How to define mysql union query

1. Also called connection query, it connects data in multiple tables to obtain a result set. When one table cannot satisfy the query results, you need to use a union query.

2. Premise, there must be logical correlation between joint tables.

Example

-- 示例：
select orders.order_id, orders.amt, customer.cust_name, customer.tel_no
from orders, customer
where orders.cust_id = customer.cust_id;
 
-- 起别名
select a.order_id, a.amt, b.cust_name, b.tel_no
from orders a, customer b
where a.cust_id = b.cust_id;

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