Vfp9与sql数据库连通后，下面语句中rq3、rq4前为何要加?写才对,该怎么解决

Vfp9与sql数据库连通后，下面语句中rq3、rq4前为何要加?写才对
Vfp9与sql数据库连通后，下面语句中rq3、rq4前为何要加?写才对

rq3=DTOC(rq1,1) &&日期转字符,格式20090101
rq4=DTOC(rq2,1) &&日期转字符
ss1=sqlexec(nhandle,"select * from history where CONVERT(varchar(12),xdate,112)>=?rq3 and CONVERT(varchar(12),xdate,112)<=?rq4 and LEFT(xcode,4)=5301 ","history")

?rq3 ?rq4前删除?,语名无反应.虽然我能从服务器取数了,但想知道原因.
多谢各位老师指教


------解决方案--------------------------------------------------------
在VFP中，传入变量前面要加？

or

ss1=sqlexec(nhandle,"select * from history where CONVERT(varchar(12),xdate,112)>="+rq3+" and CONVERT(varchar(12),xdate,112) <="+rq4+"and LEFT(xcode,4)='5301' ","history")
------解决方案--------------------------------------------------------

探讨

在VFP中，传入变量前面要加？

or

ss1=sqlexec(nhandle,"select * from history where CONVERT(varchar(12),xdate,112)>="+rq3+" and  CONVERT(varchar(12),xdate,112) <="+rq4+"and LEFT(xcode,4)='5301' ","history")

------解决方案--------------------------------------------------------
ss1=sqlexec(nhandle,"select * from history where CONVERT(varchar(12),xdate,112)>='" + rq3+ "' and CONVERT(varchar(12),xdate,112) <='" + rq4+ "' and LEFT(xcode,4)=5301 ","history")
------解决方案--------------------------------------------------------
ss1=sqlexec(nhandle,"select * from history where CONVERT(varchar(12),xdate,112)>='"+rq3+"' and CONVERT(varchar(12),xdate,112) <='"+rq4+"' and LEFT(xcode,4)='5301' ","history")