performance - SQL: selecting unique values based on conditions -


i have table containing 5 columns. first column contains id, 2 columns contain parameters ids values 0 or 1, third column contains parameter need output, last column contains date. same id can appear in several rows different parameters:

id        parameter1      parameter2        parameter3       date  001       0               1                                01.01.2010 001       0               1                 b                02.01.2010 001       1               0                 c                01.01.2010 001       1               1                 d                01.01.2010 002       0               1                                01.01.2010

for each unique id want return value in parameter3, decision row return value based on values in parameter1 , parameter2 , date:

  • if there row both parameters being 0, want value in row.
  • if there no such row, want value row parameter1 0 , parameter2 1,
  • if there no such row, want row parameter1 1 , parameter2 0.
  • finally, if there no such row, want value row both parameters being 1.

if there more 1 row matching required conditions, want row recent date.

e.g., table above, id 001 want second row value b in parameter3.

what effective / fastest way accomplish this? tried 2 approaches far:

the first 1 select distinct ids , loop through distinct ids, using select statement id in where clause , loop through rows matching id while storing necessary values in variables.:

foreach     select distinct id i_id table1         foreach             let o_case = 5             select case                 when parameter1 = 0 , parameter2 = 0 1                 when parameter1 = 0 , parameter2 = 1 2                 when parameter1 = 1 , parameter2 = 0 3                 when parameter1 = 1 , parameter2 = 1 4                 end, parameter3, date                 i_case, i_p3, i_date                 table3                 table3.id = i_id                  if i_case < o_case                      let o_p3, o_case, o_date = i_p3, i_case, i_date;                     else ( if i_case = o_case , i_date > o_date                         let o_p3, o_date = i_p3, i_date;                     end if;                 end if;         end foreach;         insert table_output values(i_id; o_p3); end foreach;

the second approach left join table 4 times on id , apply different combinations of parameter1 & parameter2 described above in left joins, selecting output via nested nvl clauses: 

select id,      nvl(t1.parameter3,          nvl(t2.parameter3,             nvl(t3.parameter3,                 nvl(t4.parameter3)))) parameter3 table1 t0     left join table1 t1         on t0.id = t1.id , t1.parameter1 = 0 , t1.parameter2 = 0         , t1.date = (select max(date) table1 t1a t1a.id = t1.id)             left join table1 t2         on t0.id = t2.id , t2.parameter1 = 0 , t2.parameter2 = 1         , t2.date = (select max(date) table1 t2a t2a.id = t1.id)     left join table1 t3         on t0.id = t3.id , t3.parameter1 = 1 , t3.parameter2 = 0         , t3.date = (select max(date) table1 t3a t3a.id = t3.id)     left join table1 t4         on t0.id = t4.id , t4.parameter1 = 1 , t4.parameter2 = 1         , t4.date = (select max(date) table1 t4a t4a.id = t4.id)

both approaches worked, however, table long, slow. recommend?

ps: dbms ibm informix 10, unfortunately restricts range of available functions lot.

i'm not sure if wanted, work:

select id, parameter3 (     select id, parameter3, rank() on (             partition id, parameter3             order parameter1 asc, parameter2 asc, date desc         )     tab ) x x.rank = 1;

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