急急，表的栏位合并

Question

一張表里的兩個欄位和並後，是否可以使里面重復的數據不丟失
   例如表b
                        type_1                type_2
                           a                          b
                           a                          b
                           c
                           c
希望合併後得到
                         type
                           a
                          a
                           c
                          c
                          b
                         b
但是我用 select  b.type_1 as type union select b.type_2 as  type
 得到的是
                     type
                       a
                       b
                       c
請指教，

Answer 1

select  b.type_1 union select b.type_2