如何解决 MySQL 缺乏 FULL OUTER JOIN 功能的问题？

MySQL 本身不支持 FULL OUTER JOIN。 这会在尝试使用它时导致语法错误。

解决方案：使用 UNION

要复制 FULL OUTER JOIN 的行为，MySQL 需要使用 UNION 子句的解决方法。这结合了 LEFT JOIN 和 RIGHT JOIN 的结果。

实现此目标的方法如下：

<code class="language-sql">SELECT airline, airports.icao_code, continent, country, province, city, website 
FROM airlines 
LEFT JOIN airports ON airlines.iaco_code = airports.iaco_code
LEFT JOIN cities ON airports.city_id = cities.city_id
LEFT JOIN provinces ON cities.province_id = provinces.province_id
LEFT JOIN countries ON cities.country_id = countries.country_id
LEFT JOIN continents ON countries.continent_id = continents.continent_id
UNION
SELECT airline, airports.icao_code, continent, country, province, city, website 
FROM airlines 
RIGHT JOIN airports ON airlines.iaco_code = airports.iaco_code
RIGHT JOIN cities ON airports.city_id = cities.city_id
RIGHT JOIN provinces ON cities.province_id = provinces.province_id
RIGHT JOIN countries ON cities.country_id = countries.country_id
RIGHT JOIN continents ON countries.continent_id = continents.continent_id;</code>

此查询有效地合并了 LEFT JOIN（左表中的所有行，匹配右表中的行）和 RIGHT JOIN（右表中的所有行，匹配左表中的行）的结果，提供像 FULL OUTER JOIN 一样的完整数据集。 请注意，重复项将通过 UNION 操作删除。 如果需要保留重复项，请使用 UNION ALL.

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