[英]Oracle How to simplify a select count(*) from (subquery)?
我被要求嘗試簡化count()
查詢,但是我不知道從哪里開始,查詢是這樣的:
SELECT COUNT( 1 )
FROM (
SELECT DISTINCT a.col,b.colx,c.coly
FROM a
JOIN b on a.id = b.id
JOIN c on b.id = c.id
WHERE a.xyz = 'something'
AND b.hijk = 'something else'
AND c.id IN (
SELECT cid
FROM cwa
WHERE csid = 22921
)
ORDER BY
e.create_timestamp DESC
);
有人告訴我可以簡化SELECT COUNT(1) FROM (subquery)
,這怎么做?
我已經嘗試了幾件事,但是結果與上面的查詢不同。
除非您在rownum上過濾結果,否則子查詢中的order-by
不會有用(有時會出錯,具體取決於上下文)。 您可以用聯接替換內部子查詢:
SELECT COUNT(*)
FROM (
SELECT DISTINCT a.col,b.colx,c.coly
FROM a
JOIN b on a.id = b.id
JOIN c on b.id = c.id
JOIN cwa on c.id cwa.cid
WHERE a.xyz = 'something'
AND b.hijk = 'something else'
AND cwa.csid = 22921
);
如果您可以識別出您選擇的三列中沒有出現的字符,甚至可以不使用子查詢就可以做到這一點,因此您可以將其用作分隔符。 例如,如果您永遠不會出現波浪號,則可以執行以下操作:
SELECT COUNT(DISTINCT a.col ||'~'|| b.colx ||'~'|| c.coly)
FROM a
JOIN b on a.id = b.id
JOIN c on b.id = c.id
JOIN cwa on c.id cwa.cid
WHERE a.xyz = 'something'
AND b.hijk = 'something else'
AND cwa.csid = 22921;
盡管這是更簡單還是更清晰是一個意見問題。
由於count()
僅接受單個參數,並且您希望計算這三列的(不同的)組合,因此該機制將所有三個列連接為一個字符串,然后計算該字符串的出現次數。 添加了定界符,以便您可以區分歧義的列值,例如在CTE中使用人為的示例:
with cte (col1, col2) as (
select 'The', 'search' from dual
union all select 'These', 'arch' from dual
)
select col1, col2,
col1 || col2 as bad,
col1 ||'~'|| col2 as good
from cte;
COL1 COL2 BAD GOOD
----- ------ ----------- ------------
The search Thesearch The~search
These arch Thesearch These~arch
通過簡單的“壞”串聯,兩行看起來是相同的。 通過添加定界符以制作“好”版本,您仍然可以區分它們,因此計算不同的串聯值可獲得正確的答案:
with cte (col1, col2) as (
select 'The', 'search' from dual
union all select 'These', 'arch' from dual
)
select count(distinct col1 || col2) as bad_count,
count (distinct col1 ||'~'|| col2) as good_count
from cte;
BAD_COUNT GOOD_COUNT
---------- ----------
1 2
如果col1
以波浪號結尾,或者col2
以波浪號開頭,您將回到歧義:
with cte (col1, col2) as (
select 'The~', 'search' from dual
union all select 'The', '~search' from dual
)
select col1, col2,
col1 || col2 as bad,
col1 ||'~'|| col2 as still_bad
from cte;
COL1 COL2 BAD STILL_BAD
---- ------- ----------- ------------
The~ search The~search The~~search
The ~search The~search The~~search
因此分隔符必須是在任何值中都找不到的東西。
嘗試使用
SELECT count(DISTINCT a.col)
FROM a
JOIN b on a.id = b.id
JOIN c on b.id = c.id
WHERE a.xyz = 'something'
AND b.hijk = 'something else'
AND c.id IN (
SELECT cid
FROM cwa
WHERE csid = 22921
);
因為order by會增加您不必要的執行時間
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