Parameter Make.Sql

Database-specific definitions

type db

The database connection or file, etc.

type arg

A value sent to the database in the place of a query parameter.

type row

A tuple from the database.

val placeholder : Stdlib.Format.formatter -> int -> unit

A generic way to write placeholders for different database drivers' prepared statement parameters.

ℹ️ Placeholders are 0-indexed.

Query runners

val query : db -> string -> arg list -> (row, 'r) Query.ret -> 'r

The main function through which queries are run is the query function. This function always creates a prepared statement for each partial call to query db sql. This prepared statement can then be called with the actual arguments (if any) and the resultset row decoder:

let add_person =
  query db (sql "insert into people (name, age) values (%a, %a)" placeholder 0 placeholder 1)
let add_person name age = add_person Arg.[text name; int age] unit
  • raises Invalid_argument

    if trying to create multiple prepared statements for the same SQL query in PostgreSQL. To avoid this, just create the prepared statement once only and call it whenever needed, as shown above.

val exec_script : db -> string -> unit

exec_script db sql executes the sql script (possibly made up of multiple statements) in the database db. Note that it ignores any rows returned by any of the statements.

The script must not have a trailing semicolon.

Binding arguments

These encode OCaml data as data to be bound to the query statement.

module Arg : sig ... end

Helpers to get typed values from columns

val int : int -> row -> int
val bool : int -> row -> bool
val int64 : int -> row -> int64
val float : int -> row -> float
val text : int -> row -> string

Also handles values of all other types. Use this when SQLite can change the exact type of value it returns at runtime, e.g. for very large numbers it can return text.

val opt : (int -> row -> 'a) -> int -> row -> 'a option

opt dec col row is the optional value NULL turns to None at column col of the result row.