An exercise to execute basic SQL queries (skipped parsing)

simple-sql.swift

indirect enum Query {
    case equal(field: String, value: String)
    case notEqual(field: String, value: String)

    case not(query: Query)
    case or(first: Query, second: Query)
    case and(first: Query, second: Query)
}

func &&(lhs: Query, rhs: Query) -> Query {
    return .and(first: lhs, second: rhs)
}
func ||(lhs: Query, rhs: Query) -> Query {
    return .or(first: lhs, second: rhs)
}
prefix func !(query: Query) -> Query {
    return .not(query: query)
}
func ==(lhs: String, rhs: String) -> Query {
    return .equal(field: lhs, value: rhs)
}
func !=(lhs: String, rhs: String) -> Query {
    return .notEqual(field: lhs, value: rhs)
}

extension Query {
    func satisfied(by row: [String:String]) -> Bool {
        switch self {

        case let .equal(field: field, value: value):
            return !row.filter { $0 == (field, value) }.isEmpty

        case let .notEqual(field: field, value: value):
            return !Query.equal(field: field, value: value).satisfied(by: row)

        case let .not(query: query):
            return !query.satisfied(by: row)

        case let .or(first: first, second: second):
            return first.satisfied(by: row) || second.satisfied(by: row)

        case let .and(first: first, second: second):
            return first.satisfied(by: row) && second.satisfied(by: row)

        }
    }
}

func execute(_ query: Query) -> [String:String]? {
    let database: [[String:String]] = [
        [
            "birthyear": "2000",
            "name": "dan"
        ],
        [
            "birthyear": "1972",
            "name": "vladimir"
        ]
    ]

    for row in database where query.satisfied(by: row) {
        return row
    }

    return nil
}

let query: Query = "name" == "igor" || "birthyear" == "2000" && ("name" == "dan" || "name" == "vladimir")
print(query) // -> .or(.equal("name", "igor"), .and(.equal("birthyear", "2000"), .or(.equal("name", "dan"), .equal("name", "vladimir"))))

let matching = execute(query)
print(matching) // -> ["birthyear": "1972", "name": "vladimir"]