SQL查询一列上的多列DISTINCT

Question

I am running an SQL query as below, selecting data for each property with a LEFT JOIN selecting data mainly from table1 ('main_table') for each property and also displaying data in table 2('houses') if table2 also holds data on each property.

However I want to only show results 1 per property. I have tried various results with DISTINCT before the fields that i am selecting but it hasn't worked.

Below is the SQL and also showing a sample of the data returned. So for example, 128 Mayfield Road has 2 entries in table 2 so it is returned twice but i only want to show each house once.

Many Thanks

SELECT main_table.housenumber, main_table.streetname, main_table.rent, houses.houseID, houses.path, houses.rooms
FROM main_table
LEFT JOIN houses ON main_table.housenumber = houses.housenumber
AND main_table.streetname = houses.streetname

533  Portswood Road   57    NULL    NULL                            NULL
35   Ripstone Gardens 70    NULL    NULL                            NULL
67   Kent Road        68    NULL    NULL                            NULL
21   Bealing Close    65    NULL    NULL                            NULL
75   Broadlands Road  76    NULL    NULL                            NULL
7    Gordon Avenue    70    243     images_housing/GOR1.jpg         4 
29   Broadlands Road  74    NULL    NULL                            NULL 
10   Westbrook Way    65    NULL    NULL                            NULL 
328C Burgess Road     85    NULL    NULL                            NULL
10   Thackeray Road   68    NULL    NULL                            NULL 
128  Mayfield Road    70    311     images_housing/mayfield1.jpg    4 
128  Mayfield Road    67    311     images_housing/mayfield1.jpg    4

Answer 1

Maybe like this?

SELECT
    main_table.housenumber,
    main_table.streetname,
    max(main_table.rent)
    houses.houseID,
    houses.path,
    houses.rooms
FROM main_table
LEFT JOIN houses ON main_table.housenumber = houses.housenumber
AND main_table.streetname = houses.streetname
group by
    main_table.housenumber,
    main_table.streetname,
    houses.houseID,
    houses.path,
    houses.rooms

Answer 2

Say a house has more than one entry. If you don't care which of the possible values you get in the other columns then you can use a much maligned MySQL extension to group by:

Select
    m.housenumber,
    m.streetname, 
    m.rent, 
    h.houseID, 
    h.path, 
    h.rooms
From
    main_table m
        Left Join
    houses h
        on m.housenumber = h.housenumber and
           m.streetname = h.streetname
Group By
    m.housenumber,
    m.streetname

Most databases won't let you do this, because usually you do care which of the other possible values you get.

Answer 3

Distinct will check that all columns being selected are unique, not just one of them.

This might be helpful: SQL Group by & Max