Question

我试图将产品评论分类到图表中。为了总结结果，我将结果集分成四个卡盘：

Score >= 4.5
Score < 4.5 && Score >= 4
Score < 4 && Score >= 3
Score < 3

SQL请求如下所示：

SELECT DISTINCT COUNT(*) AS cnt
    FROM reviews 
    WHERE score >= 4.5
    AND id = 4 
    AND live = '1'
UNION
SELECT DISTINCT COUNT(*) AS cnt
    FROM reviews 
    WHERE score >= 4 
    AND score < 4.5
    AND id = 4 
    AND live = '1'
UNION
SELECT DISTINCT COUNT(*) AS cnt
    FROM reviews
    WHERE score >= 3
    AND score < 4
    AND id = 4 
    AND live = '1'
UNION
SELECT DISTINCT COUNT(*) AS cnt
    FROM reviews 
    WHERE score < 3
    AND id = 4 
    AND live = '1'

然后将其放入数组中。我注意到上面的请求返回十三行，或者计数总和等于13.我得到以下结果：

10
  2
  1

执行以下请求...

SELECT score
    FROM reviews
    WHERE live = '1'
    AND id = 4

...返回十四行。我想也许结果集可能会导致问题，但我没有看到任何问题。分数集如下：

5.0,5.0,5.0,5.0,4.9,4.9,4.9
4.8,4.8,4.7,4.4,4.2,3.4,2.6

我不确定为什么单行丢失，任何想法，也许我的语法不稳定？为什么最终的联合（score < 3）什么都不返回？

Answer 1

请注意，最后两个查询都返回1，并且由于UNION删除了重复项，10 2 1 1 - ＆gt; 10 2 1。使用UNION ALL（请参阅What is the difference between UNION and UNION ALL?）。

Answer 2

或者，跳过UNION，而不是使用CASE进行条件计数：

SELECT COUNT(case when Score >= 4.5 then 1 end) AS score_gt_4p5,
       COUNT(case when Score < 4.5 AND Score >= 4 then 1 end) AS score_4_to_4p5,
       COUNT(case when Score < 4   AND Score >= 3 then 1 end) AS score_3_to_4,
       COUNT(case when Score < 3 then 1 end) AS score_lt_3
FROM reviews
WHERE id = 4
  AND live = '1'

这样表只能读一次！

按条件联合SQL请求：丢失行？工会失败？

2 个答案: