计算mysql的唯一字段

Question

我在mysql中有一个这样的表：

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

我应该很容易，但我如何计算独特舞者的数量（在这种情况下为5）这个问题在这里有问题：MySQL Counting Distinct Values on Multiple Columns，但不完全

Answer 1

一个简单的联盟会给你一组所有的舞者：

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

如果你想要数：

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

尝试此查询

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;