s2下半年我在內部有一次部門級別的技術分享會，以本文內容分享為主。
其實有很多人問過我相同的問題，遇到需要改寫的慢sql，不知道怎麼改，改好了以後也不知道等不等價？不等價了也不知道錯在哪？這個要怎麼破？
其實都是因為絕大多數人沒有做過開發，看不懂sql，不會寫sql，沒有sql思維，下面透過幾個案例將sql思維給引出來。
1、row_number() over() 邏輯實現
這種排名的視窗函式（row_number()、rank()、dense_rank()）據我所知，除了在oracle上大資料量執行效能表現得不錯，其他各種資料庫（包括列式儲存的ap庫），效能都表現都很一般。
所以如果在專案上遇到row_number() over() 效能不理想的情況下，可以嘗試進行改寫最佳化。
PS：當然可以透過加並行hint來提升效率，但是我們這篇文章主要是講如何將資料庫提供的邏輯透過另外一種方式來實現，提升自己sql程式碼思維。
-- student 資料量
obclient [YZJ]> select count(1) from student;
+----------+
| COUNT(1) |
+----------+
| 500001 |
+----------+
1 row in set (0.002 sec)
-- 案例sql
-- S.CLASS : 學科、包含null值
-- S.S_DATE: 入學時間
SELECT * FROM (
SELECT
S.*,
row_number() OVER(PARTITION BY S.CLASS ORDER BY S.S_DATE DESC) RN
FROM student S
) WHERE RN = 1;
27 rows in set (0.639 sec)
+---------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ================================================================ |
| |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| |
| ---------------------------------------------------------------- |
| |0 |SUBPLAN SCAN |ANONYMOUS_VIEW1|4 |691395 | |
| |1 |└─WINDOW FUNCTION | |500001 |682867 | |
| |2 | └─PARTITION SORT | |500001 |249986 | |
| |3 | └─TABLE FULL SCAN|S |500001 |46352 | |
| ================================================================ |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([.ID], [.NAME], [.AGE], [.SEX], [.S_DATE], [.CLASS], [.RN]), filter([.RN = 1]), rowset=256 |
| access([.RN], [.ID], [.NAME], [.AGE], [.SEX], [.S_DATE], [.CLASS]) |
| 1 - output([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS], [T_WIN_FUN_ROW_NUMBER()]), filter(nil), rowset=256 |
| win_expr(T_WIN_FUN_ROW_NUMBER()), partition_by([S.CLASS]), order_by([S.S_DATE, DESC]), window_type(RANGE), upper(UNBOUNDED PRECEDING), lower(UNBOUNDED |
| FOLLOWING) |
| 2 - output([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), filter(nil), rowset=256 |
| sort_keys([HASH(S.CLASS), ASC], [S.CLASS, ASC], [S.S_DATE, DESC]) |
| 3 - output([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), filter(nil), rowset=256 |
| access([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([S.ID]), range(MIN ; MAX)always true |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------+
21 rows in set (0.004 sec)
1.1、改寫方式一
改寫視窗函式的邏輯，必須要將表關聯2次或者2次+，需要將另外一張關聯的表理解成一個“滑動”的視窗。
一般有兩種改法：
- 方法1、相同表 join 兩次。
- 方法2、表量子查詢實現，將子查詢的那張表理解成一個“滑動”的視窗。
SELECT s.* FROM student s -- 原表
INNER JOIN
(
SELECT
class,
max(s_date) max_date
FROM student
GROUP BY class
) s1 -- 滑動的視窗表
ON decode(s.class,NULL,'aab',s.class) = decode(s1.class,NULL,'aab',s1.class)
-- decode 處理 s表和s1視窗表的分組列class
AND s.s_date = s1.max_date
-- s表的s_date和視窗表s1的最大的日期去比較
WHERE exists(
SELECT 1 FROM student s2 where s1.max_date = s2.s_date GROUP BY s2.class having MAX(s2.ID) = S.ID
/* exists 這段比較抽象，我也是想了一會才實現邏輯。
實現 row_number() 唯一排名的資料
*/
);
27 rows in set (0.038 sec)
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ============================================================================================== |
| |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| |
| ---------------------------------------------------------------------------------------------- |
| |0 |SUBPLAN FILTER | |4 |28745 | |
| |1 |├─NESTED-LOOP JOIN | |8 |28555 | |
| |2 |│ ├─SUBPLAN SCAN |S1 |26 |27712 | |
| |3 |│ │ └─MERGE GROUP BY | |26 |27712 | |
| |4 |│ │ └─TABLE FULL SCAN |STUDENT(IDX_CLASS_SDATE_STUDENT)|500001 |14409 | |
| |5 |│ └─DISTRIBUTED TABLE RANGE SCAN |S(IDX_S_DATE_CALSS_ID) |1 |32 | |
| |6 |└─LIMIT | |1 |27 | |
| |7 | └─MERGE GROUP BY | |1 |27 | |
| |8 | └─DISTRIBUTED TABLE RANGE SCAN|S2(IDX_S_DATE_CALSS_ID) |56 |23 | |
| ============================================================================================== |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), filter([(T_OP_EXISTS, subquery(1))]), rowset=256 |
| exec_params_([S1.MAX_DATE(:0)], [S.ID(:1)]), onetime_exprs_(nil), init_plan_idxs_(nil), use_batch=false |
| 1 - output([S1.MAX_DATE], [S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), filter(nil), rowset=256 |
| conds(nil), nl_params_([S1.CLASS(:4)], [S1.MAX_DATE(:5)]), use_batch=false |
| 2 - output([S1.CLASS], [S1.MAX_DATE]), filter(nil), rowset=256 |
| access([S1.CLASS], [S1.MAX_DATE]) |
| 3 - output([STUDENT.CLASS], [T_FUN_MAX(STUDENT.S_DATE)]), filter(nil), rowset=256 |
| group([STUDENT.CLASS]), agg_func([T_FUN_MAX(STUDENT.S_DATE)]) |
| 4 - output([STUDENT.CLASS], [STUDENT.S_DATE]), filter(nil), rowset=256 |
| access([STUDENT.CLASS], [STUDENT.S_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([STUDENT.CLASS], [STUDENT.S_DATE], [STUDENT.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true |
| 5 - output([S.ID], [S.CLASS], [S.S_DATE], [S.NAME], [S.AGE], [S.SEX]), filter([ora_decode(cast(S.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('aab', |
| VARCHAR2(3 BYTE)), cast(S.CLASS, VARCHAR2(3 BYTE))) = ora_decode(cast(:4, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('aab', VARCHAR2(3 BYTE)), cast(:4, |
| VARCHAR2(3 BYTE)))]), rowset=256 |
| access([S.ID], [S.CLASS], [S.S_DATE], [S.NAME], [S.AGE], [S.SEX]), partitions(p0) |
| is_index_back=true, is_global_index=false, filter_before_indexback[true], |
| range_key([S.S_DATE], [S.CLASS], [S.ID]), range(MIN ; MAX), |
| range_cond([S.S_DATE = :5]) |
| 6 - output([1]), filter(nil), rowset=256 |
| limit(1), offset(nil) |
| 7 - output(nil), filter([T_FUN_MAX(S2.ID) = :1]), rowset=256 |
| group([S2.CLASS]), agg_func([T_FUN_MAX(S2.ID)]) |
| 8 - output([S2.ID], [S2.CLASS]), filter(nil), rowset=256 |
| access([S2.ID], [S2.CLASS]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([S2.S_DATE], [S2.CLASS], [S2.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true, |
| range_cond([:0 = S2.S_DATE]) |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
43 rows in set (0.008 sec)
1.2、改寫方式二
SELECT S1.*
FROM student S1
INNER JOIN
(
SELECT
decode(class,null,'aab',class) class,
MAX(S2.S_DATE) MAX_S_DATE,
MAX(ID) KEEP (DENSE_RANK LAST ORDER BY S_DATE) MAX_ID
FROM student S2
GROUP BY S2.CLASS
/*
改寫邏輯：
1、decode(class,null,'aab',class) class, 提前處理null值。如果有其他案例謂詞過濾條件要轉換列名，
例如：upper(列名) = upper(:1) ,可以提前在先寫個select括起來內表處理欄位，弄個別名，然後外面再過濾。
select * from (
select upper(col) col1 from tb
) where col1 = upper(:1);
2、MAX(S2.S_DATE) MAX_S_DATE 不多說，找最大日期，等價 desc
3、MAX(ID) KEEP (DENSE_RANK LAST ORDER BY S_DATE) MAX_ID 等價於上個sql 的 exists邏輯，
利用keep 唯一的特性，取出不同組裡面最大的 id + ORDER BY S_DATE LAST 最後的唯一資料。
相當於 MAX_S_DATE，和 MAX(id) keep 兩個列的資料都是一條 ，相當於rn=1。
*/
) S2
ON decode(S1.CLASS,null,'aab',S1.CLASS) = S2.CLASS /* 處理關聯列 */
AND S1.ID = S2.MAX_ID ;
-- AND S1.S_DATE = S2.MAX_S_DATE;
/* 其實用S1.ID = S2.MAX_ID 關聯就行，
S1.S_DATE = S2.MAX_S_DATE 這個寫不寫都行,如果想邏輯更嚴謹點可以寫上。
*/
27 rows in set (0.114 sec)
+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ============================================================================== |
| |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| |
| ------------------------------------------------------------------------------ |
| |0 |NESTED-LOOP JOIN | |2 |44008 | |
| |1 |├─SUBPLAN SCAN |S2 |26 |43466 | |
| |2 |│ └─MERGE GROUP BY | |26 |43466 | |
| |3 |│ └─TABLE FULL SCAN |S2(IDX_CLASS_SDATE_STUDENT)|500001 |22423 | |
| |4 |└─DISTRIBUTED TABLE GET|S1 |1 |21 | |
| ============================================================================== |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([S1.ID], [S1.NAME], [S1.AGE], [S1.SEX], [S1.S_DATE], [S1.CLASS]), filter(nil), rowset=256 |
| conds(nil), nl_params_([S2.CLASS(:3)], [S2.MAX_S_DATE(:4)], [S2.MAX_ID(:5)]), use_batch=false |
| 1 - output([S2.CLASS], [S2.MAX_S_DATE], [S2.MAX_ID]), filter(nil), rowset=256 |
| access([S2.CLASS], [S2.MAX_S_DATE], [S2.MAX_ID]) |
| 2 - output([ora_decode(cast(S2.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('aab', VARCHAR2(3 BYTE)), cast(S2.CLASS, VARCHAR2(3 BYTE)))], |
| [T_FUN_MAX(S2.S_DATE)], [T_FUN_KEEP_MAX(S2.ID) order_items(S2.S_DATE)]), filter(nil), rowset=256 |
| group([S2.CLASS]), agg_func([T_FUN_MAX(S2.S_DATE)], [T_FUN_KEEP_MAX(S2.ID) order_items(S2.S_DATE)]) |
| 3 - output([S2.ID], [S2.CLASS], [S2.S_DATE]), filter(nil), rowset=256 |
| access([S2.ID], [S2.CLASS], [S2.S_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([S2.CLASS], [S2.S_DATE], [S2.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true |
| 4 - output([S1.ID], [S1.CLASS], [S1.S_DATE], [S1.NAME], [S1.AGE], [S1.SEX]), filter([S1.S_DATE = :4], [ora_decode(cast(S1.CLASS, VARCHAR2(1 BYTE)), cast(NULL, |
| VARCHAR2(0 )), cast('aab', VARCHAR2(3 BYTE)), cast(S1.CLASS, VARCHAR2(3 BYTE))) = :3]), rowset=256 |
| access([S1.ID], [S1.CLASS], [S1.S_DATE], [S1.NAME], [S1.AGE], [S1.SEX]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false,false], |
| range_key([S1.ID]), range(MIN ; MAX), |
| range_cond([S1.ID = :5]) |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
28 rows in set (0.011 sec)
row_number() over() 的邏輯還可以用表量子查詢來實現，不過效能會更慢，實現起來很麻煩，就不在這裡演示。
2、keep() 函式邏輯實現
某些資料庫可能不支援keep函式，或者keep函式的效能並不理想，我們也可以嘗試透過另外一種方式來實現keep函式的邏輯。
2.1、keep 形式一
SELECT
s1.class,
max(s1.s_date) MAX_S_DATE,
MAX(name) KEEP (DENSE_RANK LAST ORDER BY s1.s_date) max_name,
min(s1.s_date) MIN_S_DATE,
MIN(name) KEEP (DENSE_RANK FIRST ORDER BY s1.s_date) min_name
FROM student s1
GROUP BY
s1.class;
27 rows in set (0.495 sec)
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| =================================================== |
| |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| |
| --------------------------------------------------- |
| |0 |MERGE GROUP BY | |26 |144304 | |
| |1 |└─PARTITION SORT | |500001 |107784 | |
| |2 | └─TABLE FULL SCAN|S1 |500001 |22309 | |
| =================================================== |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([S1.CLASS], [T_FUN_MAX(S1.S_DATE)], [T_FUN_KEEP_MAX(S1.NAME) order_items(S1.S_DATE)], [T_FUN_MIN(S1.S_DATE)], [T_FUN_KEEP_MIN(S1.NAME) order_items(S1.S_DATE)]), filter(nil), rowset=256 |
| group([S1.CLASS]), agg_func([T_FUN_MAX(S1.S_DATE)], [T_FUN_KEEP_MAX(S1.NAME) order_items(S1.S_DATE)], [T_FUN_MIN(S1.S_DATE)], [T_FUN_KEEP_MIN(S1.NAME) |
| order_items(S1.S_DATE)]) |
| 1 - output([S1.CLASS], [S1.S_DATE], [S1.NAME]), filter(nil), rowset=256 |
| sort_keys([HASH(S1.CLASS), ASC], [S1.CLASS, ASC]) |
| 2 - output([S1.CLASS], [S1.S_DATE], [S1.NAME]), filter(nil), rowset=256 |
| access([S1.CLASS], [S1.S_DATE], [S1.NAME]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([S1.ID]), range(MIN ; MAX)always true |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
18 rows in set (0.004 sec)
2.2、keep 形式一改寫標量子查詢
改寫邏輯很簡單，先分組找出最大、最小的 日期，然後透過日期 + class 外部標量子查詢找出最大、最小的name。
SELECT
CLASS,
MAX_S_DATE,
(SELECT max(NAME) FROM student s2 WHERE
decode(s1.CLASS,null,'bb',s1.CLASS) = decode(s2.CLASS,null,'bb',s2.CLASS) AND s2.S_DATE = s1.MAX_S_DATE) max_name,
MIN_S_DATE,
(SELECT min(NAME) FROM student s2 WHERE
decode(s1.CLASS,null,'bb',s1.CLASS) = decode(s2.CLASS,null,'bb',s2.CLASS) AND s2.S_DATE = s1.MIN_S_DATE) min_name
FROM (
SELECT
CLASS,
max(S_DATE) MAX_S_DATE,
min(S_DATE) MIN_S_DATE
FROM student
GROUP BY
CLASS
) s1 ;
27 rows in set (0.040 sec)
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ============================================================================================ |
| |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| |
| -------------------------------------------------------------------------------------------- |
| |0 |SUBPLAN FILTER | |26 |36802 | |
| |1 |├─MERGE GROUP BY | |26 |35450 | |
| |2 |│ └─TABLE FULL SCAN |STUDENT(IDX_CLASS_SDATE_STUDENT)|500001 |14409 | |
| |3 |├─SCALAR GROUP BY | |1 |26 | |
| |4 |│ └─DISTRIBUTED TABLE RANGE SCAN|S2(IDX_S_DATE_CALSS_ID) |1 |26 | |
| |5 |└─SCALAR GROUP BY | |1 |26 | |
| |6 | └─DISTRIBUTED TABLE RANGE SCAN|S2(IDX_S_DATE_CALSS_ID) |1 |26 | |
| ============================================================================================ |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([STUDENT.CLASS], [T_FUN_MAX(STUDENT.S_DATE)], [subquery(1)], [T_FUN_MIN(STUDENT.S_DATE)], [subquery(2)]), filter(nil), rowset=256 |
| exec_params_([STUDENT.CLASS(:0)], [T_FUN_MAX(STUDENT.S_DATE)(:1)], [STUDENT.CLASS(:2)], [T_FUN_MIN(STUDENT.S_DATE)(:3)]), onetime_exprs_(nil), init_plan_idxs_(nil), |
| use_batch=false |
| 1 - output([STUDENT.CLASS], [T_FUN_MAX(STUDENT.S_DATE)], [T_FUN_MIN(STUDENT.S_DATE)]), filter(nil), rowset=256 |
| group([STUDENT.CLASS]), agg_func([T_FUN_MAX(STUDENT.S_DATE)], [T_FUN_MIN(STUDENT.S_DATE)]) |
| 2 - output([STUDENT.CLASS], [STUDENT.S_DATE]), filter(nil), rowset=256 |
| access([STUDENT.CLASS], [STUDENT.S_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([STUDENT.CLASS], [STUDENT.S_DATE], [STUDENT.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true |
| 3 - output([T_FUN_MAX(S2.NAME)]), filter(nil), rowset=256 |
| group(nil), agg_func([T_FUN_MAX(S2.NAME)]) |
| 4 - output([S2.NAME]), filter([ora_decode(cast(:0, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(:0, VARCHAR2(2 BYTE))) |
| = ora_decode(cast(S2.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(S2.CLASS, VARCHAR2(2 BYTE)))]), rowset=256 |
| access([S2.ID], [S2.CLASS], [S2.NAME]), partitions(p0) |
| is_index_back=true, is_global_index=false, filter_before_indexback[true], |
| range_key([S2.S_DATE], [S2.CLASS], [S2.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true, |
| range_cond([S2.S_DATE = :1]) |
| 5 - output([T_FUN_MIN(S2.NAME)]), filter(nil), rowset=256 |
| group(nil), agg_func([T_FUN_MIN(S2.NAME)]) |
| 6 - output([S2.NAME]), filter([ora_decode(cast(:2, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(:2, VARCHAR2(2 BYTE))) |
| = ora_decode(cast(S2.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(S2.CLASS, VARCHAR2(2 BYTE)))]), rowset=256 |
| access([S2.ID], [S2.CLASS], [S2.NAME]), partitions(p0) |
| is_index_back=true, is_global_index=false, filter_before_indexback[true], |
| range_key([S2.S_DATE], [S2.CLASS], [S2.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true, |
| range_cond([S2.S_DATE = :3]) |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
38 rows in set (0.008 sec)
2.3、keep 形式一改寫left join
改成 left join 也是按照標量的邏輯，都比較簡單。
SELECT s1.CLASS,
s1.MAX_S_DATE,
max(CASE WHEN s2.S_DATE = s1.MAX_S_DATE THEN s2.NAME END) max_name,
s1.MIN_S_DATE,
min(CASE WHEN s2.S_DATE = s1.MIN_S_DATE THEN s2.NAME END ) min_name
FROM (
SELECT
CLASS,
max(S_DATE) MAX_S_DATE,
min(S_DATE) MIN_S_DATE
FROM student
GROUP BY
CLASS
)s1 LEFT JOIN student s2 ON decode(s1.CLASS,null,'bb',s1.CLASS) = decode(s2.CLASS,null,'bb',s2.CLASS)
GROUP BY
s1.CLASS,
s1.MAX_S_DATE,
s1.MIN_S_DATE;
27 rows in set (0.213 sec)
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| =================================================================================== |
| |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| |
| ----------------------------------------------------------------------------------- |
| |0 |HASH GROUP BY | |17 |200679 | |
| |1 |└─HASH OUTER JOIN | |65001 |193221 | |
| |2 | ├─SUBPLAN SCAN |S1 |26 |35450 | |
| |3 | │ └─MERGE GROUP BY | |26 |35450 | |
| |4 | │ └─TABLE FULL SCAN|STUDENT(IDX_CLASS_SDATE_STUDENT)|500001 |14409 | |
| |5 | └─TABLE FULL SCAN |S2 |500001 |22309 | |
| =================================================================================== |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([S1.CLASS], [S1.MAX_S_DATE], [T_FUN_MAX(CASE WHEN S2.S_DATE = S1.MAX_S_DATE THEN S2.NAME ELSE NULL END)], [S1.MIN_S_DATE], [T_FUN_MIN(CASE |
| WHEN S2.S_DATE = S1.MIN_S_DATE THEN S2.NAME ELSE NULL END)]), filter(nil), rowset=256 |
| group([S1.CLASS]), agg_func([T_FUN_MAX(CASE WHEN S2.S_DATE = S1.MAX_S_DATE THEN S2.NAME ELSE NULL END)], [T_FUN_MIN(CASE WHEN S2.S_DATE = S1.MIN_S_DATE |
| THEN S2.NAME ELSE NULL END)]) |
| 1 - output([S1.CLASS], [S2.S_DATE], [S2.NAME], [S1.MAX_S_DATE], [S1.MIN_S_DATE]), filter(nil), rowset=256 |
| equal_conds([ora_decode(cast(S1.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(S1.CLASS, VARCHAR2(2 BYTE))) |
| = ora_decode(cast(S2.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(S2.CLASS, VARCHAR2(2 BYTE)))]), other_conds(nil) |
| 2 - output([S1.CLASS], [S1.MAX_S_DATE], [S1.MIN_S_DATE]), filter(nil), rowset=256 |
| access([S1.CLASS], [S1.MAX_S_DATE], [S1.MIN_S_DATE]) |
| 3 - output([STUDENT.CLASS], [T_FUN_MAX(STUDENT.S_DATE)], [T_FUN_MIN(STUDENT.S_DATE)]), filter(nil), rowset=256 |
| group([STUDENT.CLASS]), agg_func([T_FUN_MAX(STUDENT.S_DATE)], [T_FUN_MIN(STUDENT.S_DATE)]) |
| 4 - output([STUDENT.CLASS], [STUDENT.S_DATE]), filter(nil), rowset=256 |
| access([STUDENT.CLASS], [STUDENT.S_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([STUDENT.CLASS], [STUDENT.S_DATE], [STUDENT.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true |
| 5 - output([S2.CLASS], [S2.S_DATE], [S2.NAME]), filter(nil), rowset=256 |
| access([S2.CLASS], [S2.S_DATE], [S2.NAME]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([S2.ID]), range(MIN ; MAX)always true |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
31 rows in set (0.005 sec)
2.4、keep 形式二
SELECT
DEPARTMENT_ID,
HIRE_DATE,
MAX(FIRST_NAME) KEEP (DENSE_RANK LAST ORDER BY HIRE_DATE) over(PARTITION BY DEPARTMENT_ID) max_name,
MIN(FIRST_NAME) KEEP (DENSE_RANK FIRST ORDER BY HIRE_DATE) over(PARTITION BY DEPARTMENT_ID) min_name
FROM EMPLOYEES ;
107 rows in set (0.002 sec)
2.5、keep 形式二改寫left join
SELECT
e1.DEPARTMENT_ID,
e1.HIRE_DATE,
e2.max_name,
e2.min_name
FROM EMPLOYEES e1 LEFT JOIN (
SELECT DEPARTMENT_ID,
MAX(CASE WHEN max_rn = 1 THEN FIRST_NAME ELSE NULL END) max_name,
MIN(CASE WHEN min_rn = 1 THEN FIRST_NAME ELSE NULL END) min_name
FROM (
SELECT
FIRST_NAME,
HIRE_DATE,
DEPARTMENT_ID,
DENSE_RANK() OVER(PARTITION BY DEPARTMENT_ID ORDER BY HIRE_DATE DESC) max_rn,
DENSE_RANK() OVER(PARTITION BY DEPARTMENT_ID ORDER BY HIRE_DATE ) min_rn
FROM EMPLOYEES
)
GROUP BY DEPARTMENT_ID
) e2 ON // 最後關聯
decode(e1.DEPARTMENT_ID, null, 'aaa', e1.DEPARTMENT_ID) =
decode(e2.DEPARTMENT_ID, null, 'aaa', e2.DEPARTMENT_ID);
107 rows in set (0.007 sec)
2.6、keep 形式二改寫標量子查詢
SELECT DEPARTMENT_ID,
HIRE_DATE,
(SELECT e2.max_name
FROM
(SELECT DEPARTMENT_ID,
max(FIRST_NAME) max_name
FROM
(SELECT x.*,
DENSE_RANK() OVER(PARTITION BY DEPARTMENT_ID
ORDER BY HIRE_DATE DESC) max_rn
FROM EMPLOYEES x )
WHERE max_rn = 1
GROUP BY DEPARTMENT_ID ) e2
WHERE decode(e1.department_id, null, 'aaa', e1.department_id) = decode(e2.department_id, null, 'aaa', e2.department_id) ) max_name,
(SELECT e2.min_name
FROM
(SELECT DEPARTMENT_ID,
min(FIRST_NAME) min_name
FROM
(SELECT x.*,
DENSE_RANK() OVER(PARTITION BY DEPARTMENT_ID
ORDER BY HIRE_DATE ) min_rn
FROM EMPLOYEES x )
WHERE min_rn = 1
GROUP BY DEPARTMENT_ID ) e2
WHERE decode(e1.department_id, null, 'aaa', e1.department_id) = decode(e2.department_id, null, 'aaa', e2.department_id) ) min_name
FROM EMPLOYEES e1;
107 rows in set (0.030 sec)
/* 改SQL 邏輯等價上面join sql, 如果會寫join版本改成表量子查詢是更簡單的操作 */
3、rank() over() 函式邏輯實現
rank() over() 函式會跳號，如果符合over()邏輯的資料有兩條的話，那麼這兩條會並列第一，然後第二條資料會加二（即顯示第三）。
這個函式在業務邏輯上比較少使用到，但是我們也可以嘗試使用其他邏輯來實現rank() over() 函式的功能。
**** 注意：以下實現的rank() over() 邏輯的程式碼效能，會比原來rank() over() 的效能更差，這裡只做功能實現，不考慮效能問題 ****
SELECT * FROM (
SELECT
FIRST_NAME,
DEPARTMENT_ID,
HIRE_DATE,
RANK() OVER(PARTITION BY DEPARTMENT_ID ORDER BY HIRE_DATE DESC) MAX_RN
FROM EMPLOYEES ) WHERE DEPARTMENT_ID = 80;
34 rows in set (0.004 sec)
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ======================================================== |
| |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| |
| -------------------------------------------------------- |
| |0 |WINDOW FUNCTION | |34 |42 | |
| |1 |└─SORT | |34 |16 | |
| |2 | └─TABLE FULL SCAN|EMPLOYEES|34 |10 | |
| ======================================================== |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE], [T_WIN_FUN_RANK()]), filter(nil), rowset=256 |
| win_expr(T_WIN_FUN_RANK()), partition_by(nil), order_by([EMPLOYEES.HIRE_DATE, DESC]), window_type(RANGE), upper(UNBOUNDED PRECEDING), lower(UNBOUNDED |
| FOLLOWING) |
| 1 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), filter(nil), rowset=256 |
| sort_keys([EMPLOYEES.HIRE_DATE, DESC]) |
| 2 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), filter([EMPLOYEES.DEPARTMENT_ID = 80]), rowset=256 |
| access([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false], |
| range_key([EMPLOYEES.EMPLOYEE_ID]), range(MIN ; MAX)always true |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
18 rows in set (0.004 sec)
3.1、改寫方式一：標量子查詢邏輯實現
SELECT
e.FIRST_NAME,
e.DEPARTMENT_ID,
e.HIRE_DATE,
(
SELECT count(HIRE_DATE) + 1 FROM EMPLOYEES e1 WHERE
DECODE(e1.DEPARTMENT_ID,NULL,'8080',e1.DEPARTMENT_ID) = DECODE(e.DEPARTMENT_ID,NULL,'8080',e.DEPARTMENT_ID) AND
e1.HIRE_DATE > e.HIRE_DATE
) MAX_RN
FROM EMPLOYEES e WHERE e.DEPARTMENT_ID = 80;
/*
重點是在標量子查詢內的 e1.HIRE_DATE > e.HIRE_DATE 邏輯，可以將 e1 表理解成一個滑動的視窗。
*/
34 rows in set (0.006 sec)
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ========================================================= |
| |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| |
| --------------------------------------------------------- |
| |0 |MERGE GROUP BY | |34 |29 | |
| |1 |└─NESTED-LOOP OUTER JOIN | |34 |22 | |
| |2 | ├─TABLE FULL SCAN |E |34 |11 | |
| |3 | └─MATERIAL | |1 |11 | |
| |4 | └─TABLE FULL SCAN |E1 |1 |11 | |
| ========================================================= |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [T_FUN_COUNT(E1.HIRE_DATE) + 1]), filter(nil), rowset=256 |
| group([E.EMPLOYEE_ID]), agg_func([T_FUN_COUNT(E1.HIRE_DATE)]) |
| 1 - output([E.EMPLOYEE_ID], [E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [E1.HIRE_DATE]), filter(nil), rowset=256 |
| conds([E1.HIRE_DATE > E.HIRE_DATE]), nl_params_(nil), use_batch=false |
| 2 - output([E.EMPLOYEE_ID], [E.DEPARTMENT_ID], [E.FIRST_NAME], [E.HIRE_DATE]), filter([E.DEPARTMENT_ID = 80]), rowset=256 |
| access([E.EMPLOYEE_ID], [E.DEPARTMENT_ID], [E.FIRST_NAME], [E.HIRE_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false], |
| range_key([E.EMPLOYEE_ID]), range(MIN ; MAX)always true |
| 3 - output([E1.HIRE_DATE]), filter(nil), rowset=256 |
| 4 - output([E1.HIRE_DATE]), filter([ora_decode(cast(E1.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 BYTE)), |
| cast(E1.DEPARTMENT_ID, VARCHAR2(40 BYTE))) = ora_decode(cast(cast(80, NUMBER(4, 0)), VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 |
| BYTE)), cast(cast(80, NUMBER(4, 0)), VARCHAR2(40 BYTE)))]), rowset=256 |
| access([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false], |
| range_key([E1.EMPLOYEE_ID]), range(MIN ; MAX)always true |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
26 rows in set (0.006 sec)
3.2、改寫方式二：LEFT JOIN 邏輯實現
會寫標量子查詢的情況下，改寫成LEFT JOIN 是很簡單的事，邏輯一樣不再贅述。
SELECT
e.FIRST_NAME,
e.DEPARTMENT_ID,
e.HIRE_DATE,
count(e1.HIRE_DATE) + 1 MAX_RN
FROM EMPLOYEES e LEFT JOIN EMPLOYEES e1 ON (
DECODE(e1.DEPARTMENT_ID,NULL,'8080',e1.DEPARTMENT_ID) = DECODE(e.DEPARTMENT_ID,NULL,'8080',e.DEPARTMENT_ID) AND e1.HIRE_DATE > e.HIRE_DATE)
WHERE e.DEPARTMENT_ID = 80
GROUP BY
e.FIRST_NAME,
e.DEPARTMENT_ID,
e.HIRE_DATE;
34 rows in set (0.012 sec)
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ========================================================== |
| |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| |
| ---------------------------------------------------------- |
| |0 |HASH GROUP BY | |34 |49 | |
| |1 |└─NESTED-LOOP OUTER JOIN | |34 |33 | |
| |2 | ├─SUBPLAN SCAN |VIEW1|34 |22 | |
| |3 | │ └─HASH GROUP BY | |34 |22 | |
| |4 | │ └─TABLE FULL SCAN |E |34 |10 | |
| |5 | └─MATERIAL | |1 |11 | |
| |6 | └─SUBPLAN SCAN |VIEW2|1 |11 | |
| |7 | └─HASH GROUP BY | |1 |11 | |
| |8 | └─TABLE FULL SCAN|E1 |1 |11 | |
| ========================================================== |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([VIEW1.E.FIRST_NAME], [VIEW1.E.DEPARTMENT_ID], [VIEW1.E.HIRE_DATE], [T_FUN_COUNT_SUM(VIEW1.T_FUN_COUNT(*) * CASE WHEN VIEW2.T_FUN_COUNT(E1.HIRE_DATE) |
| IS NOT NULL THEN VIEW2.T_FUN_COUNT(E1.HIRE_DATE) ELSE 0 END) + 1]), filter(nil), rowset=256 |
| group([VIEW1.E.HIRE_DATE], [VIEW1.E.FIRST_NAME]), agg_func([T_FUN_COUNT_SUM(VIEW1.T_FUN_COUNT(*) * CASE WHEN VIEW2.T_FUN_COUNT(E1.HIRE_DATE) IS NOT |
| NULL THEN VIEW2.T_FUN_COUNT(E1.HIRE_DATE) ELSE 0 END)]) |
| 1 - output([VIEW1.E.HIRE_DATE], [VIEW1.E.FIRST_NAME], [VIEW1.E.DEPARTMENT_ID], [VIEW2.T_FUN_COUNT(E1.HIRE_DATE)], [VIEW1.T_FUN_COUNT(*)]), filter(nil), rowset=256 |
| conds([VIEW2.E1.HIRE_DATE > VIEW1.E.HIRE_DATE]), nl_params_(nil), use_batch=false |
| 2 - output([VIEW1.E.DEPARTMENT_ID], [VIEW1.E.HIRE_DATE], [VIEW1.E.FIRST_NAME], [VIEW1.T_FUN_COUNT(*)]), filter(nil), rowset=256 |
| access([VIEW1.E.DEPARTMENT_ID], [VIEW1.E.HIRE_DATE], [VIEW1.E.FIRST_NAME], [VIEW1.T_FUN_COUNT(*)]) |
| 3 - output([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME], [T_FUN_COUNT(*)]), filter(nil), rowset=256 |
| group([E.HIRE_DATE], [E.FIRST_NAME]), agg_func([T_FUN_COUNT(*)]) |
| 4 - output([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME]), filter([E.DEPARTMENT_ID = 80]), rowset=256 |
| access([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false], |
| range_key([E.EMPLOYEE_ID]), range(MIN ; MAX)always true |
| 5 - output([VIEW2.T_FUN_COUNT(E1.HIRE_DATE)], [VIEW2.E1.HIRE_DATE]), filter(nil), rowset=256 |
| 6 - output([VIEW2.E1.HIRE_DATE], [VIEW2.T_FUN_COUNT(E1.HIRE_DATE)]), filter(nil), rowset=256 |
| access([VIEW2.E1.HIRE_DATE], [VIEW2.T_FUN_COUNT(E1.HIRE_DATE)]) |
| 7 - output([E1.HIRE_DATE], [T_FUN_COUNT(E1.HIRE_DATE)]), filter(nil), rowset=256 |
| group([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), agg_func([T_FUN_COUNT(E1.HIRE_DATE)]) |
| 8 - output([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), filter([ora_decode(cast(E1.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', |
| VARCHAR2(40 BYTE)), cast(E1.DEPARTMENT_ID, VARCHAR2(40 BYTE))) = ora_decode(cast(cast(80, NUMBER(4, 0)), VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), |
| cast('8080', VARCHAR2(40 BYTE)), cast(cast(80, NUMBER(4, 0)), VARCHAR2(40 BYTE)))]), rowset=256 |
| access([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false], |
| range_key([E1.EMPLOYEE_ID]), range(MIN ; MAX)always true |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
40 rows in set (0.011 sec)
4、 dense_rank() over() 函式邏輯實現
dense_rank() over() 函式不會跳號，如果符合over()邏輯的資料有兩條的話，那麼這兩條會並列第一，然後第二條資料會遞增加一（即顯示第二）。
這個函式在業務邏輯上也比較少用到，但是比rank()用得多，但是我們也可以嘗試使用其他邏輯來實現dense_rank() over() 函式的功能。
**** 注意：以下實現的dense_rank() over() 邏輯的程式碼效能，會比原來dense_rank() over() 的效能更差，這裡只做功能實現，不考慮效能問題 ****
SELECT * FROM (
SELECT
FIRST_NAME,
DEPARTMENT_ID,
HIRE_DATE,
dense_rank() OVER(PARTITION BY DEPARTMENT_ID ORDER BY HIRE_DATE DESC) MAX_RN
FROM EMPLOYEES ) WHERE DEPARTMENT_ID = 80;
34 rows in set (0.001 sec)
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ======================================================== |
| |ID|OPERATOR |NAME |EST.ROWS|EST.TIME(us)| |
| -------------------------------------------------------- |
| |0 |WINDOW FUNCTION | |107 |115 | |
| |1 |└─PARTITION SORT | |107 |33 | |
| |2 | └─TABLE FULL SCAN|EMPLOYEES|107 |9 | |
| ======================================================== |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE], [T_WIN_FUN_DENSE_RANK()]), filter(nil), rowset=256 |
| win_expr(T_WIN_FUN_DENSE_RANK()), partition_by([EMPLOYEES.DEPARTMENT_ID]), order_by([EMPLOYEES.HIRE_DATE, DESC]), window_type(RANGE), upper(UNBOUNDED |
| PRECEDING), lower(UNBOUNDED FOLLOWING) |
| 1 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), filter(nil), rowset=256 |
| sort_keys([HASH(EMPLOYEES.DEPARTMENT_ID), ASC], [EMPLOYEES.DEPARTMENT_ID, ASC], [EMPLOYEES.HIRE_DATE, DESC]) |
| 2 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), filter(nil), rowset=256 |
| access([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([EMPLOYEES.EMPLOYEE_ID]), range(MIN ; MAX)always true |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
18 rows in set (0.004 sec)
4.1、改寫方式一：標量子查詢邏輯實現
SELECT
e.FIRST_NAME,
e.DEPARTMENT_ID,
e.HIRE_DATE,
(SELECT COUNT(DISTINCT HIRE_DATE) +1 FROM EMPLOYEES e1 WHERE DECODE(e1.DEPARTMENT_ID,NULL,'8080',e1.DEPARTMENT_ID) = DECODE(e.DEPARTMENT_ID,NULL,'8080',e.DEPARTMENT_ID) AND e1.HIRE_DATE > e.HIRE_DATE ) MAX_RN
FROM EMPLOYEES e WHERE DEPARTMENT_ID = 80 ;
/*
COUNT(DISTINCT HIRE_DATE) 邏輯是求 e1.HIRE_DATE > e.HIRE_DATE 符合這個邏輯的 e1表的 HIRE_DATE 資料，等價 dense_rank。
*/
34 rows in set (0.006 sec)
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ========================================================= |
| |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| |
| --------------------------------------------------------- |
| |0 |MERGE GROUP BY | |34 |29 | |
| |1 |└─NESTED-LOOP OUTER JOIN | |34 |22 | |
| |2 | ├─TABLE FULL SCAN |E |34 |11 | |
| |3 | └─MATERIAL | |1 |11 | |
| |4 | └─TABLE FULL SCAN |E1 |1 |11 | |
| ========================================================= |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [T_FUN_COUNT(distinct E1.HIRE_DATE) + 1]), filter(nil), rowset=256 |
| group([E.EMPLOYEE_ID]), agg_func([T_FUN_COUNT(distinct E1.HIRE_DATE)]) |
| 1 - output([E.EMPLOYEE_ID], [E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [E1.HIRE_DATE]), filter(nil), rowset=256 |
| conds([E1.HIRE_DATE > E.HIRE_DATE]), nl_params_(nil), use_batch=false |
| 2 - output([E.EMPLOYEE_ID], [E.DEPARTMENT_ID], [E.FIRST_NAME], [E.HIRE_DATE]), filter([E.DEPARTMENT_ID = 80]), rowset=256 |
| access([E.EMPLOYEE_ID], [E.DEPARTMENT_ID], [E.FIRST_NAME], [E.HIRE_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false], |
| range_key([E.EMPLOYEE_ID]), range(MIN ; MAX)always true |
| 3 - output([E1.HIRE_DATE]), filter(nil), rowset=256 |
| 4 - output([E1.HIRE_DATE]), filter([ora_decode(cast(E1.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 BYTE)), |
| cast(E1.DEPARTMENT_ID, VARCHAR2(40 BYTE))) = ora_decode(cast(cast(80, NUMBER(4, 0)), VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 |
| BYTE)), cast(cast(80, NUMBER(4, 0)), VARCHAR2(40 BYTE)))]), rowset=256 |
| access([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false], |
| range_key([E1.EMPLOYEE_ID]), range(MIN ; MAX)always true |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
26 rows in set (0.006 sec)
4.2、改寫方式二：LEFT JOIN 邏輯實現
會寫標量子查詢的情況下，改寫成LEFT JOIN 是很簡單的事，邏輯一樣不再贅述。
SELECT
e.FIRST_NAME,
e.DEPARTMENT_ID,
e.HIRE_DATE,
count(DISTINCT e1.HIRE_DATE) + 1 MAX_RN
FROM EMPLOYEES e LEFT JOIN EMPLOYEES e1 ON (
DECODE(e1.DEPARTMENT_ID,NULL,'8080',e1.DEPARTMENT_ID) = DECODE(e.DEPARTMENT_ID,NULL,'8080',e.DEPARTMENT_ID) AND e1.HIRE_DATE > e.HIRE_DATE)
WHERE e.DEPARTMENT_ID = 80
GROUP BY
e.FIRST_NAME,
e.DEPARTMENT_ID,
e.HIRE_DATE;
34 rows in set (0.006 sec)
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ===================================================== |
| |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| |
| ----------------------------------------------------- |
| |0 |MERGE GROUP BY | |34 |76 | |
| |1 |└─PARTITION SORT | |34 |70 | |
| |2 | └─HASH OUTER JOIN | |34 |62 | |
| |3 | ├─TABLE FULL SCAN|E |34 |10 | |
| |4 | └─TABLE FULL SCAN|E1 |107 |9 | |
| ===================================================== |
| Outputs & filters: |
| ------------------------------------- |
| 0 - output([E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [T_FUN_COUNT(distinct E1.HIRE_DATE) + 1]), filter(nil), rowset=256 |
| group([E.FIRST_NAME], [E.HIRE_DATE]), agg_func([T_FUN_COUNT(distinct E1.HIRE_DATE)]) |
| 1 - output([E.FIRST_NAME], [E.HIRE_DATE], [E.DEPARTMENT_ID], [E1.HIRE_DATE]), filter(nil), rowset=256 |
| sort_keys([HASH(E.FIRST_NAME, E.HIRE_DATE), ASC], [E.FIRST_NAME, ASC], [E.HIRE_DATE, ASC]) |
| 2 - output([E.FIRST_NAME], [E.HIRE_DATE], [E.DEPARTMENT_ID], [E1.HIRE_DATE]), filter(nil), rowset=256 |
| equal_conds([ora_decode(cast(E1.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 BYTE)), cast(E1.DEPARTMENT_ID, |
| VARCHAR2(40 BYTE))) = ora_decode(cast(E.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 BYTE)), cast(E.DEPARTMENT_ID, |
| VARCHAR2(40 BYTE)))]), other_conds([E1.HIRE_DATE > E.HIRE_DATE]) |
| 3 - output([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME]), filter([E.DEPARTMENT_ID = 80]), rowset=256 |
| access([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME]), partitions(p0) |
| is_index_back=false, is_global_index=false, filter_before_indexback[false], |
| range_key([E.EMPLOYEE_ID]), range(MIN ; MAX)always true |
| 4 - output([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), filter(nil), rowset=256 |
| access([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), partitions(p0) |
| is_index_back=false, is_global_index=false, |
| range_key([E1.EMPLOYEE_ID]), range(MIN ; MAX)always true |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
27 rows in set (0.005 sec)
5、 總結
SQL語句雖然很簡單，但是要培養出SQL思維可不是一件容易的事，還是得多看，多寫，多思考。
這個是要透過不同的開發需求日積月累形成的思維邏輯，而不是一蹴而就。