Fix for bug#52051: Aggregate functions incorrectly returns

      NULL from outer join query
      
      Problem: optimising MIN/MAX() queries without GROUP BY clause
      by replacing the aggregate expression with a constant, we may set it
      to NULL disregarding the fact that there may be outer joins involved.
      
      Fix: don't replace MIN/MAX() with NULL if there're outer joins.
      
      Note: the fix itself is just
      - if (!count)
      + if (!count && !outer_tables)
          set to NULL
      
      The rest of the patch eliminates repeated code to improve speed
      and for easy maintenance of the code.

mysql-test/r/group_by.result

@@ -1790,4 +1790,24 @@ aa	b	COUNT(         b)
 1	10	1
 DROP TABLE t1, t2;
 #
+# Bug#52051: Aggregate functions incorrectly returns NULL from outer
+# join query
+#
+CREATE TABLE t1 (a INT PRIMARY KEY);
+CREATE TABLE t2 (a INT PRIMARY KEY);
+INSERT INTO t2 VALUES (1), (2);
+EXPLAIN SELECT MIN(t2.a) FROM t2 LEFT JOIN t1 ON t2.a = t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+SELECT MIN(t2.a) FROM t2 LEFT JOIN t1 ON t2.a = t1.a;
+MIN(t2.a)
+1
+EXPLAIN SELECT MAX(t2.a) FROM t2 LEFT JOIN t1 ON t2.a = t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+SELECT MAX(t2.a) FROM t2 LEFT JOIN t1 ON t2.a = t1.a;
+MAX(t2.a)
+2
+DROP TABLE t1, t2;
+#
 # End of 5.1 tests

mysql-test/t/group_by.test

@@ -1205,6 +1205,21 @@ SELECT (SELECT t2.a FROM t2 WHERE t2.a = t1.a) aa, b, COUNT(         b)
 
 DROP TABLE t1, t2;
 
+
+--echo #
+--echo # Bug#52051: Aggregate functions incorrectly returns NULL from outer
+--echo # join query
+--echo #
+CREATE TABLE t1 (a INT PRIMARY KEY);
+CREATE TABLE t2 (a INT PRIMARY KEY);
+INSERT INTO t2 VALUES (1), (2);
+EXPLAIN SELECT MIN(t2.a) FROM t2 LEFT JOIN t1 ON t2.a = t1.a;
+SELECT MIN(t2.a) FROM t2 LEFT JOIN t1 ON t2.a = t1.a;
+EXPLAIN SELECT MAX(t2.a) FROM t2 LEFT JOIN t1 ON t2.a = t1.a;
+SELECT MAX(t2.a) FROM t2 LEFT JOIN t1 ON t2.a = t1.a;
+DROP TABLE t1, t2;
+
+
 --echo #
 
 --echo # End of 5.1 tests