SET sql_mode=ORACLE;
EXPLAIN EXTENDED SELECT '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 "'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 ''   || '';
''   || ''

SELECT ''   || 'b';
''   || 'b'
b
SELECT ''   || NULL;
''   || NULL

SELECT 'a'  || '';
'a'  || ''
a
SELECT 'a'  || 'b';
'a'  || 'b'
ab
SELECT 'a'  || NULL;
'a'  || NULL
a
SELECT NULL || '';
NULL || ''

SELECT NULL || 'b';
NULL || 'b'
b
SELECT NULL || NULL;
NULL || NULL
NULL
SELECT ''   || ''    || '';
''   || ''    || ''

SELECT ''   || ''    || 'c';
''   || ''    || 'c'
c
SELECT ''   || ''    || NULL;
''   || ''    || NULL

SELECT ''   || 'b'   || '';
''   || 'b'   || ''
b
SELECT ''   || 'b'   || 'c';
''   || 'b'   || 'c'
bc
SELECT ''   || 'b'   || NULL;
''   || 'b'   || NULL
b
SELECT ''   || NULL  || '';
''   || NULL  || ''

SELECT ''   || NULL  || 'c';
''   || NULL  || 'c'
c
SELECT ''   || NULL  || NULL;
''   || NULL  || NULL

SELECT 'a'  || ''    || '';
'a'  || ''    || ''
a
SELECT 'a'  || ''    || 'c';
'a'  || ''    || 'c'
ac
SELECT 'a'  || ''    || NULL;
'a'  || ''    || NULL
a
SELECT 'a'  || 'b'   || '';
'a'  || 'b'   || ''
ab
SELECT 'a'  || 'b'   || 'c';
'a'  || 'b'   || 'c'
abc
SELECT 'a'  || 'b'   || NULL;
'a'  || 'b'   || NULL
ab
SELECT 'a'  || NULL  || '';
'a'  || NULL  || ''
a
SELECT 'a'  || NULL  || 'c';
'a'  || NULL  || 'c'
ac
SELECT 'a'  || NULL  || NULL;
'a'  || NULL  || NULL
a
SELECT NULL || ''    || '';
NULL || ''    || ''

SELECT NULL || ''    || 'c';
NULL || ''    || 'c'
c
SELECT NULL || ''    || NULL;
NULL || ''    || NULL

SELECT NULL || 'b'   || '';
NULL || 'b'   || ''
b
SELECT NULL || 'b'   || 'c';
NULL || 'b'   || 'c'
bc
SELECT NULL || 'b'   || NULL;
NULL || 'b'   || NULL
b
SELECT NULL || NULL  || '';
NULL || NULL  || ''

SELECT NULL || NULL  || 'c';
NULL || NULL  || 'c'
c
SELECT NULL || NULL  || NULL;
NULL || NULL  || NULL
NULL
CREATE TABLE t1 (a VARCHAR(10), b VARCHAR(10), c VARCHAR(10));
INSERT INTO t1 VALUES ('',   '',   '');
INSERT INTO t1 VALUES ('',   '',   'c');
INSERT INTO t1 VALUES ('',   '',   NULL);
INSERT INTO t1 VALUES ('',   'b',  '');
INSERT INTO t1 VALUES ('',   'b',  'c');
INSERT INTO t1 VALUES ('',   'b',  NULL);
INSERT INTO t1 VALUES ('',   NULL, '');
INSERT INTO t1 VALUES ('',   NULL, 'c');
INSERT INTO t1 VALUES ('',   NULL, NULL);
INSERT INTO t1 VALUES ('a',  '',   '');
INSERT INTO t1 VALUES ('a',  '',   'c');
INSERT INTO t1 VALUES ('a',  '',   NULL);
INSERT INTO t1 VALUES ('a',  'b',  '');
INSERT INTO t1 VALUES ('a',  'b',  'c');
INSERT INTO t1 VALUES ('a',  'b',  NULL);
INSERT INTO t1 VALUES ('a',  NULL, '');
INSERT INTO t1 VALUES ('a',  NULL, 'c');
INSERT INTO t1 VALUES ('a',  NULL, NULL);
INSERT INTO t1 VALUES (NULL, '',   '');
INSERT INTO t1 VALUES (NULL, '',   'c');
INSERT INTO t1 VALUES (NULL, '',   NULL);
INSERT INTO t1 VALUES (NULL, 'b',  '');
INSERT INTO t1 VALUES (NULL, 'b',  'c');
INSERT INTO t1 VALUES (NULL, 'b',  NULL);
INSERT INTO t1 VALUES (NULL, NULL, '');
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;
LENGTH(a||b||c)	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
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;