用Java理解sql的语法

select 相当于 for 循环
select id from IDArray

LinkedList a = new LinkedList();
for ( int i=0 ; i<tableA.length ; i++){
    a.add(IDArray.get("id" ) );
}

return a;

当执行子查询时，可以理解为
select id, ( select name from nameArray) as names ,from Idarray

LinkedList a = new LinkedList();
for ( int i=0 ; i<tableA.length ; i++){
    a.add(IDArray.get("id" ) );
    for(int j=0 ; j<nameArray.length ; j++){
       a.add(namesArray.get("name"));  
   }
}

很明显这里一对多了。所以执行不对。报SQL错误

所以在 select中，应该要具体确定某一个值
select id, ( select name from nameArray where nameId=id) as names from Idarray

LinkedList a = new LinkedList();
for ( int i=0 ; i<tableA.length ; i++){
   Object object =  IDArray.get("id" ) ;
  for ( int j=0 ; j<nameArray.length ; i++){
     if ( nameArray.get("nameId")==object){
       Object object2 = nameArray.get("nameId");
       a.add(object2);  
    }
}
       a.add(object);  
}

当且仅当if 条件的值搜索是单个值的时候，是对应上的

那么groupBy呢？

select id, ( select name from nameArray where nameId=id) as names from Idarray group by id

LinkedList a = new LinkedList();
for ( int i=0 ; i<tableA.length ; i++){
   Object object =  IDArray.get("id" ) ;
  for ( int j=0 ; j<nameArray.length ; i++){
     if ( nameArray.get("nameId")==object){
       Object object2 = nameArray.get("nameId");
       a.add(object2);  
    }
}
       a.add(object);  
}

LinkedList groupByArray = new LinkedList();

for ( int i=0 ; i< a. length  ; i++){
      int id = a[i].getId();
     LinkedList tempArray = new LinkedList();
     for ( int j=0 ; j<j.length ; j++){
       int temp = a[j].getId();
      if ( id==temp){
         tempArray.add(temp);
        }
    }
   groupByArray.add(tempArray);
}