SELECT PLD_LINK.ID, PLD_LINK.TITLE, PLD_LINK.URL, PLD_CATEGORY.TITLE, TLL_SORT_STATUS.status, PLD_LINK_COMMENT.DATE_ADDED
FROM PLD_LINK, PLD_CATEGORY, TLL_SORT_STATUS, PLD_LINK_COMMENT
WHERE PLD_LINK.CATEGORY_ID = PLD_CATEGORY.ID
AND PLD_LINK.ID = TLL_SORT_STATUS.link_id
AND PLD_LINK.PAGERANK BETWEEN -1 AND 2
AND PLD_LINK.CATEGORY_ID IN (1291,2113,2664)
ORDER BY COUNT(PLD_LINK.ID = PLD_LINK_COMMENT.LINK_ID) ASC
这是我的查询。我想在ORDER BY之前得到所有匹配的结果。现在，PLD_LINK.ID中的某些ID可能位于表PLD_LINK_COMMENT中，但有些可能不是。因此，当mysql到达ORDER BY时，我想根据以下标准按升序排序结果：不在PLD_LINK_COMMENT中的ID应该是第一个，之后是基于每个ID的注释数量的ID 。 （一个ID可以没有，1,2 ......评论。）
答案 0 :(得分：1)
您需要在三个主要表格和PLD_LINK_COMMENT之间进行外部联接。
外部联接意味着您获得PLD_LINK_COMMENT.LINK_ID的空值，这意味着COUNT将为PLD_LINK.ID返回0，没有匹配的链接注释，以及其他注释的相应数量的注释。
您需要按非聚合列进行分组......但这表明M.Date_Added可能不是您想要的GROUP BY;并且评论中的问题/答案显示我们需要最近的评论日期，或者一些荒谬的早期数据，例如'0001-01-01'（SQL中最早的有效日期）或'0000-00-00'（伪 - MySQL使用的日期。
因此：
SELECT K.ID, K.Title, K.URL, C.Title, S.Status,
MAX(NVL(M.Date_Added, DATE '0001-01-01')) AS MostRecentComment
FROM Pld_Link AS K JOIN Pld_Category AS C ON K.Category_ID = C.ID
JOIN Tll_Sort_Status AS C ON K.ID = S.Link_ID
OUTER JOIN Pld_Link_Comment AS M ON K.ID = M.Link_ID
WHERE K.PageRank BETWEEN -1 AND 2
AND K.Category_ID IN (1291,2113,2664)
GROUP BY K.ID, K.Title, K.URL, C.Title, S.Status
ORDER BY COUNT(M.Link_ID) ASC
您还需要您的DBMS（您说的MySQL），以允许您按SELECT列表中未列出的列进行排序。
在MacOS X 10.6.2上使用IBM Informix Dynamic Server 11.50进行测试。我使用了DATE('0001-01-01')，因为IDS有一个名为DATE的函数需要括号，我使用'0001-01-01'因为IDS不会将'0000-00-00'识别为有效日期。
SELECT K.ID, K.Title, K.URL, C.Title, S.STATUS,
MAX(NVL(M.Date_Added, DATE('0001-01-01'))) AS MostRecentComment,
FROM PLD_Link AS K JOIN PLD_Category AS C ON K.Category_ID = C.ID
JOIN Tll_Sort_Status AS S ON K.ID = S.Link_ID
LEFT OUTER JOIN PLD_Link_Comment AS M ON K.ID = M.Link_ID
WHERE K.PageRank BETWEEN -1 AND 2
AND K.Category_ID IN (1291,2113,2664)
GROUP BY K.ID, K.Title, K.URL, C.Title, S.STATUS
ORDER BY COUNT(M.Link_ID)
结果：
id title url title status mostrecent
4 #4: This should show example.com/?id=4;c=2113 Cat 2113 0 0001-01-01
3 #3: This should show example.com/?id=3;c=1291 Cat 1291 -1 2010-03-10
7 #7: This should show example.com/?id=7;c=2664 Cat 2664 3 2010-07-11
5 #5: This should show example.com/?id=5;c=2113 Cat 2113 1 2010-05-17
2 #2: This should show example.com/?id=2;c=1291 Cat 1291 -2 2010-02-13
备选，选择评论计数，以显示数据以正确的顺序显示。
SELECT K.ID, K.Title, K.URL, C.Title, S.STATUS,
MAX(NVL(M.Date_Added, DATE('0001-01-01'))) AS MostRecentComment,
COUNT(M.Link_ID) AS CommentCount
FROM PLD_Link AS K JOIN PLD_Category AS C ON K.Category_ID = C.ID
JOIN Tll_Sort_Status AS S ON K.ID = S.Link_ID
LEFT OUTER JOIN PLD_Link_Comment AS M ON K.ID = M.Link_ID
WHERE K.PageRank BETWEEN -1 AND 2
AND K.Category_ID IN (1291,2113,2664)
GROUP BY K.ID, K.Title, K.URL, C.Title, S.STATUS
ORDER BY CommentCount ASC;
结果集：
id title url title status recent count
4 #4: This should show example.com/?id=4;c=2113 Cat 2113 0 0001-01-01 0
3 #3: This should show example.com/?id=3;c=1291 Cat 1291 -1 2010-03-10 1
7 #7: This should show example.com/?id=7;c=2664 Cat 2664 3 2010-07-11 1
5 #5: This should show example.com/?id=5;c=2113 Cat 2113 1 2010-05-17 2
2 #2: This should show example.com/?id=2;c=1291 Cat 1291 -2 2010-02-13 3
CREATE TABLE PLD_Category
(
ID SERIAL NOT NULL PRIMARY KEY,
Title VARCHAR(32) NOT NULL
);
INSERT INTO PLD_Category VALUES(3961, 'Cat 3961');
INSERT INTO PLD_Category VALUES(1291, 'Cat 1291');
INSERT INTO PLD_Category VALUES(2113, 'Cat 2113');
INSERT INTO PLD_Category VALUES(2664, 'Cat 2664');
CREATE TABLE PLD_Link
(
ID SERIAL NOT NULL PRIMARY KEY,
Category_ID INTEGER NOT NULL REFERENCES PLD_Category,
Title VARCHAR(32) NOT NULL,
PageRank INTEGER NOT NULL,
URL VARCHAR(64) NOT NULL
);
INSERT INTO PLD_Link
VALUES(1, 3961, '#1: This should not', 0, 'example.com/?id=1;c=3961');
INSERT INTO PLD_Link
VALUES(2, 1291, '#2: This should show', 0, 'example.com/?id=2;c=1291');
INSERT INTO PLD_Link
VALUES(3, 1291, '#3: This should show', 0, 'example.com/?id=3;c=1291');
INSERT INTO PLD_Link
VALUES(4, 2113, '#4: This should show', 0, 'example.com/?id=4;c=2113');
INSERT INTO PLD_Link
VALUES(5, 2113, '#5: This should show', 0, 'example.com/?id=5;c=2113');
INSERT INTO PLD_Link
VALUES(6, 3961, '#6: This should not', 0, 'example.com/?id=6;c=3961');
INSERT INTO PLD_Link
VALUES(7, 2664, '#7: This should show', 0, 'example.com/?id=7;c=2664');
INSERT INTO PLD_Link
VALUES(8, 2664, '#8: This should show', 0, 'example.com/?id=8;c=2664');
CREATE TABLE TLL_Sort_Status
(
STATUS INTEGER NOT NULL,
Link_ID INTEGER NOT NULL REFERENCES PLD_Link
);
INSERT INTO TLL_Sort_Status VALUES(-3, 1);
INSERT INTO TLL_Sort_Status VALUES(-2, 2);
INSERT INTO TLL_Sort_Status VALUES(-1, 3);
INSERT INTO TLL_Sort_Status VALUES( 0, 4);
INSERT INTO TLL_Sort_Status VALUES(+1, 5);
INSERT INTO TLL_Sort_Status VALUES(+2, 6);
INSERT INTO TLL_Sort_Status VALUES(+3, 7);
INSERT INTO TLL_Sort_Status VALUES(+4, 1);
CREATE TABLE PLD_Link_Comment
(
Link_ID INTEGER NOT NULL REFERENCES PLD_Link,
Date_Added DATE NOT NULL,
COMMENT VARCHAR(64) NOT NULL
);
INSERT INTO PLD_Link_Comment VALUES(1, DATE('2010-01-11'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(2, DATE('2010-02-11'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(2, DATE('2010-02-12'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(2, DATE('2010-02-13'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(3, DATE('2010-03-10'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(5, DATE('2010-05-15'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(5, DATE('2010-05-17'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(6, DATE('2010-06-10'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(6, DATE('2010-06-12'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(6, DATE('2010-06-14'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(6, DATE('2010-06-19'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(7, DATE('2010-07-11'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(8, DATE('2010-08-12'), 'Vacuous comment');
INSERT INTO PLD_Link_Comment VALUES(8, DATE('2010-08-13'), 'Vacuous comment');