Question

I’ve got a database like this one:

I’m trying to create a query that would enable me to update the value of the status attribute inside the incident table whenever the values of all of these three attributes: tabor_vatrogasci, tabor_policija, and tabor_hitna are contained inside the izvještaj_tabora table as a value of the oznaka_tabora attribute. If, for example, the values of the tabor_vatrogasci, tabor_policija, and tabor_hitna attributes are 3, 4 and 5 respectively, the incident table should be updated if (and only if) 3, 4, and 5 are contained inside the izvještaj_tabora table.

This is what I tried, but it didn’t work:

UPDATE incident SET status='Otvoren' FROM tabor,izvjestaj_tabora
WHERE (incident.tabor_policija=tabor.oznaka
OR incident.tabor_vatrogasci=tabor.oznaka
OR incident.tabor_hitna=tabor.oznaka)
AND izvjestaj_tabora.oznaka_tabora=tabor.oznaka
AND rezultat_izvjestaja='Riješen' AND
((SELECT EXISTS(SELECT DISTINCT oznaka_tabora FROM izvjestaj_tabora)
WHERE oznaka_tabora=incident.tabor_policija) OR tabor_policija=NULL) AND
((SELECT EXISTS(SELECT DISTINCT oznaka_tabora FROM izvjestaj_tabora)
WHERE oznaka_tabora=incident.tabor_vatrogasci) OR tabor_vatrogasci=NULL) AND
((SELECT EXISTS(SELECT DISTINCT oznaka_tabora FROM izvjestaj_tabora)
WHERE oznaka_tabora=incident.tabor_hitna) OR tabor_hitna=NULL);

Does anyone have any idea on how to accomplish this?

Answer 1

According to your description the query should look something like this:

UPDATE incident i
SET    status = 'Otvoren'
WHERE (tabor_policija IS NULL OR
       EXISTS ( 
          SELECT 1 FROM izvjestaj_tabora t
          WHERE  t.oznaka_tabora = i.tabor_policija
          )
       )
AND   (tabor_vatrogasci IS NULL OR
       EXISTS ( 
          SELECT 1 FROM izvjestaj_tabora t
          WHERE  t.oznaka_tabora = i.tabor_vatrogasci
          )
       )
AND   (tabor_hitna IS NULL OR
       EXISTS ( 
          SELECT 1 FROM izvjestaj_tabora t
          WHERE  t.oznaka_tabora = i.tabor_hitna
          )
       )

_{I wonder though, why the connecting table tabor is irrelevant to the operation.}

Among other things you fell victim to two widespread misconceptions:

1)

tabor_policija=NULL

This expression aways results in NULL. Since NULL is considered “unknown”, if you compare it to anything, the outcome is “unknown” as well. I quote the manual on Comparison Operators:

Do not write expression = NULL because NULL is not “equal to” NULL.
(The null value represents an unknown value, and it is not known
whether two unknown values are equal.)

2)

EXISTS(SELECT DISTINCT oznaka_tabora FROM ...)

In an EXISTS semi-join SELECT items are completely irrelevant. (I use SELECT 1 instead). As the term implies, only existence is checked. The expression returns TRUE or FALSE, SELECT items are ignored. It is particularly pointless to add a DISTINCT clause there.