3、 左连接（left join的简写）会返回左表中的所有记录，即便右表中没有匹配的条目，也会保留左表的每一行数据，右表无对应信息时以空值填充。 4、 内连接通过公共字段匹配的值，将两个表中的相关记录进行组合。 5、 调用方式各异 6、 在FROM子句中使用INNER ...

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