Inner join the same column of a table to multiple tables - linq

Question

I have a main table Fruit, and I'd like to join it to tables ApplePrice, PearPrice, and BananaPrice.
Fruit
Id Type Date
--------------------
1 Apple 1/1
2 Apple 1/3
3 Banana 1/5
4 Pear 1/7
Common Denominator of [Apple/Pear/Banana]Price (there are many more specific fields for each table):
Date Price F1 F2 ...
-----------------------
1/1 p1
1/2 p2
....
To get the price of each piece of Fruit, I join the Fruit table with each price table separately, then concatenate the results together.
If the Price tables can't be merged into one, do you have a better approach to this problem? For example, construct one Linq query that returns all the information instead of concatenating results from multiple queries.
Appreciate your ideas.

Answer 1

You need to use join into then DefaultIfEmpty. The LINQ equivalent of a SQL LEFT JOIN.
from fruit in fruits
join ap in applePrices
on (fruit.Type + fruit.Date.ToShortDateString()) equals ("Apple" + ap.Date.ToShortDateString())
into aps
from applePrice in aps.DefaultIfEmpty()
This will give:
Fruit | Apple Price | Banana Price | Pear Price
--------+-------------+--------------+------------
Apple | applePrice | null | null
Apple | applePrice | null | null
Banana | null | bananaPrice | null
Pear | null | null | pearPrice
Then select the valid fruitPrice values by the below:
applePrice != null
? applePrice.Price
: bananaPrice != null
? bananaPrice.Price
: pearPrice != null
? pearPrice.Price
: 0 // Default value here if all 3 are null
And use LINQ to select the wanted fields.
The complete result below, I've used an anonymous class to hold my values:
var prices = from fruit in fruits
join ap in applePrices
on (fruit.Type + fruit.Date.ToShortDateString()) equals ("Apple" + ap.Date.ToShortDateString())
into aps
from applePrice in aps.DefaultIfEmpty()
join bp in bananaPrices
on (fruit.Type + fruit.Date.ToShortDateString()) equals ("Banana" + bp.Date.ToShortDateString())
into bps
from bananaPrice in bps.DefaultIfEmpty()
join pp in pearPrices
on (fruit.Type + fruit.Date.ToShortDateString()) equals ("Pear" + pp.Date.ToShortDateString())
into pps
from pearPrice in pps.DefaultIfEmpty()
select new
{
Id = fruit.Id,
Type = fruit.Type,
Date = fruit.Date,
Price =
applePrice != null
? applePrice.Price
: bananaPrice != null
? bananaPrice.Price
: pearPrice != null
? pearPrice.Price
: 0
};

Answer 2

var prices = from T in bank.students
join O in bank.dovres
on (T.code) equals
(O.codestu)
into aps
from applePrice in aps.DefaultIfEmpty()
join Y in bank.rotbes
on (T.code) equals (Y.codestu)
into bps
from bananaPrice in bps.DefaultIfEmpty()
select new
{
Id = T.code,
Type =T.name,
Date = T.family,
father=T.fathername,
T.adi_date, T.faal_date,
// = applePrice.sal + " ماه و " + O.mah + " روز", hk = Y.sal + " ماه و " + Y.mah + " روز"
hj = applePrice != null
? applePrice.sal + " ماه و " + applePrice.mah + " روز"
:"",
hj1 = bananaPrice!= null
? bananaPrice.sal + " ماه و " +bananaPrice.mah + " روز"
: "",
};
dataGridView1.DataSource = prices;
dataGridView1.Columns[0].HeaderText = "کد ";
dataGridView1.Columns[1].HeaderText = "نام";
dataGridView1.Columns[2].HeaderText = "نام خانوادگی";
dataGridView1.Columns[3].HeaderText = "نام پدر";
dataGridView1.Columns[4].HeaderText = " عضویت عادی";
dataGridView1.Columns[5].HeaderText = "عضویت فعال";
dataGridView1.Columns[6].HeaderText = "کسری بسیج";
dataGridView1.Columns[7].HeaderText = "کسری جبهه";
This is the best code for joining 3 or more tables.