分组后,统计记录条数:

  SELECT num,count(*) AS counts from test_a GROUP BY num; 

  查询结果如下:

  

 

对num去重后的数量的统计:

  SELECT count(t.counts) FROM ( SELECT num,count(*) AS counts from test_a GROUP BY num ) AS t;

  SELECT count(DISTINCT num) AS count FROM test_a;

  它俩结果一样,都是5;只是一个是子查询(嵌套),一个是内置函数 distinct();

 

 

数据库结构

 

posted on 2016-11-20 00:29  啥也不会的程序猿  阅读(46107)  评论(0编辑  收藏  举报