Cookbook database recipes

data/cookbook/sqlite-create-insert-select/00-caqti-ppx-rapper.ml

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+---
+packages:
+- name: "ppx_rapper_lwt"
+  tested_version: "3.1.0"
+  used_libraries:
+  - ppx_rapper_lwt
+- name: "ppx_rapper"
+  tested_version: "3.1.0"
+  used_libraries:
+  - ppx_rapper
+- name: "caqti-driver-sqlite3"
+  tested_version: "1.9.0"
+  used_libraries:
+  - caqti-driver-sqlite3
+- name: "caqti-lwt"
+  tested_version: "1.9.0"
+  used_libraries:
+  - caqti-lwt
+- name: "lwt"
+  tested_version: "5.7.0"
+  used_libraries:
+  - lwt
+  - lwt.unix
+discussion: |
+  The `Caqti` library permits portable programming
+    with SQLite, MariaDB, and PostgreSQL.
+    `ppx_rapper` converts annotated SQL strings into `Caqti` queries.
+    This preprocessor makes all type conversions transparent and leverages OCaml's strong typing.
+    It also checks the SQL syntax of the given query.
+    See [the `Caqti` reference page](https://github.com/paurkedal/ocaml-caqti)
+    and [the `ppx_rapper` reference page](https://github.com/roddyyaga/ppx_rapper).
+---
+
+(* The `Caqti/ppx_rapper` combo uses an Lwt environment.
+   Let operators `( let* )` and `( let*? )` are defined as usual for Lwt, to have a clean
+   notation for chaining promises. `( let*? )` extracts the result from a returned `Ok result` or
+   stops the execution in case of an `Error err` value.
+*)
+let ( let* ) = Lwt.bind
+let ( let*? ) = Lwt_result.bind
+
+(* The helper function `iter_queries` sequentially schedules a list of queries.
+   Each query is a function that takes the
+   connection handle of the database as an argument. *)
+let iter_queries queries connection =
+  List.fold_left
+    (fun a f ->
+      Lwt_result.bind a (fun () -> f connection))
+    (Lwt.return (Ok ()))
+    queries
+
+(* The `%rapper` node here makes `ppx_rapper` generate code, such that, when applying
+  the `create_employees_table () connection` function,
+  the provided SQL `CREATE` query will be run without any parameters and without
+  receiving any data from the database.
+
+  In case of successful execution of the query, we get back an `Ok ()` value, otherwise
+  we get an `Error` value.
+  *)
+let create_employees_table =
+  [%rapper
+    execute {sql| CREATE TABLE employees
+              (name VARCHAR,
+              firstname VARCHAR,
+              age INTEGER)
+            |sql}
+  ]
+
+type employee =
+  { name:string; firstname:string; age:int }
+let employees = [
+  {name = "Dupont"; firstname = "Jacques"; age = 36};
+  {name = "Legendre"; firstname = "Patrick"; age = 42}
+  ]
+
+(* For the SQL `INSERT` query, `ppx_rapper` generates a function `insert_employee (p: employee) connection`.
+  The tag `record_in` tag tells `ppx_rapper` to read the values `name`, `firstname`,
+  and `age` from the provided record value, while the `%[TYPE_NAME]{[INPUT_FIELD_NAME]}` notation specifies
+  which conversions to perform on the input values. *)
+let insert_employee =
+  [%rapper
+    execute
+    {sql| INSERT INTO employees VALUES
+        (%string{name},
+        %string{firstname},
+        %int{age})
+    |sql}
+    record_in
+  ]
+
+(* The `get_many` tag makes `ppx_rapper` generate code that queries the database and
+   receives a list of values. The `record_out` tag specifies that each list item
+   will be a record.
+
+   The `@[TYPE_NAME]{[COLUMN_NAME]}` notation specifies
+   which conversions to perform on the output values.
+   *)
+let get_all_employees =
+  [%rapper
+    get_many
+    {sql|SELECT
+        @string{name},
+        @string{firstname},
+        @int{age}
+      FROM employees
+    |sql}
+    record_out
+  ]
+
+(* Here's another example query that selects a single row via the SQL `WHERE` clause, using the `get_opt` tag.
+   This query has both input (`name`) and output values (`name`, `firstname`, `age`).
+
+   Here the absence of the `record_in` tag makes `ppx_rapper` generate code where the
+   input values are passed as named arguments.
+   The `get_opt` tag means that the result will be an option: `None` if no rows matching the criteria
+    is found, and `Some r` if a row match the criteria.
+   *)
+let get_employee_by_name =
+  [%rapper
+    get_opt
+    {sql|SELECT
+        @string{name},
+        @string{firstname},
+        @int{age}
+      FROM employees
+      WHERE name=%string{name}
+    |sql}
+    record_out
+  ]
+
+
+(* All query functions generated by `ppx_rapper` take an argument and a `connection` parameter.
+The function `insert_employee` must be called with
+a value of type `employee` and `connection`. To insert multiple records from
+a list, we use `List.map` to create a list
+of functions. Each of these functions will execute its
+associated query when called. The function `iter_queries` runs
+the queries in sequence.
+
+Note that, if you have to insert many records, it makes sense to perform a bulk insert query instead. *)
+let execute_queries connection =
+  let*? () = create_employees_table () connection in
+  let*? () =
+    iter_queries
+      (List.map insert_employee employees)
+      connection
+  in
+  let*? employees = get_all_employees () connection in
+  employees |> List.iter (fun employees ->
+    Printf.printf
+      "name=%s, firstname=%s, age=%d\n"
+      employees.name
+      employees.firstname
+      employees.age);
+  let*? employees =
+    get_employee_by_name ~name:"Dupont" connection
+  in
+  match employees with
+  | Some employees' ->
+    Printf.printf
+      "found:name=%s, firstname=%s, age=%d\n"
+      employees'.name
+      employees'.firstname
+      employees'.age;
+    Lwt_result.return ()
+  | None ->
+    print_string "Not found";
+    Lwt_result.return ()
+
+(* The main program starts by establishing an Lwt environment.
+   The function `with_connection` opens the database,
+   executes a function with the `connection` database handle,
+   and closes the database connection again, even when an exception is raised. *)
+let () =
+  match Lwt_main.run @@
+  Caqti_lwt.with_connection
+    (Uri.of_string "sqlite3:essai.sqlite")
+    execute_queries
+  with
+  | Result.Ok () ->
+    print_string "OK\n"
+  | Result.Error err ->
+    print_string (Caqti_error.show err)
+

data/cookbook/sqlite-create-insert-select/01-ezsqlite.ml

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+---
+packages:
+- name: "ezsqlite"
+  tested_version: "0.4.2"
+  used_libraries:
+  - ezsqlite
+---
+
+(* Open or create (if it doesn't exist) the SQLite database *)
+let db = Ezsqlite.load "employees.sqlite"
+
+(* Create the `employees` table.
+   The function `run_ign` ("run and ignore") executes the query and discards the database response.
+   *)
+let () =
+  Ezsqlite.run_ign db
+    "CREATE TABLE employees (
+        name VARCHAR NOT NULL,
+        firstname VARCHAR NOT NULL,
+        age INTEGER NOT NULL
+    )"
+    ()
+
+type employee =
+  { name:string; firstname:string; age:int }
+
+let employees = [
+  {name = "Dupont"; firstname = "Jacques"; age = 36};
+  {name = "Legendre"; firstname = "Patrick"; age = 42}
+  ]
+
+let () =
+(* The function `Ezsqlite.prepare` prepares the statement, so that later actual values
+  can be bound to the variables `:name`, `:firstname`, and `:age`. *)
+  let stmt = Ezsqlite.prepare db
+    {| 
+      INSERT into employees
+      VALUES (:name, :firstname, :age)
+    |}
+  in
+(* Running these `Ezsqlite` functions in sequence binds the values from the
+   `employee` record and executes the query. *)
+  let insert_employee (employee: employee) =
+    Ezsqlite.clear stmt;
+    Ezsqlite.bind_dict stmt
+      [":name", Ezsqlite.Value.Text employee.name;
+      ":firstname", Ezsqlite.Value.Text
+        employee.firstname;
+      ":age", Ezsqlite.Value.Integer
+        (Int64.of_int employee.age)];
+    Ezsqlite.exec stmt
+  in
+  employees
+  |> List.iter insert_employee 
+
+(* The `iter` function executes a query and then maps a given function over all
+   rows returned by the database.
+   *)
+let () =
+  let stmt = Ezsqlite.prepare db
+    "SELECT name, firstname, age from employees"
+  in
+  let print_employee row =
+(* The `Ezsqlite.text`, `blob`, `int64`, `int`, `double`
+   functions can be used to read the values of individual columns.
+
+   Note that this is not type-safe, since you need to provide the correct type
+   for the column here. *)
+    let name = Ezsqlite.text row 0
+    and firstname = Ezsqlite.text row 1
+    and age = Ezsqlite.int row 2
+    in
+    Printf.printf "name=%s, firstname=%s, age=%d\n"
+      name firstname age
+  in
+  Ezsqlite.iter stmt print_employee