mybatis中的count()按条件查询方式

select count(column_name) from table_name

select count(*) from table_name

select count(distinct column_name) from table_name

+-----+---------+-------+------------+
| aid | site_id | count | date       |
+-----+---------+-------+------------+
|   1 |       1 |    45 | 2016-05-10 |
|   2 |       3 |   100 | 2016-05-13 |
|   3 |       1 |   230 | 2016-05-14 |
|   4 |       2 |    10 | 2016-05-14 |
|   5 |       5 |   205 | 2016-05-14 |
|   6 |       4 |    13 | 2016-05-15 |
|   7 |       3 |   220 | 2016-05-15 |
|   8 |       5 |   545 | 2016-05-16 |
|   9 |       3 |   201 | 2016-05-17 |
+-----+---------+-------+------------+

//特定条件下指定列的数目
select count(count) as nums from table_aid
where site_id = 3
//输出结果：nums值为：521
 
//计算table_aid中总记录数
select count(*) as nums from table_aid
//输出结果：nums值为：9
 
//指定列的不同值的数目
select count(distinct site_id) as nums from table_aid
//输出结果：nums值为：5

 <resultMap id="GroupBaseResultMap" type="java.util.Map">
    <result column="station" jdbcType="VARCHAR" property="station" />
    <result column="successNum" jdbcType="VARCHAR" property="successNum" />
    <result column="totalNum" jdbcType="VARCHAR" property="totalNum"/>
  </resultMap>
  <!--任务统计:按站点    sum(if(type="fail",status,0))-->
  <select id="selectGroupByStation" resultMap="GroupBaseResultMap">
    select rfr.station, count(rfr.status='success' or null) as successNum, count(rfr.status='fail' or null) as failNum, count(1) as totalNum
    from rec_file_report rfr
    GROUP BY station
  </select>