|Title:||Incorrect output on a LEFT JOIN|
|Last Modified:||2014-12-04 16:35:32|
|Version Found In:||3.8.6|
drh added on 2014-12-04 15:40:03:
In the following SQL, the final SELECT statement should return just a single row, but beginning in SQLite 3.8.6 it returns 3 rows: <blockquote><verbatim> CREATE TABLE x1(a); INSERT INTO x1 VALUES(1); CREATE TABLE x2(b NOT NULL); -- Remove the NOT NULL and things work CREATE TABLE x3(c, d); INSERT INTO x3 VALUES('a', NULL); INSERT INTO x3 VALUES('b', NULL); INSERT INTO x3 VALUES('c', NULL); SELECT * FROM x1 LEFT JOIN x2 LEFT JOIN x3 ON x3.d = x2.b; </verbatim></blockquote> As the comment states, removing the NOT NULL constraint on the x2.b column causes the output to be correct. The problem appears to have been introduced by checkin [952868216854e83] on 2014-08-05.
drh added on 2014-12-04 16:35:32: (text/x-fossil-wiki)
The problem was that the query planner was using NOT NULL constraints on table columns to optimize out certain operations. However, for the right-hand table of a LEFT JOIN, any column can be NULL despite having a NOT NULL constraint. The sqlite3ExprCanBeNull() routine in the query planner had to be enhanced to recognize that corner case.