Building SQL with foreach

问题

I have this code:

private void button1_Click(object sender, EventArgs e)
{
    foreach(Int_String partner in partneri)
    {
        double danaBezProdaje = (DateTime.Today - Komercijalno.Partner.PoslednjaKupovina(partner._int)).TotalDays;
        if (danaBezProdaje > 31 && danaBezProdaje < 1100)
        {
            NeaktivniPartner np = new NeaktivniPartner();
            np.ppid = partner._int;
            np.naziv = partner._string;
            np.danaBezKupovine = danaBezProdaje;
            neaktivniPartneri.Add(np);
        }
    }
    dataGridView1.DataSource = M.List.ConvertToDataTable(neaktivniPartneri);
}

So in here i have List<Int_String> partneri which contains 5k rows. For each of it i run Komercijalno.Partner.PoslednjaKupovina(partner._int) which contains SQL statement which looks like this:

public static DateTime PoslednjaKupovina(int ppid)        
{
    using (FbConnection con = new FbConnection(M.Baza.connectionKomercijalno2018))
    {
        con.Open();
        using (FbCommand cmd = new FbCommand("SELECT DATUM FROM DOKUMENT WHERE PPID = @PPID ORDER BY DATUM DESC", con))
        {
            cmd.Parameters.AddWithValue("@PPID", ppid);

            FbDataReader dr = cmd.ExecuteReader();

            if (dr.Read())
                return Convert.ToDateTime(dr[0]);
            else
                return new DateTime(1, 1, 1);
        }
    }
}

So SQL statement is executed 5k times which is way too slow.

So how could i pass my List<Int_String> or just array of ints to sql command so there i execute one time (inside foreach or how else) and return dataAdapter so i execute SQL only once.

DataAdapter returned should look like this:

| ppid     | datum    |

So let's say i have Table PARTNER (from which i populate List partneri with SELECT PPID, NAZIV FROM PARTNER) with this data in it:

| PPID    | NAZIV    |
| 1       | name001  |
| 2       | name002  |
| 3       | name003  |
| 4       | name004  |

And let's say my DOKUMENT table has this data:

| ID      | PPID    | DATE       |
| 1       | 2       | 12.03.2018 |
| 2       | 3       | 12.03.2018 |
| 3       | 2       | 05.03.2018 |
| 4       | 2       | 03.04.2018 |
| 5       | 1       | 26.03.2018 |
| 6       | 4       | 21.02.2018 |
| 7       | 4       | 06.05.2018 |

And output i want is:

| PPID    | DATE       |
| 1       | 26.03.2018 |
| 2       | 03.04.2018 |
| 3       | 12.03.2018 |
| 4       | 06.05.2018 |

回答1:

You can replace all your logic with a single query that does:

select d.ppid, max(d.datum)
from partner p
inner join dokument d
    on d.ppid = p.ppid
group by d.ppid

You may need to add a where-clause with additional conditions to select which partners you want to have.

回答2:

Making it as an answer, for there is already very many comments below the question, so it would just be lost in the noise there.

So how could i pass my List or just array of ints to sql command so there i execute one time

That question was discussed here: Filter sql based on C# List instead of a filter table

Depending upon how typical(frequent) is the query and how large is the data there are few ways.

Global Temporary Tables
Imitation of GTT using UNION ALL and several select {constants} from RDB$database
Flattening the list into one long string of special format and using reversed LIKE matching.

PS. For your specific task Mark's answer is definitely much better: you do not have to fetch lot of data from the server only to pass it back in another query. I just wanted to link a question, related to a part of this one, that was already answered.

来源：https://stackoverflow.com/questions/50363866/building-sql-with-foreach

标签

sql

winforms

firebird