Trying to understand a query (LEFT JOIN and subquery) - sql

Question

I've been trying to understand the behavior of a query but i dont fully understand what is going on.
Take a look:
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'
)
)
Can you please explain why it is using a subquery ,why the LEFT JOIN is important in these cases and the condition entity_type_code = 'catalog_product_info'?
Thanks

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
...
The query starts by pulling out table product_entity. The LEFT JOIN allows the query to access the record(s) in table product_entity_varchar whose entity_id is equal to the value of the column that has the same name in product_entity.
In the resultset, the value of column value from table product_entity_varchar is displayed, under alias name.
The keyword LEFT makes the relation optional ; it there is no matching record in product_entity_varchar, the name will simply appear as NULL in the output. If it were an [INNER] JOIN, then the relation would be mandatory : unmatched records would be filtered out, and would not appear in the output.

Answer 2

OK. Let's clarify what is LEFT OUTER JOIN in SQL.
LEFT JOIN queries could be understood as just syntax abbreviation of UNION ALL of INNER JOIN-ed and NOT EXIST-ed queries.
Let's decompose LEFT JOIN-ed query:
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'
)
)
Is equivalent to:
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'
)
)
)
I hope this will help to understand query, although it's impossible without data to answer "why it is using a subquery" etc.