F# insert a list into SQL Server - sql

Question

I'm stuck at the moment trying to figure out a method of inserting into SQL Server from F#.
I have an F# function that iterates through all files inside a folder following a user-defined pattern. Then I can use the returned data to put in a list or (ideally) insert into a database.
I already have a working insert-into-sql function that works properly:
let execNonQuery conn s =
let comm =
new SqlCeCommand(s, conn)
try
comm.ExecuteNonQuery() |> ignore
with e ->
printf "Error : %A\n" e
let string = "insert into MyTable (MyColumn) values ('test .. again')"
execNonQuery conn string; // works
I'm trying to get this method to work properly:
let rec getAllFiles dir pattern =
seq { yield! Directory.EnumerateFiles(dir, pattern)
for d in Directory.EnumerateDirectories(dir) do
yield! getAllFiles d pattern }
let getApplications (dir : string) (extension : string) =
getAllFiles dir extension
//|> Seq.toList // If I need to create a list of returned values
|> Seq.iter (fun s -> SQLInsertString s) // This does not work as it complains about the function not being of type unit
If I use Seq.toList only and call the function as per below, it works:
getApplications "C:\Admin" "*.txt" // works
The other thing I don't understand is how can you create a working insert command that takes in a string for Value. For example:
let SQLInsertString s = "insert into MyTable (MyColumn) values (%s)" //does not work

Answer 1

You're almost there. The problem is sqlInsertString returns string which is not legal to use in Seq.iter.
What you're doing with sqlInsertString is to create a string using string formats. It fits nicely with sprintf function:
let sqlInsertString s =
sprintf "insert into MyTable (MyColumn) values (%s)" s
Now you can use execNonQuery on results of sqlInsertString to actually insert data into database. Since execNonQuery returns unit, it could be easily used in Seq.iter:
// Assuming conn is a global and already defined variable.
let getApplications (dir : string) (extension : string) =
getAllFiles dir extension
|> Seq.iter (fun s -> execNonQuery conn (sqlInsertString s))
Since type annotation is redundant, your code could be rewritten in a more idiomatic way:
let getApplications dir extension conn =
getAllFiles dir extension
|> Seq.iter (sqlInsertString >> execNonQuery conn)

Answer 2

The best way to pass parameters to a query is to use SqlCeParameter. This is easier than composing strings (because you don't need to encode strings and escape quotes) and it is also safer, because you avoid SQL injection attack. Here is a basic sample:
let sqlInsertString value =
// Create and open connection ('use' makes sure it gets closed at the end)
use conn = new SqlCeConnection("...");
conn.Open()
// Create a command with a parameter named '#str'
let cmd = new SqlCeCommand("INSERT INTO MyTable (MyColumn) values (#str)", conn)
// Create parameter '#str' with string value 'value' and add it to the command
let param = new SqlCeParameter("#str", SqlDbType.NVarChar, value)
cmd.Parameters.Add(param)
// Now run the command (exception handling omitted)
cmd.ExecuteNonQuery() |> ignore
Using this function, you should now be able to use Seq.iter. The function takes a string to be inserted and returns unit (no value), so it can be passed to Seq.iter:
let getApplications (dir : string) (extension : string) =
getAllFiles dir extension
|> Seq.iter (fun s -> sqlInsertString s)
Alternatively, you can write the last line just as |> Seq.iter sqlInsertString. If you do that, you're basically saying that the argument s should be directly passed to the sqlInsertString function.