Sign Up in Go

So far only the owner and the staff have keys, and they are written into the code. Customers need accounts of their own: they sign up with an email and a password, and the shop keeps each one as a row in a new table, users.

A table is created once with CREATE TABLE IF NOT EXISTS, so the shop can start again and again. A column marked UNIQUE never holds the same value twice. To know whether a value is taken, look for it with a SELECT: db.QueryRow(…).Scan(…) gives a nil error when it found a row, and sql.ErrNoRows when nothing matches. Here is the same idea for a made-up coupons table:

db.Exec("CREATE TABLE IF NOT EXISTS coupons (id INTEGER PRIMARY KEY, code TEXT NOT NULL UNIQUE)")
var couponID int
err := db.QueryRow("SELECT id FROM coupons WHERE code = ?", "SPRING10").Scan(&couponID)
if err == sql.ErrNoRows {
	fmt.Println("No such coupon")
}

An email that is already registered gets 409 Conflict: the request clashes with what the server already has. A new user gets 201 Created with their id and email. The answer never sends the password back.

These are the requests your code must answer. /check/users is a route the checker adds to show what the table holds:

POST /register  {"email": "ana@example.com", "password": "secret123"}  →  201  {"email": "ana@example.com", "id": 1}
POST /register  {"email": "ana@example.com", "password": "hello456"}   →  409  {"error": "Email already registered"}
POST /register  {"email": "ben@example.com", "password": "secret123"}  →  201  {"email": "ben@example.com", "id": 2}
GET /check/users  →  200  [{"email": "ana@example.com", "id": 1, "password": "secret123"}, {"email": "ben@example.com", "id": 2, "password": "secret123"}]

Your task: create the users table, then add a POST /register handler. When the email is taken, answer {"error": "Email already registered"} with 409 and return; otherwise save the user and answer 201 with its id and email.

Assignment
Follow the Coding Tutorial to create the users table and the sign-up handler.

Hint
Look for the email before the INSERT: a nil error from Scan means a user already has it. res.LastInsertId() is the id the database gave the new row. Go won't build while a variable you declared is still unused, so between some steps the editor shows a build error until the next line uses it.