在Android中使用WHERE子句联接3个表

Question

I have problem with SQL in Android (SQLite). I have three tables in relation many-to-many, suppose A, B and C, each has 30 records. I want to join these tables and after that use WHERE clause. I have written something like this code

SELECT *
FROM A
   JOIN B ON A.id=B.photoid 
   JOIN C ON B.urlid=C.id 
WHERE C.type=[SOME VALUE]

I have also tried this

SELECT *
FROM A
   LEFT OUTER JOIN B ON A.id=B.photoid 
   LEFT OUTER JOIN C ON B.urlid=C.id AND C.type=[SOME VALUE]

and this

SELECT *
FROM A
   CROSS JOIN B
   CROSS JOIN C
WHERE A.id=B.photoid AND B.urlid=C.id AND C.type=[SOME VALUE]

These statements don't work in Android. They return 300 records. What should I change? Is it bug or I do something wrong?

Answer 1

Hard to tell without any additional data, but keep in mind that a JOIN operation will produce a row for each combination of a value in any of the tables involved. If you had 3 tables, each with 30 rows and you wouldn't be filtering any result, it should produce 27000 rows, but you're filtering by A.id=B.photoid and B.urlid=C.id , which might be producing a smaller set of rows.

In my opinion, that's not anything related to SQLite, but to your query instead. I'd try to make your set of values much smaller (for example, 5 per table) and run the same query, and see which results are being returned which shouldn't be, and try to modify your query to make it discard all those unwanted results.