sql的sum函数输出（请详细回答，解决了立马采纳）

请问怎么才能把gr_totalfee的和在控制台输出出来

代码1：

select sum(gr_totalfee) as gr_totalfeevalue
from core_bill

代码2：
public interface BillMapper {
int deleteByPrimaryKey(String id);

int insert(Bill record);

int insertSelective(Bill record);

Bill selectByPrimaryKey(String id);

Bill selectByPrimaryKey1();

代码3：
@Autowired
BillMapper billMapper;

@RequestMapping("/toBill")
public String toBill() {
    //Bill admin =   billMapper.selectByPrimaryKey1();
    Bill a = billMapper.selectByPrimaryKey1();
    //System.out.println("自费:"+admin.getGrTotalfee());
    //System.out.println("a:"+a);
    System.out.println("a的值:"+a);
    return "index";
}


输出的结果：
INFO impl.AbstractPoolBackedDataSource - Initializing c3p0 pool... com.mchange.v2.c3p0.ComboPooledDataSource [ acquireIncrement -> 3, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, dataSourceName -> 1bqub5u9k1r7py4mnnjruz|7441cf0d, debugUnreturnedConnectionStackTraces -> false, description -> null, driverClass -> com.mysql.jdbc.Driver, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 1bqub5u9k1r7py4mnnjruz|7441cf0d, idleConnectionTestPeriod -> 0, initialPoolSize -> 3, jdbcUrl -> jdbc:mysql://rdsw49x2tpph3bfc8ldvo.mysql.rds.aliyuncs.com/ykf_test_yufabu?useUnicode=true&characterEncoding=utf-8, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 0, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 10, maxStatements -> 0, maxStatementsPerConnection -> 0, minPoolSize -> 2, numHelperThreads -> 3, preferredTestQuery -> SELECT 1 FROM dual, properties -> {user=******, password=******}, propertyCycle -> 0, statementCacheNumDeferredCloseThreads -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, userOverrides -> {}, usesTraditionalReflectiveProxies -> false ]

com.dabay.service.web.manager.persistence.dao.BillMapper.selectByPrimaryKey1:select sum(gr_totalfee) as gr_totalfeevalue from core_bill:1458ms
a的值:null
09-Dec-2016 21:44:51.292 信息 [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory E:\tomcat\webapps\manager
09-Dec-2016 21:44:51.336 信息 [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory E:\tomcat\webapps\manager has finished in 43 ms
com.dabay.service.web.manager.persistence.dao.BillMapper.selectByPrimaryKey1:select sum(gr_totalfee) as gr_totalfeevalue from core_bill:97ms
a的值:null
com.dabay.service.web.manager.persistence.dao.BillMapper.selectByPrimaryKey1:select sum(gr_totalfee) as gr_totalfeevalue from core_bill:124ms
a的值:null

如果你的bill 对象 有gr_totalfeevalue属性 可以syso（a.gr_totalfeevalue）

重点在于你的selectByPrimaryKey1这个方法的SQL，返回的记录为空。