SQLite查询中的奇怪结果

Question

I´m getting a strange result from a SQLite query. The query is the next one:

SELECT rule FROM rules
WHERE idRule = (SELECT idRuleForeign FROM rulesXfilter
                 WHERE idFilterForeign = (SELECT idFilter FROM filters
                                          WHERE name = 'Filter1'));

Now, let´s suppose that I have the following tables with a few rows on it.

      filters                rules              rulesXfilter
idFilter   name       idRule     rule      idRuleForeign  idFilterForeign
   1      Filter1       1         Rule1          1              1
   2      Filter2       2         Rule2          2              1
                        3         Rule3          3              1
                                                 2              2

What I get is {Rule1}, although I think I should get {Rule1, Rule2, Rule3}

What am I doing wrong?

Answer 1

Select idRuleForeign... returns multiple results, yes ( {1, 2, 3} ). However, you then say "give me the rule where idRule = {SET}", and sql doesnt like this. I believe what is happening is that it is instead taking the first result only and giving you that.

The solution is to use joins. Inner selects like that, while work most of the time, can REALLY slow down your query. If I got my syntax correct, the following should do what you need:

SELECT r.rule FROM rules r
    JOIN rulesXfilter rf ON r.idRule = rf.idRuleForeign
    JOIN filters f ON f.idFilter = rf.idFilterForeign
WHERE f.name = 'Filter1'