java - 为什么 PostgreSQL 查询后部分 Java 代码没有执行？

Question

我正在编写一个生成随机数据并用它填充 PostgreSQL 表的程序。您会发现我尝试执行三个简单 SQL 查询的代码。前两个查询效果很好，但第三个查询效果不佳。实际上，我的代码的一部分似乎在第二次查询后根本没有执行，因为控制台中没有打印“yes”。

我尝试通过注释掉第二个查询执行行来编译我的代码，然后执行代码的末尾。有什么想法吗？

import java.sql.*;

public class Main {

    public static void main(String[] args) {

        //connection to DB
        Class.forName("org.postgresql.Driver");
        Connection con = DriverManager.getConnection("jdbc:postgresql://localhost:5432/Benerator","postgres","newPassword");

        //print the first two columns of table bank_card_people
        PreparedStatement stmt = con.prepareStatement("select * from public.bank_card_people");
        ResultSet res = stmt.executeQuery();
        while(res.next()){
            System.out.println(res.getString(1)+ " " + res.getString(2));}

        //add a line to the same table
        String SQL = "insert into public.bank_card_people (\"first-name\", \"last-name\", \"card-number\") VALUES ('example','example','example')";
        PreparedStatement stmt2 = con.prepareStatement(SQL);
        stmt2.executeQuery();

        // is supposed to print all the databases
        PreparedStatement stmt3 = con.prepareStatement("SELECT datname FROM pg_database WHERE datistemplate = false");
        ResultSet res2 = stmt3.executeQuery();
        System.out.println("yes");
        while(res2.next()){
            System.out.println(res2.getString(1));}

这是输出:

User One
User Two
User Three
example example
example example
No results were returned by the query.

这是我注释掉该行时的输出:stmt2.executeQuery();

User One
User Two
User Three
example example
example example
yes
postgres
Benerator

Answer 1

INSERT 语句不能由 executeQuery 执行，而是由 executeUpdate 执行。

此外，Connection、Statement 和 ResultSet 必须是 .close()d，最好使用 try-with-resources 来完成，这样即使出现异常或返回也能保证关闭。

它还有助于命名，因为它引入了新 block 。

    //connection to DB
    Class.forName("org.postgresql.Driver");
    try (Connection con = DriverManager.getConnection("jdbc:postgresql://localhost:5432/Benerator","postgres","newPassword")) {

        //print the first two columns of table bank_card_people
        try (PreparedStatement stmt = con.prepareStatement("select * from public.bank_card_people");
                ResultSet res = stmt.executeQuery()) {
            while (res.next()) {
                System.out.println(res.getString(1)+ " " + res.getString(2));
            }
        }

        //add a line to the same table
        String sql = "insert into public.bank_card_people (\"first-name\", \"last-name\", \"card-number\") VALUES ('example','example','example')";
        try (PreparedStatement stmt2 = con.prepareStatement(sql)) {
            int updateCount = stmt2.executeUpdate();
        }

        // is supposed to print all the databases
        try (PreparedStatement stmt3 = con.prepareStatement("SELECT datname FROM pg_database WHERE datistemplate = false"));
                ResultSet res2 = stmt3.executeQuery()) {
            System.out.println("yes");
            while(res2.next()){
                System.out.println(res2.getString(1));
            }
        }
    }