Reputation: 1135
I have two tables. The parent table has a single date column, and child table has 2 date columns (From / To). I need to make a left join from parent to child where parent's date column is between one in child. In sql this would look something like this:
select p.cob, count(*) from parent p
left join child c on p.cob between c.effective and c.expiry
group by p.cob
How does one write this in linq - I'm a bit stuck here....
Upvotes: 2
Views: 5793
Reputation: 113
Old but maybe it will be helpfull for others. It made my query 7 times faster on specific amount of test data!
I had (28seconds)
from st in subtable
join t in table on st.IdTable equals t.Id
Where t.date >= myStartDate
&& t.date <= myEndDate
&& st == some_other_conditions
select st
I changed it to this (4seconds)
from st in subtable
join t in table on new { parent = st.IdTable, from = 1, to = 1 } equals new { parent = t.Id, from = (t.date >= myStartDate ? 1 : 0), to = (t.date <= myEndDate ? 1 : 0 )
Where st == some_other_conditions
select st
It checks other conditions after creating result of join so all the rest works on smaller amount of data.
Hope it helps
Upvotes: 2
Reputation: 57093
I need to make a left join from parent to child
There is always more than one way to express a requirement in SQL and I'm sure the same is true of linq e.g.
SELECT p.cob, COUNT(*)
FROM parent p
WHERE EXISTS (
SELECT *
FROM child c
WHERE p.cob BETWEEN c.effective AND c.expiry
)
GROUP
BY p.cob
UNION
SELECT p.cob, 0
FROM parent p
WHERE NOT EXISTS (
SELECT *
FROM child c
WHERE p.cob BETWEEN c.effective AND c.expiry
);
A good optimizer will spot the repeated derived table, that the two union'd tables are mutually exclusive, etc.
Upvotes: -1
Reputation: 26694
This should be what you are looking for
var query = from p in context.Parent
from c in context.Child.Where(x => p.cob >= x.effective)
.Where(x => p.cob <= x.expiry)
.DefaultIfEmpty()
group p by p.cob into pg
select new
{
cob = pg.Key,
count = pg.Count()
};
Upvotes: 4
Reputation: 7176
If I read your question correctly, the between part would be handled like this.
where p.cob >= c.effective and p.cob <= c.expiry
Upvotes: 0