mysql leftjion 和inner join

MySQL中的LEFT JOIN和INNER JOIN是用于连接两个或多个表的操作。

LEFT JOIN返回左表中的所有记录以及与右表匹配的记录。如果左表中的记录没有与右表匹配的记录，则右表中的字段将显示为NULL。示例SQL语句如下：SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id;

INNER JOIN返回左表和右表中匹配的记录，即只返回两个表中都存在的记录。示例SQL语句如下：SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id;

至于LEFT JOIN和INNER JOIN的查询性能，通常来说INNER JOIN比LEFT JOIN更快。这是因为INNER JOIN在连接时只返回匹配的记录，而LEFT JOIN需要返回左表中的所有记录。然而，实际查询的性能还受到多个因素的影响，例如表的大小、索引的使用等。因此，在具体情况下，性能可能会有所不同。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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