13. 計(jì)算記錄條數(shù)
和一般的觀點(diǎn)相反, count(*) 比count(1)稍快 , 當(dāng)然如果可以通過索引檢索,對索引列的計(jì)數(shù)仍舊是最快的. 例如 COUNT(EMPNO)
(譯者按: 在CSDN論壇中,曾經(jīng)對此有過相當(dāng)熱烈的討論, 作者的觀點(diǎn)并不十分準(zhǔn)確,通過實(shí)際的測試,上述三種方法并沒有顯著的性能差別)
14. 用Where子句替換HAVING子句
避免使用HAVING子句, HAVING 只會在檢索出所有記錄之后才對結(jié)果集進(jìn)行過濾. 這個(gè)處理需要排序,總計(jì)等操作. 如果能通過WHERE子句限制記錄的數(shù)目,那就能減少這方面的開銷.
例如:
低效:
SELECT REGION，AVG(LOG_SIZE)
FROM LOCATION
GROUP BY REGION
HAVING REGION REGION != ‘SYDNEY’
AND REGION != ‘PERTH’
高效
SELECT REGION，AVG(LOG_SIZE)
FROM LOCATION
WHERE REGION REGION != ‘SYDNEY’
AND REGION != ‘PERTH’
GROUP BY REGION
(譯者按: HAVING 中的條件一般用于對一些集合函數(shù)的比較,如COUNT() 等等. 除此而外,一般的條件應(yīng)該寫在WHERE子句中)
15. 減少對表的查詢
在含有子查詢的SQL語句中,要特別注意減少對表的查詢.
例如:
低效
SELECT TAB_NAME
FROM TABLES
WHERE TAB_NAME = ( SELECT TAB_NAME
FROM TAB_COLUMNS
WHERE VERSION = 604)
AND DB_VER= ( SELECT DB_VER
FROM TAB_COLUMNS
WHERE VERSION = 604)
高效
SELECT TAB_NAME
FROM TABLES
WHERE (TAB_NAME,DB_VER)
= ( SELECT TAB_NAME,DB_VER)
FROM TAB_COLUMNS
WHERE VERSION = 604)
Update 多個(gè)Column 例子:
低效:
UPDATE EMP
SET EMP_CAT = (SELECT MAX(CATEGORY) FROM EMP_CATEGORIES),
SAL_RANGE = (SELECT MAX(SAL_RANGE) FROM EMP_CATEGORIES)
WHERE EMP_DEPT = 0020;
高效:
UPDATE EMP
SET (EMP_CAT, SAL_RANGE)
= (SELECT MAX(CATEGORY) , MAX(SAL_RANGE)
FROM EMP_CATEGORIES)
WHERE EMP_DEPT = 0020;
16. 通過內(nèi)部函數(shù)提高SQL效率.
SELECT H.EMPNO,E.ENAME,H.HIST_TYPE,T.TYPE_DESC,COUNT(*)
FROM HISTORY_TYPE T,EMP E,EMP_HISTORY H
WHERE H.EMPNO = E.EMPNO
AND H.HIST_TYPE = T.HIST_TYPE
GROUP BY H.EMPNO,E.ENAME,H.HIST_TYPE,T.TYPE_DESC;
通過調(diào)用下面的函數(shù)可以提高效率.
FUNCTION LOOKUP_HIST_TYPE(TYP IN NUMBER) RETURN VARCHAR2
AS
TDESC VARCHAR2(30);
CURSOR C1 IS
SELECT TYPE_DESC
FROM HISTORY_TYPE
WHERE HIST_TYPE = TYP;
BEGIN
OPEN C1;
FETCH C1 INTO TDESC;
CLOSE C1;
RETURN (NVL(TDESC,’?’));
END;
FUNCTION LOOKUP_EMP(EMP IN NUMBER) RETURN VARCHAR2
AS
ENAME VARCHAR2(30);
CURSOR C1 IS
SELECT ENAME
FROM EMP
WHERE EMPNO=EMP;
BEGIN
OPEN C1;
FETCH C1 INTO ENAME;
CLOSE C1;
RETURN (NVL(ENAME,’?’));
END;
SELECT H.EMPNO,LOOKUP_EMP(H.EMPNO),
H.HIST_TYPE,LOOKUP_HIST_TYPE(H.HIST_TYPE),COUNT(*)
FROM EMP_HISTORY H
GROUP BY H.EMPNO , H.HIST_TYPE;
(譯者按: 經(jīng)常在論壇中看到如 ’能不能用一個(gè)SQL寫出….’ 的貼子, 殊不知復(fù)雜的SQL往往犧牲了執(zhí)行效率. 能夠掌握上面的運(yùn)用函數(shù)解決問題的方法在實(shí)際工作中是非常有意義的)
(待續(xù))