How do you check for matching value in third column based on distinct combinations of other two columns?

问题

I have a table with Building name, such as A, B, C. These building names could be repeated. I have another column which has the floor. Such as floor 1 ,floor 2. Again there could be multiple floor 1 for every building. There is a third column which has object present such as television, bricks, fans. I want to check for every combination of building with corresponding floors, such as Building A - floor 1, Building A- floor 2, if an object 'brick' exists then 'wall' must exist.

EXAMPLE DATA: For every building and location, if 'WALL' exists , 'WINDOW', 'WINDOW1' or 'WINDOW2' must exist, ELSE FLAG

BUILDING  LOCATION  OBJECT   
A         FLOOR1    WALL
A         FLOOR1    WINDOW  
A         FLOOR2    WALL  
B         FLOOR1    WALL  
C         FLOOR1    WALL  
C         FLOOR1    WINDOW

DESIRED OUPUT

BUILDING  LOCATION  ACTION  

A         FLOOR2    FLAG
B         FLOOR1    FLAG

I have tried using GROUP BY, DISTINCT, WHERE EXISTS, but I cant seem to come up with the right logic.

回答1:

You can group by building, location for the rows where object in ('WALL', 'WINDOW'):

select building, location, 'FLAG' action
from tablename
where object in ('WALL', 'WINDOW')
group by building, location
having count(distinct object) < 2

The condition count(distinct object) < 2 in the having clause returns combination of building, location where 'WALL' and 'WINDOW' do not both exist.
See the demo.
Results:

| building | location | action |
| -------- | -------- | ------ |
| A        | FLOOR2   | FLAG   |
| B        | FLOOR1   | FLAG   |

Or with NOT EXISTS:

select t.building, t.location, 'FLAG' action
from tablename t
where object in ('WALL', 'WINDOW')
and not exists (
  select 1 from tablename
  where building = t.building and location = t.location and object <> t.object
)

See the demo.

回答2:

nested tables is what you want. Something like

select column_3
from (
  select *
  from table
  where table.column_3="brick"
) as e
join table t on t.id = e.id
where table.column_3="window"

fyi: I reccomend you use this as a start, but for your exact case, id imagine this needs to be modified

来源：https://stackoverflow.com/questions/56600496/how-do-you-check-for-matching-value-in-third-column-based-on-distinct-combinatio

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mysql

mysql-workbench