('district 2', 'type B', 'AUS', '2020-03-30', '2020-04-05', 5.21)
    ,('district 2', 'type B', 'AUS', '2020-03-30', '2020-04-05', 1.2)
    ,('district 2', 'type B', 'AUS', '2020-03-30', '2020-04-05', 1.4);

with TopEdits as (
    select 
        edit_num = rank() over (partition by district_distribution, sample_type, collection_location, date_sample_collected order by last_edited_date desc)
        ,district_distribution
        ,sample_type
        ,collection_location
        ,date_sample_collected
        ,last_edited_date
        ,chlorine_mgl
    from @bactisurvey
)
select
    district_distribution
    ,sample_type
    ,collection_location
    ,date_sample_collected
    ,min_chlorine = min(chlorine_mgl)
    ,max_chrloine = max(chlorine_mgl)
from TopEdits
where
    edit_num = 1
group by
    district_distribution
    ,sample_type
    ,collection_location
    ,date_sample_collected

district_distribution	sample_type	collection_location	date_sample_collected	min_chlorine	max_chrloine
district 1	type A	AUS	2020-03-30 00:00:00.000	1.21	1.44
district 2	type B	AUS	2020-03-30 00:00:00.000	1.11	5.21