Java: Cannot use the "USE" keyword from MySQL?

Huy :

I have two SQL files:

query1.sql

SELECT * FROM laptop_store.gpu;

query2.sql

USE laptop_store;
SELECT * FROM gpu

Excecuting both in MySQL Workbench 8.0 CE will display the same result :

When I copy everything from two SQL code and run it in java

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;

public class NewClass {
    static String queryString1 = "SELECT * FROM laptop_store.gpu";

    static String queryString2 = "USE laptop_store;\n" +
                                 "SELECT * FROM gpu";

    public static void main(String[] args) {
       try{      
          Class.forName("com.mysql.cj.jdbc.Driver");  
          Connection con = DriverManager.getConnection( 
              "jdbc:mysql://localhost:3306/laptop_store","root","tomnisa123");          

          Statement statement = con.createStatement();   

          //Change SQL code here:
          ResultSet rs = statement.executeQuery(queryString1); 

          ResultSetMetaData rsmd = rs.getMetaData();

          int colCount = rsmd.getColumnCount();

            while(rs.next()) {
                for (int i = 1; i <= colCount; i++){          
                    System.out.print(rs.getString(i) + "  ");
                }
                System.out.println();
            }

          con.close();  

       } catch(Exception e){ 
        System.out.println(e);
       }  
    }  
}

Only the first one is success

.

But the second one shows an error:

java.sql.SQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT * FROM gpu' at line 2

Why I cannot use the "USE" keyword from MySQL?

Stephen C :

Apparently, the problem is that you are trying to execute two SQL statements in a single executeQuery call. You can't do that. Run the "use" in a separate execute calls.

But the question is why are you using the use statement?

• If you are using it for no particular reasons ... don't.

• If you are using it to make sure that you are using the correct database, that is unnecessary. The database / schema to be used is specified in the connection URL.

   "jdbc:mysql://localhost:3306/laptop_store"
                                ^^^^^^^^^^^^
  

• If you are using it to dynamically switch between databases / schemas, it may be better to use a separate connection for each database.