How to use JOIN correctly - sql

Question

Can someone help me with this? I am new to SQL.
I want to return one record for every record in Product_Entity.
Product_Entity is linked to Entity_Varchar by the common coilumn entity_id.
I want each record in the results to include Entity_Varchar.value, but only when Entity_Varchar.attribute_id = ‘73’ as there will be many records in Entity_Varchar that will match on entity_id, but only one where also Entity_Varchar.attribute_id=’73’
SELECT
Product_Entity.sku AS SKU,
Entity_Varchar.value AS Description
FROM
Product_Entity
JOIN
Entity_Varchar ON Entity_Varchar.entity_id = Entity_Varchar.entity_id
WHERE
Entity_Varchar.attribute_id = '73'

Answer 1

Your query basically looks right. However, if you want all rows in the first table, then use a LEFT JOIN:
SELECT pe.sku AS SKU, ev.value AS Description
FROM Product_Entity pe LEFT JOIN
Entity_Varchar ev
ON ev.entity_id = pe.entity_id AND
ev.attribute_id = '73';
Rows that don't match will have NULL for the description.

Answer 2

Your query is correct you just misspelled table name. Instead of Product_Entity you used Entity_Varchar in both side of on clause.
Corrected query:
SELECT Product_Entity.sku AS SKU, Entity_Varchar.value AS Description FROM Product_Entity inner Join Entity_Varchar
ON Product_Entity.entity_id = Entity_Varchar.entity_id
WHERE Entity_Varchar.attribute_id = '73'
For better readability you can use table aliaces.
SELECT pe.sku AS SKU, ev.value AS Description
FROM Product_Entity pe inner Join Entity_Varchar ev
ON pe.entity_id = ev.entity_id
WHERE ev.attribute_id = '73'
If you want to select rows from Product_Entity table even when there is no row in Entity_Varchar table with same entity_id and for attribute_id 73 you need to use left join instead of inner join.
SELECT pe.sku AS SKU, ev.value AS Description
FROM Product_Entity pe left Join Entity_Varchar ev
ON pe.entity_id = ev.entity_id
WHERE ev.attribute_id = '73'

Answer 3

If I understand you need, you want every record from Product_Entity, whether or not it has an attribute_id = 73 record on Entity_Varchar. But if there is an attibute_id = 73 record on Entity_Varchar, you want the description from that row also.
First, you need a LEFT OUTER JOIN to get all of the records from Product_Entity, not filtered by the contents of Entity_Varchar.
Then, you need to filter Entity_Varchar. But if you place this filter in the WHERE clause, it causes the join to become an INNER JOIN. (I see error a lot. I call it a "LEFT INNER JOIN". Of course, that's not a real thing.) To overcome this, put the filter in the join logic. Then it's filtered before being joined.
SELECT Product_Entity.sku AS SKU
, Entity_Varchar.value AS Description
FROM Product_Entity
LEFT OUTER JOIN Entity_Varchar ON Entity_Varchar.entity_id = Entity_Varchar.entity_id
AND Entity_Varchar.attribute_id = '73'

Answer 4

Your query is correct you just misspelled table name. Instead of Product_Entity you used Entity_Varchar in both side of on clause.
SELECT Product_Entity.sku AS SKU, Entity_Varchar.value AS Description FROM Product_Entity inner Join Entity_Varchar
ON Product_Entity.entity_id = Entity_Varchar.entity_id
WHERE Entity_Varchar.attribute_id = '73'