mysql学习打卡day13

今日成果：

SELECT 
    p.product_id, name, oi.quantity
FROM
    products p
        LEFT JOIN
    order_items oi ON p.product_id = oi.product_id;
-- 查询所有产品信息   
-- LEFT JOIN 左连接：返回所有左表的结果 


select o.order_date,o.order_id,c.first_name as customer,sh.name as shipper,os.name as status from orders o
left join customers c on o.customer_id = c.customer_id
left join shippers sh on o.shipper_id = sh.shipper_id
left join order_statuses os on o.status = os.order_status_id;
-- 查询指定的订单结果
-- 多表外连接；内连接：只返回非空的结果 ；外连接：返回包含空的结果

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