计数唯一字段 mysql

Question

我在 mysql 中有这样一个 table:

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

我应该很简单，但是我如何计算唯一舞者的数量（在本例中为 5）问题看起来像在这里被质疑：MySQL Counting Distinct Values on Multiple Columns，但不完全是

Answer 1

一个简单的联合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

如果你想要计数：

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

试试这个查询

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;

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