SQL中count(1)、count(＊) 与 count(列名)的区别详细解释

　　import java.sql.Connection;

　　import java.sql.DriverManager;

　　import java.sql.Statement;

　　import java.sql.ResultSet;

　　import java.sql.SQLException;

　　public class CountExample {

　　private static final String JDBC_URL = "jdbc:mysql://localhost:3306/yourdatabase";

　　private static final String JDBC_USER = "yourusername";

　　private static final String JDBC_PASSWORD = "yourpassword";

　　public static void main(String[] args) {

　　try (Connection conn = DriverManager.getConnection(JDBC_URL, JDBC_USER, JDBC_PASSWORD);

　　Statement stmt = conn.createStatement()) {

　　// 使用 COUNT(1)

　　String count1SQL = "SELECT COUNT(1) AS total FROM employees";

　　ResultSet rs1 = stmt.executeQuery(count1SQL);

　　if (rs1.next()) {

　　int total1 = rs1.getInt("total");

　　System.out.println("Total rows (COUNT(1)): " + total1);

　　}

　　// 使用 COUNT(*)

　　String countAllSQL = "SELECT COUNT(*) AS total FROM employees";

　　ResultSet rsAll = stmt.executeQuery(countAllSQL);

　　if (rsAll.next()) {

　　int totalAll = rsAll.getInt("total");

　　System.out.println("Total rows (COUNT(*)): " + totalAll);

　　}

　　// 使用 COUNT(column)

　　String countColumnSQL = "SELECT COUNT(department_id) AS total FROM employees";

　　ResultSet rsColumn = stmt.executeQuery(countColumnSQL);

　　if (rsColumn.next()) {

　　int totalColumn = rsColumn.getInt("total");

　　System.out.println("Total rows (COUNT(department_id)): " + totalColumn);

　　}

　　} catch (SQLException e) {

　　e.printStackTrace();

　　}

　　}

　　}