为什么MySQL在FULL OUTER JOIN上报语法错误？

Question

SELECT airline, airports.icao_code, continent, country, province, city, website 

FROM airlines 
FULL OUTER JOIN airports ON airlines.iaco_code = airports.iaco_code
FULL OUTER JOIN cities ON airports.city_id = cities.city_id
FULL OUTER JOIN provinces ON cities.province_id = provinces.province_id
FULL OUTER JOIN countries ON cities.country_id = countries.country_id
FULL OUTER JOIN continents ON countries.continent_id = continents.continent_id

据说

你的 SQL 语法有错误；检查与您的 MySQL 服务器版本对应的手册，了解在 airlines.iaco_code = airports.iaco_code 上使用 near 'outer join

airports

的正确语法 第 4 行的全外连接

语法对我来说是正确的。我以前从未做过很多连接，但我需要一个表中的那些列，这些列被各种 id 交叉引用。

Answer 1

MySQL中没有

FULL OUTER JOIN

。见7.2.12。外连接简化和12.2.8.1。加入语法：

您可以模拟

FULL OUTER JOIN

使用 UNION（从 MySQL 4.0.0 开始）：

有两个表 t1，t2：

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id

三个表 t1, t2, t3:

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
RIGHT JOIN t3 ON t2.id = t3.id

Answer 2

cletus 的回答不太正确。

UNION

将删除

FULL OUTER JOIN

将包含的重复记录。如果您需要使用类似的副本：

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
LEFT JOIN t4 ON t3.id = t4.id
UNION ALL
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
LEFT JOIN t4 ON t3.id = t4.id
WHERE t1.id IS NULL
UNION ALL
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
RIGHT JOIN t3 ON t2.id = t3.id
LEFT JOIN t4 ON t3.id = t4.id
WHERE t2.id IS NULL
UNION ALL
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
RIGHT JOIN t3 ON t2.id = t3.id
RIGHT JOIN t4 ON t3.id = t4.id
WHERE t3.id IS NULL;

Answer 3

我刚刚为此做了一个技巧：

(select 1 from DUAL) d
LEFT OUTER JOIN t1 ON t1.id = t2.id
LEFT OUTER JOIN t2 ON t1.id = t2.id

关键是，来自 dual 的查询确定了一个固定点，mysql 可以将其他 2 个表外连接到那个

Answer 4

需要

FULL OUTER JOIN

三张表t1,t2,t3的时候补充case即可。你可以依次使 t1、t2、t3 左连接其余两个表，然后合并。

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t1.id = t3.id
UNION
SELECT * FROM t2
LEFT JOIN t1 ON t2.id = t1.id
LEFT JOIN t3 ON t2.id = t3.id
UNION
SELECT * FROM t3
LEFT JOIN t1 ON t3.id = t1.id
LEFT JOIN t2 ON t3.id = t2.id

Answer 5

只是为了补充此处提供的答案，我想分享这个连接 4 个表的示例，在所有

LEFT JOIN

语句上使用

SELECT

，以及一个更形象的示例。

SELECT doctor.name, professor.name, singer.name, actor.name
FROM doctor
LEFT JOIN professor ON professor.id = doctor.id
LEFT JOIN singer ON singer.id = doctor.id
LEFT JOIN actor ON actor.id = doctor.id
UNION
SELECT doctor.name, professor.name, singer.name, actor.name
FROM professor
LEFT JOIN doctor ON doctor.id = professor.id
LEFT JOIN singer ON singer.id = professor.id
LEFT JOIN actor ON actor.id = professor.id
UNION
SELECT doctor.name, professor.name, singer.name, actor.name
FROM singer
LEFT JOIN doctor ON doctor.id = singer.id
LEFT JOIN professor ON professor.id = singer.id
LEFT JOIN actor ON actor.id = singer.id
UNION
SELECT doctor.name, professor.name, singer.name, actor.name
FROM actor
LEFT JOIN doctor ON doctor.id = actor.id
LEFT JOIN professor ON professor.id = actor.id
LEFT JOIN singer ON singer.id = actor.id;