SQL Natural連接和嵌套SELECT查詢

Question

什麼可能會使這兩個查詢不同？

該帖子確實主要是代碼。

SELECT DISTINCT S.name 
FROM 
    student S NATURAL JOIN taking NATURAL JOIN 
    (select * from class where classnum ='121') 
WHERE 
    department='CMPSC' 
    AND semester='Spring 2013'; 



SELECT DISTINCT S.name 
FROM 
    student S NATURAL JOIN taking NATURAL JOIN class 
WHERE 
    department='CMPSC' 
    AND semester='Spring 2013' 
    AND classnum='121'; 

謝謝！

編輯：

至於用於解釋命令請求的響應：我不得不這樣做對ORACLE，所以我不知道這是預期的結果：

這是第一個查詢：

Plan hash value: 3259400360 

------------------------------------------------------ 
| Id | Operation      | Name   | 
------------------------------------------------------ 
| 0 | SELECT STATEMENT    |    | 
| 1 | HASH UNIQUE     |    | 
| 2 | NESTED LOOPS    |    | 
| 3 | NESTED LOOPS    |    | 
| 4 |  TABLE ACCESS FULL   | CLASS  | 
| 5 |  INDEX FULL SCAN   | SYS_C0099014 | 
| 6 | TABLE ACCESS BY INDEX ROWID| STUDENT  | 
| 7 |  INDEX UNIQUE SCAN   | SYS_C0098998 | 
------------------------------------------------------ 

Predicate Information (identified by operation id): 
--------------------------------------------------- 

    4 - filter("CLASSNUM"=121 AND "CLASS"."SEMESTER"='Spring 2013' AND 
       "CLASS"."DEPARTMENT"='CMPSC') 

PLAN_TABLE_OUTPUT 
-------------------------------------------------------------------------------- 
    5 - access("TAKING"."SCHEDULENUM"="CLASS"."SCHEDULENUM" AND 
       "TAKING"."SEMESTER"='Spring 2013') 
     filter("TAKING"."SEMESTER"='Spring 2013' AND 
       "TAKING"."SCHEDULENUM"="CLASS"."SCHEDULENUM") 
    7 - access("S"."STUDENTNUM"="TAKING"."STUDENTNUM") 

第二個查詢：

PLAN_TABLE_OUTPUT 
-------------------------------------------------------------------------------- 
Plan hash value: 226170808 

------------------------------------------------------- 
| Id | Operation      | Name   | 
------------------------------------------------------- 
| 0 | SELECT STATEMENT    |    | 
| 1 | HASH UNIQUE     |    | 
| 2 | HASH JOIN     |    | 
| 3 | MERGE JOIN     |    | 
| 4 |  TABLE ACCESS BY INDEX ROWID| STUDENT  | 
| 5 |  INDEX FULL SCAN   | SYS_C0098998 | 
| 6 |  SORT JOIN     |    | 
| 7 |  INDEX FULL SCAN   | SYS_C0099014 | 
| 8 | TABLE ACCESS FULL   | CLASS  | 
------------------------------------------------------- 

Predicate Information (identified by operation id): 
--------------------------------------------------- 

PLAN_TABLE_OUTPUT 
-------------------------------------------------------------------------------- 

    2 - access("TAKING"."SEMESTER"="CLASS"."SEMESTER") 
    6 - access("S"."STUDENTNUM"="TAKING"."STUDENTNUM") 
     filter("S"."STUDENTNUM"="TAKING"."STUDENTNUM") 
    7 - access("TAKING"."SEMESTER"='Spring 2013') 
     filter("TAKING"."SEMESTER"='Spring 2013') 
    8 - filter("CLASS"."CLASSNUM"=121 AND "CLASS"."SEMESTER"='Spring 2013' AND 
       "CLASS"."DEPARTMENT"='CMPSC') 

Answer 1

如果性能是問題，這是貝科用你擊敗索引。當執行子查詢時，服務器可以使用索引來檢索結果;然而，它不能索引子查詢的結果，導致更差的性能。一般來說，嘗試「優化」優化器效果不佳 - 您最終會遇到難以閱讀的查詢，並執行完全相同的操作（優化器會將查詢優化爲與原始操作相同的內容），或者您最終會出現這樣的情況，表現更糟糕。應該使用JOIN子句來定義連接，而不是限制數據。

優化器將相同執行以下查詢：

SELECT * 
FROM foo f 
    JOIN bar b 
    ON f.id = b.id 
     AND b.col = 'baz' 

和

SELECT * 
FROM foo f 
    JOIN bar b 
    ON f.id = b.id 
WHERE b.col = 'baz' 

而

SELECT * 
FROM foo f 
    JOIN (
    SELECT * 
    FROM bar b 
    WHERE col = 'baz' 
) AS b 
    ON f.id = b.id 

在MySQL基本上產生非索引存儲器內臨時表。它執行與前面兩個sql服務器中的相同。