1

I have the following query:

  SELECT (CASE WHEN cc.id IS NULL THEN dc.id ELSE cc.id END) AS id,
 l.CALL,l.QSO_DATE,cc.prefix,l.state,l.QSL_RCVD,l.Band,l.Mode,
(CASE  WHEN CC.Country IS NULL THEN dc.Country ELSE cc.Country END) AS country,
(CASE  WHEN CC.Country IS NULL THEN 'Y' ELSE 'N' END) AS 'deleted',
(CASE  WHEN CC.flags IS NULL THEN dc.flags ELSE cc.flags END) AS 'flag'
FROM lotw l
LEFT OUTER JOIN CountryCode AS cc on cc.code = l.dxcc 
LEFT OUTER JOIN DeleteCountry AS dc on dc.code = l.dxcc 
WHERE l.DXCC > '' 

GROUP BY CC.Country,dc.Country  ORDER BY cc.COUNTRY,dc.COUNTRY.

which returns a set of unique country that matches the countrycode table code field. the speed is good less than a second.

I needed to expand this so the query select's the earliest QSLRDATE from the LOTW table. so I modify the query like this:

SELECT (CASE WHEN cc.id IS NULL THEN dc.id ELSE cc.id END) AS id, l.CALL,l.QSO_DATE,cc.prefix,l.state,l.QSL_RCVD,l.Band,l.Mode,
(CASE  WHEN CC.Country IS NULL THEN dc.Country ELSE cc.Country END) AS country,
(CASE  WHEN CC.Country IS NULL THEN 'Y' ELSE 'N' END) AS 'deleted',
(CASE  WHEN CC.flags IS NULL THEN dc.flags ELSE cc.flags END) AS 'flag'
FROM lotw l
LEFT OUTER JOIN CountryCode AS cc on cc.code = l.dxcc 
LEFT OUTER JOIN DeleteCountry AS dc on dc.code = l.dxcc 
WHERE l.QSLRDATE = (SELECT MIN(l2.QSLRDATE) FROM LOTW l2 WHERE l2.DXCC=l.DXCC) AND l.DXCC > '' 

GROUP BY CC.Country,dc.Country  ORDER BY cc.COUNTRY,dc.COUNTRY

this works but the performance went from less than a second to 15 seconds. The sub query I added:

l.QSLRDATE = (SELECT MIN(l2.QSLRDATE) FROM LOTW l2 WHERE l2.DXCC=l.DXCC)

can't be the best way to accomplish what I want. any help would be great.

4

1 回答 1

0

尝试加入子查询，而不是在 WHERE 子句中使用相关的子查询。

SELECT
  (CASE WHEN cc.id IS NULL THEN dc.id ELSE cc.id END) AS id,
  l.CALL,l.QSO_DATE,cc.prefix,l.state,l.QSL_RCVD,l.Band,l.Mode,
  (CASE  WHEN CC.Country IS NULL THEN dc.Country ELSE cc.Country END) AS country,
  (CASE  WHEN CC.Country IS NULL THEN 'Y' ELSE 'N' END) AS 'deleted',
  (CASE  WHEN CC.flags IS NULL THEN dc.flags ELSE cc.flags END) AS 'flag'
FROM
  (SELECT dxcc, MIN(qslrdate) AS qslrdate FROM lotw GROUP BY dxcc) AS qslr
     INNER JOIN lotw AS l ON l.dxcc = qslr.dxcc AND l.qslrdate = qslr.qslrdate
LEFT OUTER JOIN CountryCode AS cc on cc.code = l.dxcc 
LEFT OUTER JOIN DeleteCountry AS dc on dc.code = l.dxcc 
WHERE l.DXCC > ''
GROUP BY CC.Country,dc.Country  ORDER BY cc.COUNTRY,dc.COUNTRY

此外，确保lotw有一个索引(dxcc, qslrdate)以确保子查询和连接都执行良好。

事实上，请确保您首先拥有该索引，然后重试您的查询。然后，如果性能仍然很差，请尝试上述查询。

于 2013-07-05T21:03:37.097 回答