MDEV-12143 sql_mode=ORACLE: make the CONCAT function ignore NULL arguments

mysql-test/suite/compat/oracle/r/func_concat.result

@@ -4,6 +4,11 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
 Warnings:
 Note	1003	select 'a' || 'b' || 'c' AS "'a'||'b'||'c'"
+EXPLAIN EXTENDED SELECT CONCAT('a'||'b'||'c');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+Warnings:
+Note	1003	select 'a' || 'b' || 'c' AS "CONCAT('a'||'b'||'c')"
 SELECT ''   || '';
 ''   || ''
 
@@ -169,4 +174,33 @@ NULL	NULL
 2	ab
 2	ab
 3	abc
+SELECT LENGTH(CONCAT(a||b||c)), CONCAT(a||b||c) FROM t1 ORDER BY a,b,c;
+LENGTH(CONCAT(a||b||c))	CONCAT(a||b||c)
+NULL	NULL
+0	
+1	c
+0	
+0	
+1	c
+1	b
+1	b
+2	bc
+0	
+0	
+1	c
+0	
+0	
+1	c
+1	b
+1	b
+2	bc
+1	a
+1	a
+2	ac
+1	a
+1	a
+2	ac
+2	ab
+2	ab
+3	abc
 DROP TABLE t1;

mysql-test/suite/compat/oracle/t/func_concat.test

@@ -5,6 +5,7 @@
 SET sql_mode=ORACLE;
 
 EXPLAIN EXTENDED SELECT 'a'||'b'||'c';
+EXPLAIN EXTENDED SELECT CONCAT('a'||'b'||'c');
 
 SELECT ''   || '';
 SELECT ''   || 'b';
@@ -79,5 +80,6 @@ INSERT INTO t1 VALUES (NULL, NULL, 'c');
 INSERT INTO t1 VALUES (NULL, NULL, NULL);
 
 SELECT LENGTH(a||b||c), a||b||c FROM t1 ORDER BY a,b,c;
+SELECT LENGTH(CONCAT(a||b||c)), CONCAT(a||b||c) FROM t1 ORDER BY a,b,c;
 
 DROP TABLE t1;

sql/item_create.cc

@@ -3857,7 +3857,9 @@ Create_func_concat::create_native(THD *thd, LEX_STRING name,
     return NULL;
   }
 
-  return new (thd->mem_root) Item_func_concat(thd, *item_list);
+  return thd->variables.sql_mode & MODE_ORACLE ?
+    new (thd->mem_root) Item_func_concat_operator_oracle(thd, *item_list) :
+    new (thd->mem_root) Item_func_concat(thd, *item_list);
 }
 
 Create_func_decode_histogram Create_func_decode_histogram::s_singleton;

sql/item_strfunc.h

@@ -297,6 +297,9 @@ class Item_func_concat :public Item_str_func
 class Item_func_concat_operator_oracle :public Item_func_concat
 {
 public:
+  Item_func_concat_operator_oracle(THD *thd, List<Item> &list)
+   :Item_func_concat(thd, list)
+  { }
   Item_func_concat_operator_oracle(THD *thd, Item *a, Item *b)
    :Item_func_concat(thd, a, b)
   { }