SQLite：如何限制 GROUP_CONCAT 的子查询

Question

我有一个提供 GROUP_CONCAT 的子查询。除了 LIMIT 之外，一切都很好，它没有达到我想要的效果。

这是代码：

CREATE TABLE IF NOT EXISTS Animal (
    id  TEXT    PRIMARY KEY,
    diet    TEXT
    );
INSERT INTO Animal VALUES ('monkey',    'herbivore');
INSERT INTO Animal VALUES ('goat',  'herbivore');
INSERT INTO Animal VALUES ('cat',   'carnivore');
INSERT INTO Animal VALUES ('dog',   'omnivore');
INSERT INTO Animal VALUES ('human', 'omnivore');

CREATE TABLE IF NOT EXISTS Domicile (
    id  TEXT    PRIMARY KEY REFERENCES Animal,
    home    TEXT
    );

INSERT INTO Domicile VALUES ('monkey',  'jungle');
INSERT INTO Domicile VALUES ('goat',    'farmyard');
INSERT INTO Domicile VALUES ('cat', 'house');
INSERT INTO Domicile VALUES ('dog', 'garden');
INSERT INTO Domicile VALUES ('human',   'house');

CREATE TABLE IF NOT EXISTS Food (
    name    TEXT    PRIMARY KEY
    );

INSERT INTO Food VALUES ('fruit');
INSERT INTO Food VALUES ('meat');
INSERT INTO Food VALUES ('fish');
INSERT INTO Food VALUES ('bread');
INSERT INTO Food VALUES ('sausages');

CREATE TABLE IF NOT EXISTS Eats (
    id  TEXT    REFERENCES Animal,
    name    TEXT    REFERENCES Food
    );

INSERT INTO Eats VALUES ('monkey',  'fruit');
INSERT INTO Eats VALUES ('monkey',  'bread');
INSERT INTO Eats VALUES ('goat',    'fruit');
INSERT INTO Eats VALUES ('cat',     'meat');
INSERT INTO Eats VALUES ('cat',     'sausages');
INSERT INTO Eats VALUES ('dog',     'meat');
INSERT INTO Eats VALUES ('dog',     'bread');
INSERT INTO Eats VALUES ('dog',     'sausages');
INSERT INTO Eats VALUES ('human',   'fruit');
INSERT INTO Eats VALUES ('human',   'meat');
INSERT INTO Eats VALUES ('human',   'fish');
INSERT INTO Eats VALUES ('human',   'bread');
INSERT INTO Eats VALUES ('human',   'sausages');

.mode column

SELECT 'TEST 4: JOIN Eats using a subquery to order the names.';
SELECT      Animal.id,
        Animal.diet,
        GROUP_CONCAT (Eats.name) as names,
        Domicile.home
FROM        Animal
        INNER JOIN Domicile USING (id)
        INNER JOIN (
            SELECT * FROM Eats
            ORDER BY name
            ) as Eats USING (id)
GROUP BY    Animal.id
ORDER BY    Animal.id
;
SELECT 'TEST 4 result: names in order, but not limited to 3.';

SELECT 'TEST 5: LIMIT Eats subquery to 3.';
SELECT      Animal.id,
        Animal.diet,
        GROUP_CONCAT (Eats.name) as names,
        Domicile.home
FROM        Animal
        INNER JOIN Domicile USING (id)
        INNER JOIN (
            SELECT * FROM Eats
            ORDER BY name
            LIMIT 3
            ) as Eats USING (id)
GROUP BY    Animal.id
ORDER BY    Animal.id
;
SELECT 'TEST 5 result: LIMIT has applied to wrong SELECT?.';

我想要的是这样的输出：

id      diet       names                home
------  ---------  -------------------  --------
cat     carnivore  meat,sausages        house
dog     omnivore   bread,meat,sausages  garden
goat    herbivore  fruit                farmyard
human   omnivore   bread,fish,fruit     house
monkey  herbivore  bread,fruit          jungle

备注：

1. 按“id”排序的行。
2. 并非所有可能的组合都存在于 Eats 中。 （例如山羊只吃水果）。
3. 'names' 列按字母顺序排列，最多包含 3 个元素 （否则“人类”将是面包、鱼、水果、肉、香肠）。

我从两次测试中得到的是这样的：

'TEST 4: JOIN Eats using a subquery to order the names.'
--------------------------------------------------------
TEST 4: JOIN Eats using a subquery to order the names.  
id      diet       names                           home    
------  ---------  ------------------------------  --------
cat     carnivore  meat,sausages                   house   
dog     omnivore   bread,meat,sausages             garden  
goat    herbivore  fruit                           farmyard
human   omnivore   bread,fish,fruit,meat,sausages  house   
monkey  herbivore  bread,fruit                     jungle  
'TEST 4 result: names in order, but not limited to 3.'
------------------------------------------------------
TEST 4 result: names in order, but not limited to 3.  
'TEST 5: LIMIT Eats subquery to 3.'
-----------------------------------
TEST 5: LIMIT Eats subquery to 3.  
id      diet       names  home  
------  ---------  -----  ------
dog     omnivore   bread  garden
human   omnivore   bread  house 
monkey  herbivore  bread  jungle
'TEST 5 result: LIMIT has applied to wrong SELECT?.'
----------------------------------------------------
TEST 5 result: LIMIT has applied to wrong SELECT?.  

测试 4 和 5 的源之间的唯一区别是测试 5 子查询中的 LIMIT。

有没有办法将名称限制为 3 个元素？ （我希望该限制能够轻松更改。）

Answer 1

我相信以下是解决方案：-

WITH
    cte1 AS (
        SELECT 
            animal.*,
            domicile.home,
            eats.name 
        FROM animal 
        JOIN eats ON eats.id = animal.id 
        JOIN domicile ON domicile.id = animal.id
        ORDER BY animal.id,eats.id
        ),
    cte2 AS (
        SELECT *, (
            SELECT count(*) FROM cte1 AS cte1_c 
            WHERE cte1_c.name <= cte1.name AND cte1_c.id = cte1.id ORDER BY id,name
        ) AS c 
        FROM cte1
    )
SELECT id,diet,group_concat(name) AS names,home FROM cte2 WHERE c <= 3 GROUP BY id;

结果是：-

通过更改最后一个 WHERE 子句中的数字，可以轻松更改多少个 names

WHERE c <= 3