按條件聯合SQL請求：丟失行？聯盟失敗？

Question

我想分類產品的評論放在圖表中。總結成績，我打破的結果集分爲四個夾頭：

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

的SQL請求可以在下面看到：

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' 

這一操作將在陣列中放。我注意到上面的請求返回行，或計數的總和等於13.我得到的結果如下：

做以下請求......

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

...返回14行。我想也許結果集可能會導致問題，但我沒有看到任何結果。該組得分如下：

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

I」米不知道爲什麼單行缺失，任何想法，也許我的語法不符合？爲什麼最後的聯盟（score < 3）沒有任何回報？

Answer 1

注意最後兩個查詢都返回1，並且因爲UNION刪除重複項，所以10 2 1 1→10 2 1。使用UNION ALL（見What is the difference between UNION and UNION ALL?）。

Answer 2

另外，跳過UNION，而是使用CASE做conditonional計數：

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' 

這樣的表將只讀取一次！