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SQLite3 custom function (UDF)

2022-05-15 07:09:11Donghai Chen Guangjian

package main

import (
    "database/sql"
    "fmt"
    "log"
    "math"
    "math/rand"

    sqlite "github.com/mattn/go-sqlite3"
)

// Computes x^y
func pow(x, y int64) int64 {
    return int64(math.Pow(float64(x), float64(y)))
}

// Computes the bitwise exclusive-or of all its arguments
func xor(xs ...int64) int64 {
    var ret int64
    for _, x := range xs {
        ret ^= x
    }
    return ret
}

// Returns a random number. It's actually deterministic here because
// we don't seed the RNG, but it's an example of a non-pure function
// from SQLite's POV.
func getrand() int64 {
    return rand.Int63()
}

// Computes the standard deviation of a GROUPed BY set of values
type stddev struct {
    xs []int64
    // Running average calculation
    sum int64
    n   int64
}

func newStddev() *stddev { return &stddev{} }

func (s *stddev) Step(x int64) {
    s.xs = append(s.xs, x)
    s.sum += x
    s.n++
}

func (s *stddev) Done() float64 {
    mean := float64(s.sum) / float64(s.n)
    var sqDiff []float64
    for _, x := range s.xs {
        sqDiff = append(sqDiff, math.Pow(float64(x)-mean, 2))
    }
    var dev float64
    for _, x := range sqDiff {
        dev += x
    }
    dev /= float64(len(sqDiff))
    return math.Sqrt(dev)
}

func main() {
    sql.Register("sqlite3_custom", &sqlite.SQLiteDriver{
        ConnectHook: func(conn *sqlite.SQLiteConn) error {
            if err := conn.RegisterFunc("pow", pow, true); err != nil {
                return err
            }
            if err := conn.RegisterFunc("xor", xor, true); err != nil {
                return err
            }
            if err := conn.RegisterFunc("rand", getrand, false); err != nil {
                return err
            }
            if err := conn.RegisterAggregator("stddev", newStddev, true); err != nil {
                return err
            }
            return nil
        },
    })

    db, err := sql.Open("sqlite3_custom", ":memory:")
    if err != nil {
        log.Fatal("Failed to open database:", err)
    }
    defer db.Close()

    var i int64
    err = db.QueryRow("SELECT pow(2,3)").Scan(&i)
    if err != nil {
        log.Fatal("POW query error:", err)
    }
    fmt.Println("pow(2,3) =", i) // 8

    err = db.QueryRow("SELECT xor(1,2,3,4,5,6)").Scan(&i)
    if err != nil {
        log.Fatal("XOR query error:", err)
    }
    fmt.Println("xor(1,2,3,4,5) =", i) // 7

    err = db.QueryRow("SELECT rand()").Scan(&i)
    if err != nil {
        log.Fatal("RAND query error:", err)
    }
    fmt.Println("rand() =", i) // pseudorandom

    _, err = db.Exec("create table foo (department integer, profits integer)")
    if err != nil {
        log.Fatal("Failed to create table:", err)
    }
    _, err = db.Exec("insert into foo values (1, 10), (1, 20), (1, 45), (2, 42), (2, 115)")
    if err != nil {
        log.Fatal("Failed to insert records:", err)
    }

    rows, err := db.Query("select department, stddev(profits) from foo group by department")
    if err != nil {
        log.Fatal("STDDEV query error:", err)
    }
    defer rows.Close()
    for rows.Next() {
        var dept int64
        var dev float64
        if err := rows.Scan(&dept, &dev); err != nil {
            log.Fatal(err)
        }
        fmt.Printf("dept=%d stddev=%f\n", dept, dev)
    }
    if err := rows.Err(); err != nil {
        log.Fatal(err)
    }
}

copyright notice
author[Donghai Chen Guangjian],Please bring the original link to reprint, thank you.
https://en.chowdera.com/2022/131/202205102230274649.html

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