solutions
:: Selection Access
Selection Access
Once you have selected data from the database, it returns a set of rows containing queried columns values. Each row loaded with DB_SELECTION is stored in a DB_RESULT object. The easiest way to access the data is thus to refer to DB_RESULT objects themselves.
Note: Take a look at the Database/Eiffel objects Coupling to learn advanced data handling features.
To use DB_RESULT, process in 2 steps:
Retrieving DB_RESULT objects
DB_SELECTION class provides different ways to customize result loading:
- You want to access an unique row: DB_RESULT object is accessible via cursor:
selection: DB_SELECTION
my_result: DB_RESULT
...
selection.query ("...")
if selection.is_ok then
selection.load_result
my_result := selection.cursor
end
- You want to load a complete list of rows: DB_SELECTION can store DB_RESULT objects in a list. To do this, you have mainly to provide a LIST object to DB_SELECTION with set_container:
selection: DB_SELECTION
container: ARRAYED_LIST [DB_RESULT]
...
create container.make (Max_results)
...
selection.set_container (container)
selection.load_result
...
from
container.start
until
container.after
loop
...
end
Tip: Provide DB_SELECTION with the LIST structure convenient for what you need to do with the results.
- You want to select part of the result set: you can set an action in DB_SELECTION that will be executed each time a row is loaded. This action can for instance manipulate current row and define a stop condition.
- You need to define a descendant of class ACTION and set it to DB_SELECTION :
class
MY_ACTION
inherit
ACTION
redefine
execute, found
end
...
execute
do
i := i + 1
end
...
found: BOOLEAN
do
Result := i >= Max_result
end
- Then set action to DB_SELECTION :
selection: DB_SELECTION
action: MY_ACTION
...
selection.set_action (action)
selection.query ("...")
if selection.is_ok then
selection.load_result
end
Accessing content of DB_RESULT
A DB_RESULT object merely carries data retrieved from the database. You have to convert it to a DB_TUPLE to access data within the retrieved row conveniently, i.e. mostly the column values: selection: DB_SELECTION
tuple: DB_TUPLE
...
create tuple.make_from_cursor (selection.cursor)
if tuple.count >= 2 and then tuple.column_name (2).is_equal ("Firstname") then
io.putstring (tuple.item (2).out)
end
See Also:
Performing a database selection.
Coupling database data and Eiffel objects.