Exists 和 in

SELECT
    *
FROM
    `user`
WHERE
    `user`.id IN (
        SELECT
            `order`.user_id
        FROM
            `order`
    )

List resultSet=[];
Array A=(select * from A); 
Array B=(select id from B);
for(int i=0;i<A.length;i++) {    
    for(int j=0;j<B.length;j++) {       
            if(A[i].id==B[j].id) {          
                    resultSet.add(A[i]);          
                    break;         
     }     
  } } 
return resultSet;

SELECT
    `user`.*
FROM
    `user`
WHERE
    EXISTS (
        SELECT
            `order`.user_id
        FROM
            `order`
        WHERE
            `user`.id = `order`.user_id
    )

List resultSet[];
Array A = (SELECT 'user.*' FROM  'user');
for(int i = 0; i < A.length(); ++i){
   if(EXIST(A[i])){
        resultSet.add(A[i]);
    }
    <=> SELECT 'order.user_id' FROM 'order' WHERE 'user.id' = 'order.user_id';
}