2個查詢的比較（MySQL）

Question

我有兩個查詢。他們是一樣的嗎？

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，哪一個更有效？

Answer 1

在我看來都是相同的，因爲我從這個語法知道：

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

多表語法：

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] 

至於大家關心哪些的他們更有效率？這取決於情況意味着何時何地我們正在使用這個。

閱讀此更好的體驗：http://dev.mysql.com/doc/refman/5.1/en/delete.html