关于联合查询

老师.这种是联合查询吗?

可以这样去查询的吗?之前课程有教吗?

我记得之前教的联合查询是:

UNION和UNION ALL的呀

例如:

SELECT * FROM tb1 UNION SELECT * FROM tb2

和

SELECT * FROM tb1 UNION ALL SELECT * FROM tb2

另外,在看课程之后,我也自己尝试自己写代码,我是这样写的

package com.mason.dao.impl;
 
import com.mason.dao.ProductDao;
import com.mason.domain.Category;
import com.mason.domain.Product;
import com.mason.utils.JDBCUtils;
 
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.List;
 
public class ProductDaoImpl implements ProductDao {
    @Override
    public List<Product> findAll() {
        System.out.println("ProductDao的findAll方法");
        Connection conn = null;
        ResultSet rs = null;
        PreparedStatement pstmt = null;
        List<Product> list = null;
        try {
            //获得连接
            conn = JDBCUtils.getConnection();
            //编写SQL
            String sql = "SELECT product.*,category.* FROM product JOIN category ON product.cid=category.cid";
            //预编译SQL
            pstmt = conn.prepareStatement(sql);
            //执行SQL
            rs = pstmt.executeQuery();
            //遍历结果集
            list = new ArrayList();
            while (rs.next()) {
                Product product = new Product();
                product.setPid(rs.getInt("pid"));
                product.setPname(rs.getString("pname"));
                product.setAuthor(rs.getString("author"));
                product.setPrice(rs.getDouble("price"));
                product.setDescription(rs.getString("filename"));
                product.setPath(rs.getString("path"));
                Category category = new Category();
                category.setCid(rs.getInt("cid"));
                category.setCname(rs.getString("cname"));
                category.setCdesc(rs.getString("cdesc"));
                product.setCategory(category);
                list.add(product);
            }
            for (Product p :
                    list) {
                System.out.println("商品ID:"+p.getPid()+"   商品名:"+p.getPname()+"  商品分类名称:"+p.getCategory().getCname());
            }
            return list;
 
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            //释放资源
            JDBCUtils.release(rs, pstmt, conn);
        }
        return null;
    }
}

我的查询语句是用了内连接

1	`String sql =` `"SELECT product.,category. FROM product JOIN category ON product.cid=category.cid";`

然后遍历结果集以及封装数据是这样

while (rs.next()) {
    Product product = new Product();
    product.setPid(rs.getInt("pid"));
    product.setPname(rs.getString("pname"));
    product.setAuthor(rs.getString("author"));
    product.setPrice(rs.getDouble("price"));
    product.setDescription(rs.getString("filename"));
    product.setPath(rs.getString("path"));
    Category category = new Category();
    category.setCid(rs.getInt("cid"));
    category.setCname(rs.getString("cname"));
    category.setCdesc(rs.getString("cdesc"));
    product.setCategory(category);
    list.add(product);
}