The following query:

SELECT * FROM ( SELECT t2.* FROM t1 , t2 ) AS alias1 WHERE b IS NULL;

returns rows for which b IS NOT NULL

MariaDB [test]> explain SELECT * FROM ( SELECT t2.* FROM t1 , t2 ) AS alias1 WHERE b IS NULL ;
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minimal optimizer switch:derived_merge=on
full optimizer_switch:
index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=off,firstmatch=off,loosescan=off,materialization=off,in_to_exists=on,semijoin=off,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=off,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=off,semijoin_with_cache=off,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on

bzr version-info
revision-id: <email address hidden>
date: 2011-08-09 18:34:26 +0300
build-date: 2011-08-10 13:04:33 +0300
revno: 3148
branch-nick: maria-5.3

test case:

CREATE TABLE t1 (a int) ;
INSERT INTO t1 VALUES (1),(1);

CREATE TABLE t2 (b int) ;
INSERT INTO t2 VALUES (NULL),(6);

SET SESSION optimizer_switch='derived_merge=on';

SELECT * FROM ( SELECT t2.* FROM t1 , t2 ) AS alias1 WHERE b IS NULL;