Stars Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)Total Submission(s): 15951 Accepted Submission(s): 5945 Problem Description Astronomers often examine star maps where stars are represented by points on a pla
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