How to use mysql union to implement full outer join query

1. Union is not a method of multi-table connection query. It combines the query results of multiple query sentences into one result and removes duplicate data.

2. Full outer join queries the data of the left table and the right table, and then connects according to the connection conditions.

Example

# 用左外的A union 右外的B
SELECT * FROM t_category c LEFT OUTER JOIN t_product p ON c.cid = p.cno
union
SELECT * FROM t_category c RIGHT  OUTER JOIN t_product p ON c.cid = p.cno

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