Question

Table_A has entity_id

Table_B has a related entity_id with attribute_id = 1

Table_B has a related entity_id with attribute_id = 2

  SELECT Table_B.value
    FROM  Table_A
    LEFT JOIN Table_B
    ON Table_A.entity_id = Table_B.entity_id
    WHERE Table_A.date =  '2012-7-30 00:00:00'
    AND Table_B.attribute_id = 1

I would like to get attribute_id = 1 and attribute_id = 2 into one query.

Answer 1

You can join Table_B a second time and use AS to give it a new name (an alias).

SELECT Table_B.value, Table_B2.value
FROM  Table_A
LEFT JOIN Table_B
ON Table_A.entity_id = Table_B.entity_id
LEFT JOIN Table_B AS Table_B2
ON Table_A.entity_id = Table_B2.entity_id
WHERE Table_A.date =  '2012-7-30 00:00:00'
AND Table_B.attribute_id = 1
AND Table_B2.attribute_id = 2

You can also move the attribute criteria to make it a little clearer what you’re up to:

SELECT Table_B.value, Table_B2.value
FROM  Table_A
LEFT JOIN Table_B
ON Table_A.entity_id = Table_B.entity_id AND Table_B.attribute_id = 1
LEFT JOIN Table_B AS Table_B2
ON Table_A.entity_id = Table_B2.entity_id AND Table_B2.attribute_id = 2
WHERE Table_A.date =  '2012-7-30 00:00:00'