SQL SELECT criteria in another table - sql

Question

I have 2 related tables:
messages
--------
mid subject
--- -----------------
1 Hello world
2 Bye world
3 The third message
4 Last one
properties
----------
pid mid name value
--- --- ---------------- -----------
1 1 read false
2 1 importance high
3 2 read false
4 2 importance low
5 3 read true
6 3 importance low
7 4 read false
8 4 importance high
And I need to get from messages using the criteria on the properties table.
Eg: if I have a criteria like return unread (read=false) high prio (importance=high) messages it should return
mid subject
--- -----------------
1 Hello world
4 Last one
How could I get this with a SELECT clause (MySQL dialect)?

Answer 1

In SQL, any expression in a WHERE clause can only reference one row at a time. So you need some way of getting multiple rows from your properties table onto one row of result. You do this with self-joins:
SELECT ...
FROM messages AS m
JOIN properties AS pRead
ON m.mid = pRead.mid AND pRead.name = 'read'
JOIN properties AS pImportance
ON m.mid = pImportance.mid AND pImportance.name = 'importance'
WHERE pRead.value = 'false' AND pImportance.value = 'high';
This shows how awkward it is to use the EAV antipattern. Compare with using conventional attributes, where one attribute belongs in one column:
SELECT ...
FROM messages AS m
WHERE m.read = 'false' AND m.importance = 'high';
By the way, both answers from #Abe Miessler and #Thomas match more mid's than you want. They match all mid's where read=false OR where importance=high. You need to combine these properties with the equivalent of AND.

Answer 2

I believe the query below will work.
UPDATE: #Gratzy is right, this query won't work, take a look at the structure changes I suggested.
SELECT DISTINCT m.id as mid, m.subject
FROM message as m
INNER JOIN properties as p
ON m.mid = p.mid
where (p.name = 'read' and p.value = 'false') or (p.name = 'importance' AND p.value = 'high')
The structure of your properties table seems a little off to me though...
Would it be possible to structure the table like this:
messages
--------
mid subject Read Importance
--- ----------------- --------- ------------
1 Hello world false 3
2 Bye world false 1
3 The third message true 1
4 Last one false 3
importance
----------
iid importanceName
--- --------------
1 low
2 medium
3 high
and use this query:
SELECT m.id as mid, m.subject
FROM message as m
where m.read = false AND m.importance = 3

Answer 3

Clearly, you are using an EAV (Entity-Attribute-Value) schema. One of the many reasons for avoiding such a structure is that it makes queries more difficult. However, for the example you gave, you could do something like:
Select ...
From messages As M
Where Exists (
Select 1
From Properties As P1
Where P1.mid = M.mid
And P1.name = 'unread' And P1.value = 'false'
)
And Exists (
Select 1
From Properties As P2
Where P2.mid = M.mid
And P2.name = 'importance' And P2.value = 'high'
)
A more succinct solution would be:
Select ...
From messages As M
Where Exists (
Select 1
From Properties As P1
Where P1.mid = M.mid
And ((P1.name = 'unread' And P1.value = 'false')
Or (P1.name = 'importance' And P1.value = 'high'))
Having Count(*) = 2
)

Answer 4

Select m.mid, m.subject
from properties p
inner join properties p1 on p.mid = p1.mid
inner join messages m on p.mid = m.mid
where
p.name = 'read'
and p.value = 'false'
and p1.name = 'importance'
and p2.value = 'high'
I prefer to put my filter criteria in the where clause and leave my join's to elements that are in both tables and are the actual criteria for the join.

Answer 5

Another way might be (untested) to use a derived table to hold the criteria that all messages must meet then use the standard relational division technique of double NOT EXISTS
SELECT mid,
subject
FROM messages m
WHERE NOT EXISTS
( SELECT *
FROM ( SELECT 'read' AS name,
'false' AS value
UNION ALL
SELECT 'importance' AS name,
'high' AS value
)
c
WHERE NOT EXISTS
(SELECT *
FROM properties P
WHERE p.mid = m.mid
AND p.name =c.name
AND p.value=c.value
)
)

Answer 6

If you want to keep your existing data model, then go with Bill Karwin's first suggestion. Run it with this select clause to understand what it's doing:
select m.*, r.value as read, i.value as importance
from message m
join properties r
on r.mid = m.mid and r.name = 'read'
join properties i
on i.mid = m.mid and i.name = 'importance'
where r.value = 'false' and i.value = 'high';
But if you go this way, there are a few constraints you should put in place to avoid storing and retrieving bad data:
A unique index on message(mid) and a unique index on properties(pid), both of which I'm sure you have already.
A unique index on properties(mid, name) so that each property can only be defined once for a message -- otherwise you may get duplicate results from your query. This will also help your query performance by allowing an index access for both joins.