SQL UNION 操作符

SQL UNION 操作符( 默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL)

select 1 as id,1 as userid, '一班' as class
union
select 1 as id,1 as userid, '一班' as class
union
select 4 as id,2 as userid, '二班' as class
union
select 5 as id,2 as userid, '二班' as class