計數獨特領域的MySQL

Question

我有一個這樣的表在MySQL：

dancer1 | dancer2 
------------------ 
jeff | Julia 
john | Megan 
jeff | Vanessa 

我應該很容易，但我怎麼算獨特的舞者的數量（在這種情況下5）這個問題看起來像在這裏questionned ：MySQL Counting Distinct Values on Multiple Columns，但並不完全

Answer 1

一個簡單的聯盟會給你一套所有的舞者：

select DISTINCT dancer1 as dancer from table 
union 
select DISTINCT dancer2 as dancer from table 

如果你想計數：

select count(all_dancers.dancer) from ( 
    select DISTINCT dancer1 as dancer from table 
    union 
    select DISTINCT dancer2 as dancer from table 
) all_dancers 

Answer 2

嘗試此查詢

select count(distinct dancer1,dancer2 ) from table 

Answer 3

SELECT COUNT(DISTINCT dancer) 
    FROM 
    (SELECT dancer1 dancer 
     FROM my_table 
     UNION 
     SELECT dancer2 
     FROM my_table 
    ) x;