Help with Delphi 7, ADO, & MS Access SQL Statement - Part Deuce - sql

Question

I need help understanding why my SQL does not work. Or, if i need to write it differently to get the results i need. As the title suggests, I am using Delphi 7, with ADO components, and a MS Access 2000 database. You can see my table structure from Part I here:
Help with Delphi 7, ADO, & MS Access SQL Statement
The SQL i am currently using to get all knowledge based on keywords is as follows:
select * from (knowledge K
inner join knowledge_keywords KKW on KKW.knowledgeid = K.id)
inner join keywords KW on KW.id = KKW.keywordid
where (KW.keyword = 'job') AND (KW.keyword = 'task')
However, this does not return and results, when there is clearly both of those words in the knowledge_keywords table with the same knowledge id.
However, if i do the same SQL with an OR instead of an AND, i get the two records i expected
select * from (knowledge K
inner join knowledge_keywords KKW on KKW.knowledgeid = K.id)
inner join keywords KW on KW.id = KKW.keywordid
where (KW.keyword = 'job') AND (KW.keyword = 'task')
thanks for any help

Answer 1

Think about it this way: How many records are there in knowledge_keywords for which it is true both that keyword = 'job' AND keyword = 'task'. There are no such records. When you use AND you're asking for records that satisfy both the first condition AND the second condition at the same time. When you use OR, you're asking for records that satisfy one condition OR the other one (or both).
In this case, OR expresses what you want. AND expresses something different.
You can also use KW.keyword IN ('job', 'task') which is more concise and, perhaps, clearer.

Answer 2

I think the first query won't return any result, does it? That's because 'and' in speech differs from 'and' in programming. When you say, you want the keywords 'job' and 'task', you actually mean you want the rows where keyword is either 'job' or 'task'. A keyword cannot be both 'job' and 'task' so that query won't return any rows. You could replace the OR with an IN in the form of
WHERE KW.Keyword in ('job', 'task')
But this probably won't give you the result you want. I suspect you need to find articles that match both keywords.
To check if a knowledgebase has both keywords, you might need something like this (although I'm not sure if Access accepts this:
select
*
from
knowledge K
where
exists
(select 'x' from
knowledge_keywords KKW
inner join keywords KW on KW.id = KKW.keywordid
where
KKW.knowledgeid = K.id and
KW.keyword = 'job')
and exists
(select 'x' from
knowledge_keywords KKW
inner join keywords KW on KW.id = KKW.keywordid
where
KKW.knowledgeid = K.id and
KW.keyboard = 'task') and
[edit]
A different approach, that might work better in Access (I'm sorry I can't test it) is by using a count like this. I made a small assumption about the fields in K for this example.
This way, you join each keyword in the list. For a knowledge base article that has both 'job' and 'task' it will return two rows at first. These rows are then grouped on the Knowledge fields, and the rows are counted. Only the articles where count matches the total number of keywords are returned.
Possible problem: When an article has the same keyword (job) linked twice, it is still returned. This can be solved by preventing that from happening using unique constraints.
select
K.ID,
K.Title,
K.Content
from
knowledge K
inner join knowledge_keywords KKW on KKW.knowledgeid = K.id)
inner join keywords KW on KW.id = KKW.keywordid
where
KW.keyword in ('job', 'task')
group by
K.ID,
K.Title,
K.Content
having
count(*) = 2 /* Number of keywords */