Question

我有两张这样的表：

summary(id, status, value, time, aperson) and long_summary(id, who, comment, alltext)

因为我尝试了下面的查询，如：

SELECT l.id,summary.status
FROM  long_summary l INNER JOIN summary ON l.id = summary.id INNER JOIN long_summary ON summary.aperson = long_summary.who 
WHERE summary.status IN('old','new','waiting')
AND summary.value IN ('') 
AND summary.time >= DATE_SUB(NOW(),INTERVAL 2 MONTH)
AND l.alltext LIKE '% relational database management system %' ORDER BY FIELD(summary.status,'old','new','waiting'),summary.time DESC

在运行它时会提供正确的输出但是已经多次返回的行如下：

(1,'old')
(1,'old')
(1,'old')
(2,'new')
(2,'new')
(3,'new')
(4,'waiting')
(4,'waiting')

对于那个i have used SELECT DISTINCT and after that it is giving correct output without repetition of the single row。但我没有得到我做错了什么？你能建议吗？

Answer 1

查看正在发生的事情的方法是（a）创建一个精简的测试用例，然后（b）执行SELECT *，这样你就可以看到你在其他列中得到的内容，这会导致重复。

所以，我这样做了：

CREATE TABLE summary(id, status, value, time, aperson);
INSERT INTO "summary" VALUES(1,'old',23,'time0','joe');
INSERT INTO "summary" VALUES(2,'new',42,'time1','bob');
INSERT INTO "summary" VALUES(3,'new',32,'time2','mike');
CREATE TABLE long_summary(id, who, comment, alltext);
INSERT INTO "long_summary" VALUES(1,'someone','i say!','some text');
INSERT INTO "long_summary" VALUES(1,'joe','joe likes','some text');
INSERT INTO "long_summary" VALUES(2,'joe','joe likes bob','some text');
INSERT INTO "long_summary" VALUES(3,'joe','joe likes mike','some text');
INSERT INTO "long_summary" VALUES(1,'bob','nice one, joe','some text');
INSERT INTO "long_summary" VALUES(2,'bob','nice one, me','some text');
INSERT INTO "long_summary" VALUES(2,'bob','double nice one, me','some text');
INSERT INTO "long_summary" VALUES(3,'bob','nice one, mike','some text');
COMMIT;

然后我提出了一个更简单的查询版本：

SELECT l.id,summary.status
FROM  long_summary l INNER JOIN summary ON l.id = summary.id INNER JOIN long_summary ON summary.aperson = long_summary.who 
WHERE summary.status IN('old','new','waiting')

其他任何东西都不会让事情变得更糟，对吧？所以这是无关紧要的。运行时我能得到什么？ 9份1|old份和12份2|new。

所以，让我们改变它以查看整行：

SELECT *
FROM  long_summary l INNER JOIN summary ON l.id = summary.id INNER JOIN long_summary ON summary.aperson = long_summary.who 
WHERE summary.status IN('old','new','waiting')

1|bob|nice one, joe|some text|1|old|23|time0|joe|1|joe|joe likes|some text
1|bob|nice one, joe|some text|1|old|23|time0|joe|2|joe|joe likes bob|some text
1|bob|nice one, joe|some text|1|old|23|time0|joe|3|joe|joe likes mike|some text
1|joe|joe likes|some text|1|old|23|time0|joe|1|joe|joe likes|some text
1|joe|joe likes|some text|1|old|23|time0|joe|2|joe|joe likes bob|some text
1|joe|joe likes|some text|1|old|23|time0|joe|3|joe|joe likes mike|some text
1|someone|i say!|some text|1|old|23|time0|joe|1|joe|joe likes|some text
1|someone|i say!|some text|1|old|23|time0|joe|2|joe|joe likes bob|some text
1|someone|i say!|some text|1|old|23|time0|joe|3|joe|joe likes mike|some text
2|bob|double nice one, me|some text|2|new|42|time1|bob|1|bob|nice one, joe|some text
2|bob|double nice one, me|some text|2|new|42|time1|bob|2|bob|double nice one, me|some text
2|bob|double nice one, me|some text|2|new|42|time1|bob|2|bob|nice one, me|some text
2|bob|double nice one, me|some text|2|new|42|time1|bob|3|bob|nice one, mike|some text
2|bob|nice one, me|some text|2|new|42|time1|bob|1|bob|nice one, joe|some text
2|bob|nice one, me|some text|2|new|42|time1|bob|2|bob|double nice one, me|some text
2|bob|nice one, me|some text|2|new|42|time1|bob|2|bob|nice one, me|some text
2|bob|nice one, me|some text|2|new|42|time1|bob|3|bob|nice one, mike|some text
2|joe|joe likes bob|some text|2|new|42|time1|bob|1|bob|nice one, joe|some text
2|joe|joe likes bob|some text|2|new|42|time1|bob|2|bob|double nice one, me|some text
2|joe|joe likes bob|some text|2|new|42|time1|bob|2|bob|nice one, me|some text
2|joe|joe likes bob|some text|2|new|42|time1|bob|3|bob|nice one, mike|some text

好的，现在你可以看到问题，让我们看看为什么正在发生。这些行中的每一行都应该吗？换句话说，如果前三个组合中至少有一个存在，你应该看到1|old吗？

如果没有，哪些不应该造成它？您需要在WHERE或JOIN中过滤掉某些内容。

如果是这样，那么您需要GROUP BY将相关字段合并在一起，或者在某处合并OR，更可能是第一个。

逐步通过所有提出相同问题的小组。如果您在实际显示的列上需要GROUP BY，则只需使用SELECT DISTINCT即可。

您也可能想退后一步，询问您是否真的需要一个完整的JOIN long_summary以及带有long_summary的摘要。这是8 * 3 * 8 = 192行，你已经过滤到21。这是否有意义，或者你只是期望，比方说，24行过滤？如果是后者，那你就错了JOIN。这些JOIN中的任何一个都不应该存在，或者它应该是一对一而不是一对多JOIN，或者其他错误。< / p>

顺便说一句，您可以从上面的测试中看出我使用的是sqlite3而不是mysql，因为它更容易获取和运行。我怀疑这有什么不同，但如果你在mysql测试并看到不同的结果，请务必告诉我。

多次返回单行

1 个答案: