有两表a和b,两字段完全相同:(idint,namevarchar(10)...)select * from a,b 的查询结果
时间: 2024-05-29 11:08:32 浏览: 62
会返回一个笛卡尔积,即a表和b表的所有行组合,这并不是我们想要的结果。正确的做法应该是使用JOIN连接两个表,并且指定连接条件,例如:select * from a inner join b on a.id=b.id and a.name=b.name。这样就可以获得两个表中按照指定条件匹配的行集合了。
相关问题
根据idint(11) NOT NULL book_idint(11) NOT NULL user_idint(11) NOT NULL order_timedatetime NOT NULL quantityint(11) NOT NULL unit_pricedecimal(10,2) NOT NULL total_amountdecimal(10,2) NOT NULL编写 java类
可以使用以下代码来编写一个 Java 类来表示一个订单:
```java
import java.util.Date;
public class Order {
private int bookId;
private int userId;
private Date orderTime;
private int quantity;
private double unitPrice;
private double totalAmount;
public Order(int bookId, int userId, Date orderTime, int quantity, double unitPrice, double totalAmount) {
this.bookId = bookId;
this.userId = userId;
this.orderTime = orderTime;
this.quantity = quantity;
this.unitPrice = unitPrice;
this.totalAmount = totalAmount;
}
public int getBookId() {
return bookId;
}
public void setBookId(int bookId) {
this.bookId = bookId;
}
public int getUserId() {
return userId;
}
public void setUserId(int userId) {
this.userId = userId;
}
public Date getOrderTime() {
return orderTime;
}
public void setOrderTime(Date orderTime) {
this.orderTime = orderTime;
}
public int getQuantity() {
return quantity;
}
public void setQuantity(int quantity) {
this.quantity = quantity;
}
public double getUnitPrice() {
return unitPrice;
}
public void setUnitPrice(double unitPrice) {
this.unitPrice = unitPrice;
}
public double getTotalAmount() {
return totalAmount;
}
public void setTotalAmount(double totalAmount) {
this.totalAmount = totalAmount;
}
}
```
注:以上代码仅为示例,实际应用中可能需要添加其他属性和方法。
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