Use Java to understand SQL syntax, and use Java to understand SQL syntax

Use Java to understand SQL syntax, and use Java to understand SQL syntax

Select is equivalent to for Loop
Select id from IDArray

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

When executing a subquery, it can be understood
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"));     }}

It is obvious that there is a one-to-many relationship. So the execution is incorrect. SQL error reported

Therefore, in select, you must determine a specific value.
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);  }

When and only when the value of the if condition is a single value, it corresponds

What about groupBy?

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);}

Join the study exchange group 569772982 to learn and exchange.