SQL JOIN Query not returning expected results - mysql

Question

2I know I am having a simple issue .. But I cannot for the life of me solve it .. Here is what I am trying to do. I have 3 tables and some sample data:
customer_entity_varchar
entity_id attribute_id value
'2' '5' 'John'
'2' '7' 'Smith'
'2' '336' 'ADELANTO'
'3' '5' 'Jane'
'3' '7' 'Doe'
'3' '336' 'ADELANTO'
'4' '5' 'Peter'
'4' '7' 'Griffin'
'4' '336' 'Not ADELANTO'
customer_entity
entity_id email
'2' 'jsmith#whatever.com'
'3' 'janed#thisthat.com'
'4' 'peterg#notanemail.com'
What I am trying to come up with first name, last name and email for everyone that matches a certain district which is attribut_id = '336'. What I am trying is this:
SELECT CE.email as email,
max(case when CEV.attribute_id = '5' then CEV.value end) as FirstName,
max(case when CEV.attribute_id = '7' then CEV.value end) as LastName
FROM customer_entity_varchar CEV
LEFT JOIN customer_entity CE
ON ( CE.entity_id = CEV.entity_id)
WHERE CEV.value ='ADELANTO'
AND CEV.attribute_id='336'
My hopes for a result are:
email FirstName LastName
jsmith#whatever.com John Smith
janed#thisthat.com Jane Doe
However what I am getting back is a SINGLE row -- where email has a value, however both FirstName and LastName are blank. Is my logic flawed?

Answer 1

I would probably solve this like this. It's a solution that favours readability.
WITH firstNames AS
(SELECT entity_id, Value FROM customer_entity_char WHERE attribute_id = '5')
lastNames AS
(SELECT entity_id, Value FROM customer_entity_char WHERE attribute_id = '7')
districts AS
(SELECT entity_id, Value FROM customer_entity_char WHERE attribute_id = '336')
SELECT ce.email, fn.Value, ln.Value, d.Value FROM firstNames fn,lastNames ln, districts d
INNER JOIN customer_entity ce
WHERE fn.entity_id = ln.entity_id AND ln.entity_id = d.entity_id AND ce.entity_id = d.entity_id
AND d.Value = 'ADELANTO';

Answer 2

The WHERE condition in your query is excluding rows with attribute_id 5,7 and so it will not give value containing first and last name.
Try this
SELECT CE.email as email,
max(case when CEV.attribute_id = '5' then CEV.value end) as FirstName,
max(case when CEV.attribute_id = '7' then CEV.value end) as LastName
FROM
(
SELECT entity_id,attribute_id,value
FROM customer_entity_varchar
WHERE entity_id IN (
SELECT entity_id
FROM customer_entity_varchar
WHERE value ='ADELANTO'
AND attribute_id='336'
)
AND attribute_id IN ('5','7')
)As CEV
INNER JOIN customer_entity CE
ON CE.entity_id = CEV.entity_id
GROUP BY CEV.entity_id
SQL Fiddle Demo

Answer 3

You're dealing with an entity-attribute-value table. That means you're going to suffer the joys of an endless number of outer joins in every query. This is why EAV sucks and you're told not to do it unless you have to.
The single query fix:
SELECT DISTINCT
-- ^^^^You need this because you're cross joining the crap out of the EAV table.
CEV.entity_id,
CE.email,
CEV5.value "FirstName",
CEV7.value "LastName"
FROM customer_entity_varchar CEV
-- ^^^^ This is the base table that determines what entities exist. It makes sure you always have your entity_id even if attributes are missing.
LEFT JOIN customer_entity_varchar CEV5
ON CEV5.entity_id = CEV.entity_id AND CEV5.attribute_id = 5
-- ^^^^ This is the join that lets you access attribute 5, FirstName
LEFT JOIN customer_entity_varchar CEV7
ON CEV7.entity_id = CEV.entity_id AND CEV7.attribute_id = 7
-- ^^^^ This is the join that lets you access attribute 7, LastName
LEFT JOIN customer_entity_varchar CEV336
ON CEV336.entity_id = CEV.entity_id AND CEV336.attribute_id = 336
-- ^^^^ This is the join that lets you access attribute 336, City?
LEFT JOIN customer_entity CE
ON CE.entity_id = CEV.entity_id
-- ^^^^ Here's the other table, joined to the base table so we're sure it joins when partial data exists.
WHERE CEV336.value = 'ADELANTO'
Here's how to do it more easily.
First, make a view for your EAV table:
CREATE VIEW vw_customer_entity_varchar AS
SELECT DISTINCT
CEV.entity_id
CEV5.value "FirstName",
CEV7.value "LastName",
CEV336.value "City?"
FROM customer_entity_varchar CEV
LEFT JOIN customer_entity_varchar CEV5
ON CEV5.entity_id = CEV.entity_id AND CEV5.attribute_id = 5
LEFT JOIN customer_entity_varchar CEV7
ON CEV7.entity_id = CEV.entity_id AND CEV7.attribute_id = 7
LEFT JOIN customer_entity_varchar CEV336
ON CEV336.entity_id = CEV.entity_id AND CEV336.attribute_id = 336
Add additional joins for every field in your EAV table.
Then you can treat this view as a normal table, except it has terrible performance, and can't use it for INSERT, UPDATE, or DELETE.
Welcome to EAV hell.

Answer 4

First, do you really have those tick marks around all your values? In other words, are the quotations stored in the db?
As a couple of folk have pointed out, this database design is really a pain to query. Basically, you'll have to take two passes through your customer_entity_varchar table. One pass is to get the entity_id for whoever your're interested in (aliased as T1 in my query), and one to get their attributes (cev in the query). You will filter for this 'adelanto' value against the t1 query, which will give you the entity_ids. Then you can join that back to the CEV alias to get the actual information you want. (Hopefully all that made sense).
I think this what you are looking for:
SQL Fiddle
SELECT CE.email as email,
max(case when CEV.attribute_id = '5' then CEV.value end) as FirstName,
max(case when CEV.attribute_id = '7' then CEV.value end) as LastName
FROM customer_entity_varchar CEV
inner join (select distinct entity_id, value from customer_entity_varchar where attribute_id = 336) t1
on t1.entity_id = cev.entity_id
LEFT JOIN customer_entity CE
ON ( CE.entity_id = CEV.entity_id)
and t1.value = 'ADELANTO'
group by
ce.entity_id
EDIT: As MPelletier also pointed out, you need to include a group by. I can't stand that MySQL will allow you to submit that query without it.