Question

我有两个问题。它们是一样的吗？

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，哪一个更有效？

Answer 1

在我看来，两者都是相同的，因为我从这个语法中知道：

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]

关注哪一个更有效率？这取决于情况意味着我们何时何地使用它。