group by by multiple colums

Question

我在一個表（A）中有一個東西，它有兩個其他類別（C1，C2） 來自不同表格（MC）的category_id。

我想指出在特定類別中呈現多少東西。實況數據的

實施例：

表阿

main_key (unique) stuff_id (non unique) 
    1     7 
    2     6 
    3     3 

表C1

category_id main_key (it is FK for A table) 
    1    1 
    1    2 
    3    1 

表C2

category_id main_key (it is FK for A table) 
    2    3 
    2    1 

表MC

category_id category_name 
    1    blablbl 
    2    asas 
    3    asasa 
... 

之間

關係： C1和A爲多對一 C2和A爲多對一 C1或C2和MC爲多對一

在當前的例子中，我想看到最終結果作爲

stuff_qnt  category_id  category_name 
2     1     blablbl 
2     2     asas 
1     3     asasa 

我如何用一個單一的查詢來實現它？

我的查詢是：

SELECT count(A.stuff_id) as stuff_qnt, MC.category_id, MC.category_name 
FROM A 
LEFT JOIN C1 using(main_key) 
LEFT JOIN C2 using(main_key) 
LEFT JOIN MC ON (C1.category_id = MC.category_id AND C2.category_id = MC.category_id) 
GROUP BY C1.category_id, C2.category_id 

但它讓我錯誤的結果，是我做錯了嗎？

Answer 1

SELECT COUNT(*) AS Stuff_Qnt, C.Category_ID, MC.Category_Name 
    FROM MC 
    JOIN (SELECT C1.Category_ID 
      FROM A 
      JOIN C1 ON A.Main_Key = C1.Main_Key 
     UNION ALL 
     SELECT C2.Category_ID 
      FROM A 
      JOIN C2 ON A.Main_Key = C2.Main_Key 
     ) AS C 
    ON C.Category_ID = MC.Category_ID 
GROUP BY C.Category_ID, MC.Category_Name 
ORDER BY C.Category_ID, Stuff_Qnt; 

你需要從C1和C2與加盟一個類別加盟 - 一個類別，你肯定不希望一個笛卡爾乘積，如你與LEFT OUTER JOIN得到，所以你拿將類別ID列表進行聯合，然後彙總並加入MC。

CREATE TABLE A 
(
    main_key INTEGER NOT NULL PRIMARY KEY, 
    stuff_id INTEGER NOT NULL 
); 
INSERT INTO A VALUES(1, 7); 
INSERT INTO A VALUES(2, 6); 
INSERT INTO A VALUES(3, 3); 

CREATE TABLE MC 
(
    category_id INTEGER NOT NULL PRIMARY KEY, 
    category_name VARCHAR(10) NOT NULL 
); 
INSERT INTO mc VALUES(1, "blablbl"); 
INSERT INTO mc VALUES(2, "asas"); 
INSERT INTO mc VALUES(3, "asasa"); 

CREATE TABLE C1 
(
    category_id INTEGER NOT NULL REFERENCES mc, 
    main_key INTEGER NOT NULL REFERENCES a 
); 
INSERT INTO c1 VALUES(1, 1); 
INSERT INTO c1 VALUES(1, 2); 
INSERT INTO c1 VALUES(3, 1); 

CREATE TABLE C2 
(
    category_id INTEGER NOT NULL REFERENCES mc, 
    main_key INTEGER NOT NULL REFERENCES a 
); 
INSERT INTO c2 VALUES(2, 3); 
INSERT INTO c2 VALUES(2, 1); 

SELECT COUNT(*) AS Stuff_Qnt, C.Category_ID, MC.Category_Name 
    FROM MC 
    JOIN (SELECT C1.Category_ID 
      FROM A 
      JOIN C1 ON A.Main_Key = C1.Main_Key 
     UNION ALL 
     SELECT C2.Category_ID 
      FROM A 
      JOIN C2 ON A.Main_Key = C2.Main_Key 
     ) AS C 
    ON C.Category_ID = MC.Category_ID 
GROUP BY C.Category_ID, MC.Category_Name 
ORDER BY C.Category_ID, Stuff_Qnt; 

輸出：

2 1 blablbl 
2 2 asas 
1 3 asasa 

---

這裏是一個進一步的測試在兩個額外行，而在C1和C2對應的行。有兩個查詢測試，我的和querydkkumargoyal。

CREATE TABLE A(main_key INTEGER NOT NULL PRIMARY KEY, stuff_id INTEGER NOT NULL); 
INSERT INTO A VALUES(1, 7); 
INSERT INTO A VALUES(2, 6); 
INSERT INTO A VALUES(3, 3); 
INSERT INTO A VALUES(4, 3); 
INSERT INTO A VALUES(5, 3); 

CREATE TABLE MC(category_id INTEGER NOT NULL PRIMARY KEY, category_name VARCHAR(10) NOT NULL); 
INSERT INTO mc VALUES(1, "blablbl"); 
INSERT INTO mc VALUES(2, "asas"); 
INSERT INTO mc VALUES(3, "asasa"); 

CREATE TABLE C1(category_id INTEGER NOT NULL REFERENCES mc, main_key INTEGER NOT NULL REFERENCES a); 
INSERT INTO c1 VALUES(1, 1); 
INSERT INTO c1 VALUES(1, 2); 
INSERT INTO c1 VALUES(3, 1); 
INSERT INTO c1 VALUES(3, 4); 
INSERT INTO c1 VALUES(1, 4); 
INSERT INTO c1 VALUES(1, 5); 

CREATE TABLE C2(category_id INTEGER NOT NULL REFERENCES mc, main_key INTEGER NOT NULL REFERENCES a); 

INSERT INTO c2 VALUES(2, 3); 
INSERT INTO c2 VALUES(2, 1); 
INSERT INTO c2 VALUES(2, 5); 

SELECT COUNT(*) AS Stuff_Qnt, C.Category_ID, MC.Category_Name 
    FROM MC 
    JOIN (SELECT C1.Category_ID 
      FROM A 
      JOIN C1 ON A.Main_Key = C1.Main_Key 
     UNION ALL 
     SELECT C2.Category_ID 
      FROM A 
      JOIN C2 ON A.Main_Key = C2.Main_Key 
     ) AS C 
    ON C.Category_ID = MC.Category_ID 
GROUP BY C.Category_ID, MC.Category_Name 
ORDER BY C.Category_ID, Stuff_Qnt; 

-- Query by dkkumargoyal 
SELECT COUNT(DISTINCT A.stuff_id) AS stuff_qnt, MC.category_id, MC.category_name 
FROM A 
LEFT JOIN C1 on a.main_key = c1.main_key -- USING(main_key) 
LEFT JOIN C2 on a.main_key = c2.main_key -- USING(main_key) 
LEFT JOIN MC ON (C1.category_id = MC.category_id OR C2.category_id = MC.category_id) 
GROUP BY MC.category_id, mc.category_name 
ORDER BY MC.Category_id, stuff_qnt;  -- stuff_qnt added for standard compatibility 

所做的更改對代碼在測試DBMS（Informix 11.70.FC6）上工作是必需的。

結果1：

4 1 blablbl 
3 2 asas 
2 3 asasa 

結果2：

3 1 blablbl 
2 2 asas 
2 3 asasa 

我認爲我的結果是正確的，其他沒有，主要是因爲可供選擇取決於當問題規定，它A.Stuff_ID是唯一不是唯一的（並且額外的數據行使其不唯一）。

Answer 2

試試這個SQL。

SELECT COUNT(DISTINCT A.stuff_id) AS stuff_qnt, MC.category_id, MC.category_name 
FROM A 
LEFT JOIN C1 USING(main_key) 
LEFT JOIN C2 USING(main_key) 
LEFT JOIN MC ON (C1.category_id = MC.category_id OR C2.category_id = MC.category_id) 
GROUP BY MC.category_id 

Answer 3

請看一看這個參考：

• SQLFIDDLE

查詢：

select m.category_id, m.category_name, count(a.stuff_id) as cntstuff from 
(select * from c1 
union all 
select * from c2) as c 
join a 
on a.main_key = c.main_key 
join mc m 
on m.category_id = c.category_id 
group by m.category_id 
order by m.category_id 
; 

結果：

CATEGORY_ID  CATEGORY_NAME CNTSTUFF 
1    blablbl   2 
2    asas   2 
3    asasa   1