在给定情况下 SQL 中的查询优化

Question

这些是给定的表格：

create table products 
(productID int not null auto_increment,
 Name varchar(30),
 Price float ,
 CoffeOrigin varchar(30),
 primary key (productID));

create table customers
(customerID int not null auto_increment,
First_Name varchar(30),
 Last_Name varchar(30),
 Gender varchar(2) not null CHECK (Gender IN ('M', 'F')),
 ContactNumber bigint,
primary key (customerID));

create table orders
(orderID int not null auto_increment,
productID int,
customerID int,
Date_Time datetime,
primary key(orderID),
foreign key(customerID) references customers(customerID),
foreign key(productID) references products(productID));

问题是：

Write a optimized query to find out the name of all the customerIDs who ordered coffee that originated from ‘Costa Rica’ or ‘Indonesia’.

我的尝试：

select customers.customerID, first_name, last_name from customers where customerID in
(select customerID from orders where productid in
(select productid from products where coffeorigin = "costa rica" or "indonesia"));

我的老师说它可以进一步优化，但我看不出有什么办法。请帮帮我。

Answer 1

而不是这些嵌套的 in 子查询，我会推荐 exists 和一个带有 join:

的相关子查询

select c.customerID, c.first_name, c.last_name 
from customers c 
where exists (
    select 1
    from orders o
    inner join products p on p.productID = o.productID
    where p.coffeorigin in ('costa rica', 'indonesia') and o.customerID = c.customerID
);

在给定情况下 SQL 中的查询优化

Query Optimization in SQL in given case

mysql

sql

inner-join

subquery