Question

由于某种原因，我的SELECT查询返回每行的重复项。以下是我的表格的简要分类：

这个查询没问题......

***QUERY***
SELECT
  categories_name AS category,
  products_model AS model,
  products_status AS status,
  products_quantity AS qty
FROM products p
LEFT JOIN categories c
ON c.categories_id = p.master_categories_id
LEFT JOIN categories_description cd
USING (categories_id)
WHERE p.products_quantity > 0
AND p.products_status = FALSE;


***OUTPUT***
+-------------+------------------------+--------+-----+
| category    | model                  | status | qty |
+-------------+------------------------+--------+-----+
| Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Batteries   | BATT-LHP203            |      0 |   1 |
| Batteries   | BATT-LIB201            |      0 |   1 |
| Refurbished | UC*DELL-OPT-760-DC-IBC |      0 |   9 |
| Desktop     | HDD-320-SATA-3.5-PULL  |      0 |  12 |
| Refurbished | UL*ASUS-S56CM-SH51     |      0 |   1 |
+-------------+------------------------+--------+-----+

但是，categories表包含parent_id，表示您在上面看到的类别的父类别（例如，Cables＆gt; Network）。这是我尝试包含所述主类别的名称：

***QUERY***
SELECT
  parent_name AS parent,
  categories_name AS category,
  products_model AS model,
  products_status AS status,
  products_quantity AS qty
FROM products p
LEFT JOIN categories c
ON c.categories_id = p.master_categories_id
LEFT JOIN categories_description cd
USING (categories_id)
LEFT JOIN (
  SELECT
    parent_id,
    categories_name AS parent_name
  FROM categories c
  LEFT JOIN categories_description cd
  ON cd.categories_id = c.parent_id
) par
USING (parent_id)
WHERE p.products_quantity > 0
AND p.products_status = FALSE;

***OUTPUT***
+--------------+-------------+------------------------+--------+-----+
| parent       | category    | model                  | status | qty |
+--------------+-------------+------------------------+--------+-----+
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-10-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Cables       | Network     | CP-C-03-CAT6X0         |      0 |   5 |
| Laptop Parts | Batteries   | BATT-LHP203            |      0 |   1 |
| Laptop Parts | Batteries   | BATT-LHP203            |      0 |   1 |
| Laptop Parts | Batteries   | BATT-LHP203            |      0 |   1 |
| Laptop Parts | Batteries   | BATT-LHP203            |      0 |   1 |
| Laptop Parts | Batteries   | BATT-LIB201            |      0 |   1 |
| Laptop Parts | Batteries   | BATT-LIB201            |      0 |   1 |
| Laptop Parts | Batteries   | BATT-LIB201            |      0 |   1 |
| Laptop Parts | Batteries   | BATT-LIB201            |      0 |   1 |
| Desktop PCs  | Refurbished | UC*DELL-OPT-760-DC-IBC |      0 |   9 |
| Desktop PCs  | Refurbished | UC*DELL-OPT-760-DC-IBC |      0 |   9 |
| Hard Drives  | Desktop     | HDD-320-SATA-3.5-PULL  |      0 |  12 |
| Hard Drives  | Desktop     | HDD-320-SATA-3.5-PULL  |      0 |  12 |
| Hard Drives  | Desktop     | HDD-320-SATA-3.5-PULL  |      0 |  12 |
| Hard Drives  | Desktop     | HDD-320-SATA-3.5-PULL  |      0 |  12 |
| Laptops      | Refurbished | UL*ASUS-S56CM-SH51     |      0 |   1 |
| Laptops      | Refurbished | UL*ASUS-S56CM-SH51     |      0 |   1 |
+--------------+-------------+------------------------+--------+-----+

如您所见，每行重复多次。我做错了什么？

Answer 1

使用select distinct是一个坏习惯。有时这是必要的，但不是在这种情况下。为什么删除重复项的额外工作，当它足够简单来修复查询时？

from子句应如下所示：

FROM products p left join
     categories c
     ON c.categories_id = p.master_categories_id left join
     categories_description cd
     USING (categories_id) left join
     categories_description cdp
     on cdp.categories_id = c.parent_id

您的附加子查询不仅会引入额外的处理开销，还会引入重复项和不必要的查询复杂性。

Answer 2

使用DISTINCT删除重复项

SELECT DISTINCT ...

所有MySQL输出行都是重复的

2 个答案: