关于hql中部分关联字段为空时，不报错而直接查不出字段为空的记录

 select b.listNo,b.appGNo,b.trGno,b.complex,b.commName,b.commSpec,b.tradeUnit,b.tradeQty,b.unit,b.qty,b.note,                 c.listNo,c.appGNo,c.trGno,c.complex,c.commName,c.commSpec,c.tradeUnit,c.tradeQty,c.unit,c.qty,c.note,                 a.companyCode,a.companyName,a.billNo,                  a.seqNo,a.appNo,a.declareState ,a.messageIndentureType                  from  MessageIndentureHead a , KHMessageIndenture b , GYSMessageIndenture c  where a.id = c.messageIndentureHead.id and a.id = b.messageIndentureHead.id                  and ((a.messageIndentureType = 1 and b.listNo = c.outNo)                  or (a.messageIndentureType = 0 and c.listNo = b.outNo))

如上hql代码，查询，会出现先TradeUnit为字段空的记录将不会被查询出来。（MessageIndentureHead拥有对tradeunit的unit表的单向一对多关联）

注意到hibernate根据hql输出的sql如下：

select khmessagei1_.listNo as col_0_0_, khmessagei1_.appGNo as col_1_0_, khmessagei1_.trGno as col_2_0_, khmessagei1_.complex as col_3_0_, khmessagei1_.commName as col_4_0_, khmessagei1_.commSpec as col_5_0_, khmessagei1_.tradeUnit_code as col_6_0_, khmessagei1_.tradeQty as col_7_0_, khmessagei1_.unit_code as col_8_0_, khmessagei1_.qty as col_9_0_, khmessagei1_.note as col_10_0_, gysmessage2_.listNo as col_11_0_, gysmessage2_.appGNo as col_12_0_, gysmessage2_.trGno as col_13_0_, gysmessage2_.complex as col_14_0_, gysmessage2_.commName as col_15_0_, gysmessage2_.commSpec as col_16_0_, gysmessage2_.tradeUnit_code as col_17_0_, gysmessage2_.tradeQty as col_18_0_, gysmessage2_.unit_code as col_19_0_, gysmessage2_.qty as col_20_0_, gysmessage2_.note as col_21_0_, messageind0_.companyCode as col_22_0_, messageind0_.companyName as col_23_0_, messageind0_.billNo as col_24_0_, messageind0_.seqNo as col_25_0_, messageind0_.appNo as col_26_0_, messageind0_.declareState as col_27_0_, messageind0_.messageIndentureType as col_28_0_, unit3_.code as code90_0_, unit4_.code as code90_1_, unit5_.code as code90_2_, unit6_.code as code90_3_, unit3_.isOut as isOut90_0_, unit3_.modifyDate as modifyDate90_0_, unit3_.modifyUser as modifyUser90_0_, unit3_.name as name90_0_, unit4_.isOut as isOut90_1_, unit4_.modifyDate as modifyDate90_1_, unit4_.modifyUser as modifyUser90_1_, unit4_.name as name90_1_, unit5_.isOut as isOut90_2_, unit5_.modifyDate as modifyDate90_2_, unit5_.modifyUser as modifyUser90_2_, unit5_.name as name90_2_, unit6_.isOut as isOut90_3_, unit6_.modifyDate as modifyDate90_3_, unit6_.modifyUser as modifyUser90_3_, unit6_.name as name90_3_ from MessageIndentureHead messageind0_ cross join KHMessageIndenture khmessagei1_ inner join Unit unit3_ on khmessagei1_.tradeUnit_code=unit3_.code inner join Unit unit4_ on khmessagei1_.unit_code=unit4_.code cross join GYSMessageIndenture gysmessage2_ inner join Unit unit5_ on gysmessage2_.tradeUnit_code=unit5_.code inner join Unit unit6_ on gysmessage2_.unit_code=unit6_.code where messageind0_.id=gysmessage2_.messageIndentureHead_id and messageind0_.id=khmessagei1_.messageIndentureHead_id and (messageind0_.messageIndentureType=1 and khmessagei1_.listNo=gysmessage2_.outNo or messageind0_.messageIndentureType=0 and gysmessage2_.listNo=khmessagei1_.outNo)

注意到hibernate输出的sql是对unit表使用内连接的，这就解释了为何该字段为空的记录不会输出。

原因：

使用a.xx，去查出a关联的表中的数据会导致隐式是用内连接（原因据说是与‘n+1’问题有关）

网上找的资料：

对于Hibernate3.2.3以后的版本，如果关联实体是单个实体或单个的组件属性，HQL依然可以似乎用英文点号（.）来隐式连接关联实体或组件；但如果关联实体是集合（包括1-N关联、N-N关联和集合元素时组件等），则必须使用xxx join来显示连接关联实体或组件。

解决办法：

显示声明为外连接连接该些字段

 select b.listNo,b.appGNo,b.trGno,b.complex,b.commName,b.commSpec,uc,b.tradeQty,ud,b.qty,b.note, c.listNo,c.appGNo,c.trGno,c.complex,c.commName,c.commSpec,ua,c.tradeQty,ub,null,c.qty,c.note, a.companyCode,a.companyName,a.billNo,  a.seqNo,a.appNo,a.declareState ,a.messageIndentureType  from  MessageIndentureHead a , KHMessageIndenture b , GYSMessageIndenture cleft join c.tradeUnit ua left join c.unit ub left join b.tradeUnit uc left join b.unit ud  where a.id = c.messageIndentureHead.id and a.id = b.messageIndentureHead.id