mysql - 2个查询的比较(MySQL)

Question

我有两个疑问。它们相同吗？

DELETE FROM t1 
USING t1 
LEFT JOIN t2 using(key) 
WHERE t2.key is null

DELETE t1 
FROM t1  
LEFT JOIN t2 using(key) 
WHERE t2.key is null

如果是，哪一个效率更高？

Answer 1

在我看来，两者是相同的，正如我从以下语法中了解到的:

DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
    [WHERE where_condition]
    [ORDER BY ...]
    [LIMIT row_count]

多表语法:

DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
    tbl_name[.*] [, tbl_name[.*]] ...
    FROM table_references
    [WHERE where_condition]

或者:

DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
    FROM tbl_name[.*] [, tbl_name[.*]] ...
    USING table_references
    [WHERE where_condition]

至于关心哪一个更有效率？这取决于我们使用它的地点和时间的情况。

阅读此内容以获得更好的体验:http://dev.mysql.com/doc/refman/5.1/en/delete.html