class_A 資料表
s_id | name | gender | age
------------- | -------------
1 | Amy | female | 18
2 | John | male | 54
3 | David | male | 32
4 | Joyce | female |44
5 | Joe | male | 10
6 | Iris | female | 10

class_B 資料表
s_id | name | gender | age
------------- | -------------
1 | Cindy | female | 11
2 | Jack | male | 5
3 | John | male | 12
4 | Alice | female | 33
5 | Joe | male | 49

Union

• 將資料表的column合併起來。其合併的資料型別需相同，若是需要合併多個column時，下SQL時各個column順序也要對應正確。
• 不同table中，若是有相同的record，結果只會出現一條record
  Union vs. Join
  Union:做垂直合併，將不同資料表的column合併到一起
  Join:做水平合併，將不同資料表的row合併到一起

select name as students  from class_A 
union
select name from class_B

結果：

Union all

Union會把重複的record去除，而Union all則是將所有資料保留下來

select name as students  from class_A 
union
select name from class_B

結果：