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| 1. 内连接：Inner join
SELECT LEFT(a.title, 20) AS ArticleTitle, LEFT(c.content, 20) AS CommentContent
FROM article a
INNER JOIN comment c ON a.id = c.article_id
LIMIT 2;
返回左右均匹配的行
2. 外连接：LEFT JOIN RIGHT JOIN
（1）. 左连接
SELECT LEFT(a.title, 20) AS ArticleTitle, LEFT(c.content, 20) AS CommentContent
FROM article a
LEFT JOIN comment c ON a.id = c.article_id
LIMIT 2;
会返回左边的全部行，右边无匹配行则返回null
（2）. 右连接
SELECT LEFT(a.title, 20) AS ArticleTitle, RIGHT(c.content, 20) AS CommentContent
FROM comment c
RIGHT JOIN article a ON a.id = c.article_id
LIMIT 2;
会返回右边的全部行，左边无匹配行则返回null
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