Java-MySql：查询以获取列的所有值

Question

SELECT col_name1, col_name2, col_name3 from Table_Name;

I am using above SQL query and I have to get all the values in col_name3 which has more than 5 value.

I am using :

while(rs.next()){
    String value1 = rs.getString("col_name3");
    String value2 = rs.getString("col_name3");
}

But all value in the String value1 and value2 is the last value of the col_name3. Why?

Answer 1

This problem is because if you need to store all of the data you can't do it with only a 'String' object but with a Collection maybe so you will do:

List<String> list = new ArrayList();
while(rs.next()){
    String value=rs.getString("col_name3");
    list.add(value);
}

Because as are you doing you store every time a new String in value1 and value2, overwriting the previous values so you will get only the last one.

With using a Collection you can dynamically store 'String' that you get in 'ResultSet' and save them all.

Then to operate on them you can use a forEach like this:

for(String value : list){
    System.out.prinln(value);
}

This will show in console every value in 'list'.

Answer 2

If you want to get only the result of one columns then you can use a List<String>

List<String> list = new ArrayList<>();

while(rs.next()){
   list.add(rs.getString("col_name3"));
}

If you want to get all your columns then you need to change the type of your list to your Object :

List<MyObject> list = new ArrayList<>();

while(rs.next()){
   list.add(new MyObject(rs.getString("col_name1"), rs.getString("col_name2"), rs.getString("col_name3")));
}

You can create a class that contain all the columns of your Object for example :

Class MyObject{
   private String col1;
   private String col2;
   private String col3;

   //Constructor
   public MyObject(String col1, String col2, String col3){
      this.col1 = col1;
      this.col2 = col2;       
      this.col3 = col3
   }

   //getter and setters
}

EDIT

If you want to show your information you have two ways first like @Mick Mnemonic said in comment override toString() in your class MyObject like this :

@Override
public String toString() {
    return "MyObject{" + "col1=" + col1 + ", col2=" + col2 + ", col3=" + col3 + '}';
}

and your loop should look like this :

for(int i = 0; i<list.size; i++){
    System.out.println(list.get(i));
}

Or you can get element by element :

for(int i = 0; i<list.size; i++){
    System.out.println(list.get(i).getCol1() + " " + list.get(i).getCol2());
}

Answer 3

because every time you overwrite previous value. Try using a List:

List<string> myValues=new List<string>();
while(rs.next()){
    myValues.Add(rs.getString("col_name3"));   
}