Thanks Ryan. When I saw the redundant table still in the explain I was worried I had misunderstood something about left joins.
Tom ________________________________ From: sqlite-users <sqlite-users-boun...@mailinglists.sqlite.org> on behalf of R Smith <rsm...@rsweb.co.za> Sent: Monday, November 20, 2017 3:55:42 PM To: sqlite-users@mailinglists.sqlite.org Subject: Re: [sqlite] Help with left joins On 2017/11/20 5:33 PM, x wrote: >> Only if ColB, ColC and ColD are unique in their tables. Otherwise each join >> has the potential of returning multiple rows, which will carry over to the >> next joins. > Thanks David, I did say ColB, ColC & ColD were primary keys. Any kind of Query (especially of the SELECT variety) can have multiple redundancies or indeed omit-able specified values. An optimization to ensure non-used left-joins on Primary keys are not used is probably a negligible improvement. To answer your question: You are correct to assume the same output - There is no difference in the SET-Theory/Algebra producing those specific rows whether you omit the left joins or not, BUT that only holds true while the Indexes are indeed all PRIMARY (or at least Unique) and the joins are of the "LEFT" variety. Change any of those and the query output may look very different. _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
