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
