SQL在表的两个副本之间使用WHERE子句依赖关系进行左自联接
以下两句话:SQL在表的两个副本之间使用WHERE子句依赖关系进行左自联接,sql,outer-join,Sql,Outer Join,以下两句话: hello there bye! 在表中,用以下词语表示: WORD_ID SENTENCE_ID WORD WORD_NUMBER 10 1 hello 1 11 1 there 2 12 2 bye! 1 我想执行一个外部联接查询,该查询将提供以下结果: WORD1 WORD2 hello there bye!
hello there
bye!
在表中,用以下词语表示:
WORD_ID SENTENCE_ID WORD WORD_NUMBER
10 1 hello 1
11 1 there 2
12 2 bye! 1
我想执行一个外部联接查询,该查询将提供以下结果:
WORD1 WORD2
hello there
bye! NULL
注意,我可能想从句子的中间开始,所以我不能假定Word2的词数=2。如果我选择my_start_number=2,则查询应给出:
WORD1 WORD2
there NULL
我试过:
(my_start_number = 1)
select s1.word word1, s2.word word2
from sentence_words s1
left join sentence_words s2
on s1.sentence_id = s2.sentence_id
where s1.word_number = my_start_number
and (s2.word_number = s1.word_number +1 or s2.word_number is null);
只有在句子中有两个词的情况下,这才会给我一个结果。我不知道该怎么做,这并不复杂。将
单词+1
要求移动到左连接中
SELECT
s1.word word1, s2.word word2
FROM
sentence_words s1
LEFT JOIN
sentence_words s2
ON s2.sentence_id = s1.sentence_id
AND s2.word_number = s1.word_number + 1
WHERE
s1.word_number = my_start_number
NECRO编辑:
虽然上面修复了左连接的使用,但我建议根本不要使用连接
SELECT
sentence_id,
MAX(CASE WHEN pos = 0 THEN word END) AS word1,
MAX(CASE WHEN pos = 1 THEN word END) AS word2
FROM
(
SELECT
sentence_id,
word_number - MY_START_NUMBER AS pos,
word
FROM
sentence_words
)
AS offset_sentence_words
WHERE
pos IN (0, 1)
GROUP BY
sentence_id
Dems的答案绝对正确。我决定写这个答案来解释你原来的解决方案不起作用的原因。这是因为您正在尝试筛选left outter联接的以下结果集(显示所有列,某些名称缩写为fit):
现在,看看您的where子句:
where s1.word_number = my_start_number
and (s2.word_number = s1.word_number +1 or s2.word_number is null);
。。。这应该相对容易理解为什么它不起作用。例如,s2.word\u number
从不为NULL
什么在使用mssql、mysql、oracel?您是在尝试重新创建句子还是只对返回成对的单词感兴趣?
where s1.word_number = my_start_number
and (s2.word_number = s1.word_number +1 or s2.word_number is null);