sql语句，一个id对应多条数据，如何转换为一条？

Question

数据如下：
ID SEG MMM NNN
123 1 A AA
123 2 B BB
123 3 C CC
我想转换成下面的格式：
ID MMM_1 NNN_1 MMM_2 NNN_2 MMM_3 NNN_3
123 A AA B BB C CC

MMM_1，NNN_1就是SEG=1时的MMM，NNN，2和3类似。
请问怎么通过sql语句实现？

Answer 1

/*
create temporary table Tmp(ID int,SEG int, MMM varchar(10), NNN varchar(10));
insert into Tmp value
(123, 1, 'A', 'AA'),
(123, 2, 'B', 'BB'),
(123, 3, 'C', 'CC');
*/
set @sql='';
select group_concat('max(case when SEG=',SEG,' then MMM else '''' end) as ', 'MMM_',SEG,',max(case when SEG=',SEG,' then NNN else '''' end) as NNN_',SEG,'' ) into @sql from Tmp;
set @sql=concat('SELECT ID,',@sql,' from Tmp group by ID');

prepare stmt from @sql;

execute stmt ;

DEALLOCATE PREPARE stmt;

/*

ID, MMM_1, NNN_1, MMM_2, NNN_2, MMM_3, NNN_3

'123', 'A', 'AA', 'B', 'BB', 'C', 'CC'

*/