Question

我有五个表，但两个是主表。表itemy是可以在其中找到所有项目的表。桌子obchody_parovani，这是我们可以在其中找到与某些商店相关的商品的桌子。

表项

ID | nazev | cesta | druh | kvalita 
1  | nazev | cesta | 1    | 1
2  | nazev | cesta | 3    | 2
3  | nazev | cesta | 1    | 1
4  | nazev | cesta | 5    | 3

表obchody_parovani

ID | obchod | item
1  | 1      | 2
2  | 3      | 3
3  | 1      | 1

我需要识别商品是否在商店中。如果是，那么我需要商店的ID，如果不是，则需要NULL。

我想要从obchody_parovani中获得结果的结果是ID 1

ID | nazev | cesta | druh_n | kvalita_n | obchod
1  | nazev | cesta | 1      | 1         | NULL
2  | nazev | cesta | 3      | 2         | 1
3  | nazev | cesta | 1      | 1         | NULL
4  | nazev | cesta | 5      | 3         | 1

nazev，cesta，druh_n，kvalita_n不重要，我需要列阻塞。如果表包含obchod = 1的行，则结果为1，否则为NULL

我当前的SQL是：

SELECT i.id, i.nazev, i.cesta, d.druh AS druh_n, k.nazev AS kvalita_n, par.obchod FROM itemy AS i LEFT JOIN itemy_druhy AS d ON d.id = i.druh LEFT JOIN itemy_kvalita AS k ON k.id = i.kvalita LEFT JOIN itemy_rozdeleni AS r ON r.id = i.rozdeleni LEFT JOIN obchody_parovani AS par ON par.item = i.id WHERE par.obchod = 1

Answer 1

您可以尝试基于OUTER JOIN表使用Itemy。

模式（MySQL v5.7）

CREATE TABLE Itemy(
   ID INT,
   nazev VARCHAR(50),
  cesta VARCHAR(50), druh INT, kvalita INT
);


INSERT INTO Itemy VALUES (1,'nazev','cesta',1,1);
INSERT INTO Itemy VALUES (2,'nazev','cesta',3,2);
INSERT INTO Itemy VALUES (3,'nazev','cesta',1,1);
INSERT INTO Itemy VALUES (4,'nazev','cesta',5,3);

CREATE TABLE obchody_parovani(
   ID INT,
   obchod INT,
  item INT
);


INSERT INTO obchody_parovani VALUES (1, 1,2);
INSERT INTO obchody_parovani VALUES (2, 3,3);
INSERT INTO obchody_parovani VALUES (3, 1,1);

查询＃1

SELECT i.*,op.obchod 
FROM Itemy i 
LEFT JOIN obchody_parovani op on i.druh = op.obchod 
and i.kvalita = op.item
order by i.id;

| ID  | nazev | cesta | druh | kvalita | obchod |
| --- | ----- | ----- | ---- | ------- | ------ |
| 1   | nazev | cesta | 1    | 1       | 1      |
| 2   | nazev | cesta | 3    | 2       |        |
| 3   | nazev | cesta | 1    | 1       | 1      |
| 4   | nazev | cesta | 5    | 3       |        |

View on DB Fiddle

从您尝试过的查询中，我认为您可以尝试将par.obchod = 1的条件从on移到where子句，否则您将从inner join进行obchody_parovani < / p>

SELECT i.id, i.nazev, i.cesta, d.druh AS druh_n, k.nazev AS kvalita_n, par.obchod
 FROM itemy AS i 
 LEFT JOIN itemy_druhy AS d ON d.id = i.druh 
 LEFT JOIN itemy_kvalita AS k ON k.id = i.kvalita 
 LEFT JOIN itemy_rozdeleni AS r ON r.id = i.rozdeleni
 LEFT JOIN obchody_parovani AS par ON par.item = i.id and par.obchod = 1

从表中获取所有行，并根据条件从另一个表中合并一些行

1 个答案: