Question

我一直试图了解查询的行为，但我不完全了解发生了什么。

看看：

SELECT main.entity_id,main.sku,name.value AS name
FROM product_entity AS main 
LEFT JOIN product_entity_varchar AS name ON main.entity_id = name.entity_id
WHERE name.attribute_id = (
    SELECT attribute_id 
    FROM ger_attribute 
    WHERE attribute_code LIKE "name"
    AND 'entity_type_id' = (
        SELECT entity_type_id
        FROM ger_entity_type
        WHERE entity_type_code = 'catalog_product_info'
    )
)

您能解释一下为什么使用子查询的原因，为什么在这种情况下LEFT JOIN很重要以及条件 entity_type_code ='catalog_product_info'？

谢谢

Answer 1

SELECT main.entity_id,main.sku,name.value AS name
FROM product_entity AS main 
LEFT JOIN product_entity_varchar AS name ON main.entity_id = name.entity_id
...

查询从拉出表product_entity开始。 LEFT JOIN允许查询访问表product_entity_varchar中entity_id等于product_entity中具有相同名称的列的值的记录。

在结果集中，显示表value中列product_entity_varchar的值，别名为name。

关键字LEFT使关系成为可选；如果product_entity_varchar中没有匹配的记录，则该名称将在输出中仅显示为NULL。如果它是[INNER] JOIN，则该关系将是强制性的：不匹配的记录将被过滤掉，并且不会出现在输出中。

Answer 2

好。让我们澄清一下SQL中的LEFT OUTER JOIN。

LEFT JOIN查询可以理解为UNION ALL版和INNER JOIN版的NOT EXIST的语法缩写。

让我们分解LEFT JOIN版的查询：

SELECT main.entity_id,main.sku,name.value AS name
FROM 
    product_entity AS main 
    LEFT JOIN product_entity_varchar AS name ON main.entity_id = name.entity_id
WHERE 
    name.attribute_id = (
        SELECT attribute_id 
        FROM ger_attribute 
        WHERE attribute_code LIKE "name"
        AND 'entity_type_id' = (
            SELECT entity_type_id
            FROM ger_entity_type
            WHERE entity_type_code = 'catalog_product_info'
        )
)

等效于：

SELECT main.entity_id,main.sku,name.value AS name
FROM 
    product_entity AS main 
    JOIN product_entity_varchar AS name 
        ON main.entity_id = name.entity_id
WHERE 
    name.attribute_id = (
        SELECT attribute_id 
        FROM ger_attribute 
        WHERE attribute_code LIKE "name"
        AND 'entity_type_id' = (
            SELECT entity_type_id
            FROM ger_entity_type
            WHERE entity_type_code = 'catalog_product_info'
        )
    )
UNION ALL
SELECT main.entity_id,main.sku, NULL AS name -- <-- Attention!
FROM 
    product_entity AS main 
    NOT EXISTS(
        SELECT * FROM  product_entity_varchar AS name 
        WHERE 
            main.entity_id = name.entity_id
            AND name.attribute_id = (
                SELECT attribute_id 
                FROM ger_attribute 
                WHERE attribute_code LIKE "name"
                AND 'entity_type_id' = (
                    SELECT entity_type_id
                    FROM ger_entity_type
                    WHERE entity_type_code = 'catalog_product_info'
                )
        )
    )

我希望这将有助于理解查询，尽管没有数据来回答“为什么使用子查询”等也是不可能的。

试图理解查询（LEFT JOIN和子查询）

2 个答案: