drop table if exists t1;
set sql_mode="MySQL40";
select @@sql_mode;
@@sql_mode
NO_FIELD_OPTIONS,MYSQL40,HIGH_NOT_PRECEDENCE
set @@sql_mode="ANSI";
select @@sql_mode;
@@sql_mode
REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,ANSI
SELECT 'A' || 'B';
'A' || 'B'
AB
CREATE TABLE t1 (id INT, id2 int);
SELECT id,NULL,1,1.1,'a' FROM t1 GROUP BY id;
id	NULL	1	1.1	a
SELECT id FROM t1 GROUP BY id2;
id
drop table t1;
SET @@SQL_MODE="";