Java学习教程，从入门到精通，JDBC中WHERE子句的语法知识点及案例代码（107）

JDBC中WHERE子句的语法知识点及案例代码

JDBC WHERE子句语法知识点

WHERE子句用于在SQL查询中对数据进行筛选，它可以根据指定的条件过滤数据行。在JDBC中，WHERE子句通常用于SELECT、UPDATE和DELETE语句中。

常见的WHERE子句条件

1. 比较运算符
   • =：等于
   • <> 或 !=：不等于
   • >：大于
   • <：小于
   • >=：大于等于
   • <=：小于等于
2. 逻辑运算符
   • AND：逻辑与
   • OR：逻辑或
   • NOT：逻辑非
3. 特殊运算符
   • IN：用于指定多个可能的值
   • BETWEEN：用于指定一个范围
   • LIKE：用于模糊查询，常与通配符%（任意字符序列）和_（单个字符）一起使用
   • IS NULL：用于判断是否为NULL

注意事项

• 在WHERE子句中，字符串值需要用单引号'括起来。
• 对于IN子句，可以列出多个值，用逗号分隔。
• 在使用LIKE时，%和_是通配符，分别表示任意长度的字符序列和单个字符。
• 对于BETWEEN子句，范围是闭区间，即包括两端的值。

案例代码

以下是一个使用WHERE子句的JDBC代码示例。假设有一个数据库表students，包含以下字段：

• id（学生ID）
• name（学生姓名）
• age（学生年龄）
• score（学生成绩）

查询特定条件的学生信息

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class JdbcWhereExample {
    public static void main(String[] args) {
        // 数据库连接信息
        String url = "jdbc:mysql://localhost:3306/your_database_name"; // 数据库URL
        String user = "your_username"; // 数据库用户名
        String password = "your_password"; // 数据库密码
        // 查询语句
        String query = "SELECT * FROM students WHERE age > 18 AND score >= 80";
        try {
            // 加载数据库驱动
            Class.forName("com.mysql.cj.jdbc.Driver");
            // 建立连接
            Connection connection = DriverManager.getConnection(url, user, password);
            // 创建Statement对象
            Statement statement = connection.createStatement();
            // 执行查询
            ResultSet resultSet = statement.executeQuery(query);
            // 处理结果
            while (resultSet.next()) {
                int id = resultSet.getInt("id");
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");
                double score = resultSet.getDouble("score");

                // 打印结果
                System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age + ", Score: " + score);
            }
            // 关闭资源
            resultSet.close();
            statement.close();
            connection.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

使用LIKE进行模糊查询

// 查询语句
String query = "SELECT * FROM students WHERE name LIKE 'A%'";
// 执行查询
ResultSet resultSet = statement.executeQuery(query);
// 处理结果
while (resultSet.next()) {
    int id = resultSet.getInt("id");
    String name = resultSet.getString("name");
    int age = resultSet.getInt("age");
    double score = resultSet.getDouble("score");

    // 打印结果
    System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age + ", Score: " + score);
}

使用IN子句

// 查询语句
String query = "SELECT * FROM students WHERE id IN (1, 3, 5)";
// 执行查询
ResultSet resultSet = statement.executeQuery(query);
// 处理结果
while (resultSet.next()) {
    int id = resultSet.getInt("id");
    String name = resultSet.getString("name");
    int age = resultSet.getInt("age");
    double score = resultSet.getDouble("score");
    // 打印结果
    System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age + ", Score: " + score);
}

使用BETWEEN子句

// 查询语句
String query = "SELECT * FROM students WHERE score BETWEEN 70 AND 90";
// 执行查询
ResultSet resultSet = statement.executeQuery(query);
// 处理结果
while (resultSet.next()) {
    int id = resultSet.getInt("id");
    String name = resultSet.getString("name");
    int age = resultSet.getInt("age");
    double score = resultSet.getDouble("score");
    // 打印结果
    System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age + ", Score: " + score);
}

总结

• WHERE子句是SQL中非常重要的部分，用于根据条件筛选数据。
• 在JDBC中，可以通过Statement对象执行带有WHERE子句的SQL语句。
• 通过ResultSet对象可以获取查询结果，并逐行处理。
• 代码中需要注意数据库连接信息的正确性，以及SQL语句的语法正确性。

以下是一些更具体的JDBC WHERE子句的案例，涵盖不同的场景和条件，帮助你更好地理解和应用。

案例 1：查询年龄大于18岁的学生

假设我们有一个students表，包含以下数据：

id	name	age	score
1	Alice	17	85
2	Bob	19	78
3	Charlie	20	90
4	David	16	88

SQL语句

SELECT * FROM students WHERE age > 18;

JDBC代码

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

public class JdbcWhereExample {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/your_database_name";
        String user = "your_username";
        String password = "your_password";

        String query = "SELECT * FROM students WHERE age > 18";

        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            Connection connection = DriverManager.getConnection(url, user, password);
            Statement statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(query);

            while (resultSet.next()) {
                int id = resultSet.getInt("id");
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");
                double score = resultSet.getDouble("score");

                System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age + ", Score: " + score);
            }

            resultSet.close();
            statement.close();
            connection.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

输出结果

ID: 2, Name: Bob, Age: 19, Score: 78.0
ID: 3, Name: Charlie, Age: 20, Score: 90.0

案例 2：查询名字以“A”开头的学生

假设students表数据如下：

id	name	age	score
1	Alice	17	85
2	Bob	19	78
3	Charlie	20	90
4	Andrew	16	88

SQL语句

SELECT * FROM students WHERE name LIKE 'A%';

JDBC代码

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

public class JdbcWhereExample {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/your_database_name";
        String user = "your_username";
        String password = "your_password";

        String query = "SELECT * FROM students WHERE name LIKE 'A%'";

        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            Connection connection = DriverManager.getConnection(url, user, password);
            Statement statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(query);

            while (resultSet.next()) {
                int id = resultSet.getInt("id");
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");
                double score = resultSet.getDouble("score");

                System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age + ", Score: " + score);
            }

            resultSet.close();
            statement.close();
            connection.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

输出结果

ID: 1, Name: Alice, Age: 17, Score: 85.0
ID: 4, Name: Andrew, Age: 16, Score: 88.0

案例 3：查询成绩在70到90之间的学生

假设students表数据如下：

id	name	age	score
1	Alice	17	85
2	Bob	19	78
3	Charlie	20	90
4	David	16	65

SQL语句

SELECT * FROM students WHERE score BETWEEN 70 AND 90;

JDBC代码

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

public class JdbcWhereExample {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/your_database_name";
        String user = "your_username";
        String password = "your_password";

        String query = "SELECT * FROM students WHERE score BETWEEN 70 AND 90";

        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            Connection connection = DriverManager.getConnection(url, user, password);
            Statement statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(query);

            while (resultSet.next()) {
                int id = resultSet.getInt("id");
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");
                double score = resultSet.getDouble("score");

                System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age + ", Score: " + score);
            }

            resultSet.close();
            statement.close();
            connection.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

输出结果

ID: 1, Name: Alice, Age: 17, Score: 85.0
ID: 2, Name: Bob, Age: 19, Score: 78.0
ID: 3, Name: Charlie, Age: 20, Score: 90.0

案例 4：查询名字为Alice或Bob的学生

假设students表数据如下：

id	name	age	score
1	Alice	17	85
2	Bob	19	78
3	Charlie	20	90
4	David	16	65

SQL语句

SELECT * FROM students WHERE name IN ('Alice', 'Bob');

JDBC代码

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

public class JdbcWhereExample {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/your_database_name";
        String user = "your_username";
        String password = "your_password";
        String query = "SELECT * FROM students WHERE name IN ('Alice', 'Bob')";
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            Connection connection = DriverManager.getConnection(url, user, password);
            Statement statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(query);

            while (resultSet.next()) {
                int id = resultSet.getInt("id");
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");
                double score = resultSet.getDouble("score");

                System.out.println("ID: " + id + ", Name: " + name + ", Age: " + age + ", Score: " + score);
            }

            resultSet.close();
            statement.close();
            connection.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

输出结果

ID: 1, Name: Alice, Age: 17, Score: 85.0
ID: 2, Name: Bob, Age: 19, Score: 78.0