Code Generation of an Upsert / Merge for SQL Server - C#/.NET - sql-server

Question

What is the best way to generate an "UPSERT" Merge statement for SQL Server? I'm looking for a way to just put in a SQL Server connection string and then point a piece of code at a single SQL Server table.
Is it to crawl INFORMATION_SCHEMA.COLUMNS and INFORMATION_SCHEMA.TABLES for the metadata and write a small console app? other suggestions?
I'd like to be able to copy and paste the resulting code into a "const string sql" variable to then use with something like Dapper.

Answer 1

The fastest way to upsert in SQL Server in C# is to bulk insert into a temp table and issue a merge from there to the production table.
var tmpTable =
"create table #myTable (a as int, b as nvarchar(200)";
var conString = "my_connection_string";
using (var con = new SqlConnection(conString))
{
con.Open();
//Create Temp Table
var cmd = new SqlCommand(tmpTable, con);
cmd.ExecuteNonQuery();
//Build Data Set
var dt = new DataTable();
dt.Columns.Add(new DataColumn("A", typeof(int)));
dt.Columns.Add(new DataColumn("B", typeof(string)));
foreach (var myDataRow in myData)
{
var row = dt.NewRow();
row["A"] = myDataRow.A;
row["B"] = myDataRow.B;
dt.Rows.Add(row);
}
//Write to temp table
using (var bulk = new SqlBulkCopy(con))
{
bulk.BulkCopyTimeout = 0;
bulk.DestinationTableName = "#myTable";
bulk.WriteToServer(dt);
}
//Do the upsert
var mergeSql = "merge into myData as Target " +
"using #myTable as Source " +
"on " +
"Target.A=Source.A " +
"and Target.B = Source.B " +
"when matched then " +
"update set Target.A=Source.A, Target.B = Source.B, " +
"when not matched then " +
"insert (A,B) values (Source.A,Source.B);";
cmd.CommandTimeout = 0;
cmd.CommandText = mergeSql;
cmd.ExecuteNonQuery();
//Clean up the temp table
cmd.CommandText = "drop table #myTable";
cmd.ExecuteNonQuery();
}
}
I've tested several ways and this is the fastest I have came across. I use it in a production environment for inserting millions of rows daily without issue.