plsql - PL/SQL Case Statement - IS NULL -


i have sql, far add of statement in question, trying improve in order drive out system slowdowns.

in part of pl/sql there and exists clause contains and not exists clause.

both of these clauses contain case statements. after end part of case statement, there either is not null or is null statement. having used pl/sql years first time have seen syntax , freely admit not understanding it. whole of and exists clause displayed below, , appreciated.

and exists  (   select distinct projectid   milestone_history mh,         project_milestone pm   mh.milestoneid = pm.id     , mh.projectid = pr.id     , ((upper(pm.description) = 'final build invoice instructions issued') or          (upper(pm.description) = 'feasibility study required') or          (upper(pm.description) = 'project complete'))      , (case when upper(pm.description) = 'feasibility study required'                   actual                 end not null                   ,                    case                     when upper(pm.description) = 'project complete'                       mh.actual                   end null)            , not exists          (select distinct mh2.projectid           milestone_history mh2, project_milestone pm2           mh2.milestoneid = pm2.id             , mh2.projectid = mh.projectid             , case                     when upper(pm2.description) = 'project complete'  -                           mh2.actual                     end not null))

think of case...end single expression. can use parentheses better understanding:

(case when upper(pm.description) = 'feasibility study required'      actual  end) not null

this particular expression not null if , if case finds match , actual not null.

this rewritten as:

(upper(pm.description) = 'feasibility study required' , actual not null)

the following statement rewritten as:

(upper(pm.description) != 'project complete'  or pm.description null  or actual null)

the 2 statements overlapping: descrition can have 1 value. when merge them there not left:

(select distinct projectid    milestone_history mh,          project_milestone pm   mh.milestoneid = pm.id     , mh.projectid = pr.id     , upper(pm.description) = 'feasibility study required'     , actual not null     , not exists          (select distinct mh2.projectid             milestone_history mh2, project_milestone pm2            mh2.milestoneid = pm2.id              , mh2.projectid = mh.projectid              , pm2.description = 'project complete'              , mh2.actual not null) )

i think it's bit misleading use case when there single case.

either author misinformed behaviour of case or remnant code has been updated along way without being cleaned.


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