Question

背景：我是SQL的新手，并且正在使用this homework assignment开设Coursera课程。 Data file here。该数据库包含一个表frequency(docid, term, count)。

实际问题：为什么以下两个SQLite查询会返回不同的结果？（1）

SELECT count(*)
FROM(
        SELECT term
        FROM frequency
        WHERE docid = '10398_txt_earn' AND count=1
    UNION
        SELECT term
        FROM frequency
        WHERE docid = '925_txt_trade' AND count=1);

返回

coun
----
324

（2）

SELECT count(*)
FROM frequency
WHERE (docid = '10398_txt_earn' OR docid = '925_txt_trade') AND count = 1;

返回

coun
----
335

Answer 1

以下查询给出了325

SELECT count(term)
FROM(
        SELECT term
        FROM frequency
        WHERE docid = '10398_txt_earn' AND count=1
          UNION ALL
        SELECT term
        FROM frequency
        WHERE docid = '925_txt_trade' AND count=1
);

问题是UNION合并了重复的结果，因此以下查询

SELECT A.TERM
FROM (
  SELECT term
  FROM frequency
  WHERE docid = '10398_txt_earn' AND count=1
) A
JOIN (    
  SELECT term
  FROM frequency
  WHERE docid = '925_txt_trade' AND count=1
) B ON A.term = B.term;

将显示两者中的项目（其中11个）

based
costs
export
july
many
march
month
problems
reuter
speech
world

此查询也会给出相同的结果：

SELECT term
FROM frequency
WHERE docid = '10398_txt_earn' AND count=1
INTERSECT
SELECT term
FROM frequency
WHERE docid = '925_txt_trade' AND count=1;

为什么这些SQLite查询会返回不同的结果？

1 个答案: