CODEWITHSUNDEEP
mysqlsqlpostgresqlgoprepared-statement
galusben
galusben

Reputation: 6372

In golang how to support multiple sql syntax (mysql vs postgres)

My go app shall support multiple databases. Meaning, running the same binary with different databases, the database the app is working with will be determined by configuration.

The problem is, each database has it's own prepared statements syntax. Example:

db.Prepare("select p, f, t from mytable where p = $1")

Will work for postgres but will not work for mysql.

db.Prepare("select p, f, t from mytable where p = ?")

Will work for mysql but will not work for postgres.

Off curse I can solve it by editing the string on runtime or maintaining multiple queries.

Is there a better way?

I do not want to have some huge abstraction with an external library that will take control on all my db access, but if there is some light weight library that just magically fix the syntax, I am good with that.

EDIT: Summarising what I have said before, the part that bothers me is that for mysql you will have to use "?" while for postgres you will have to use $1, $2...

Cheers

Upvotes: 2

Views: 2544

Answers (2)

TomKraljevic
TomKraljevic

Reputation: 3671

I found db.Rebind() to help with this. So:

    name := "my name"
    var p = property{}
    // language=SQL
    s := "SELECT * FROM property WHERE name=?"
    err := db.Get(&p, db.Rebind(s), name)

The language comment at the top is so that IntelliJ can still syntax-check the SQL statement for me in the UI.

I also had to write separate CREATE statements for each database (my application is simultaneously supporting mysql, postgres, and sqlite).

I also found the UPDATE statement syntax between mysql and sqlite to be the same, but postgres required special handling. Since my UPDATE statements are very consistent, I was able to write a function to just kludge-translate from the mysql dialect to the postgres dialect. This is definitely not a generic solution but worked well enough for my unit and integration tests to pass. YMMV.

// RebindMore takes a MySQL SQL string and convert it to Postgres if necessary.
// The db.Rebind() handles converting '?' to '$1', but does not handle SQL statement
// syntactic changes needed by Postgres.
//
// convert: "UPDATE table_name SET a = ?, b = ?, c = ? WHERE d = ?"
// to:      "UPDATE table_name SET (a, b, c) = ROW (?, ?, ?) WHERE d = ?"
func RebindMore(db *sqlx.DB, s string) string {
    if db.DriverName() != "postgres" {
        return s
    }

    if !strings.HasPrefix(strings.ToLower(s), "update") {
        return db.Rebind(s)
    }

    // Convert a MySQL update statement into a Postgres update statement.
    var idx int
    idx = strings.Index(strings.ToLower(s), "set")
    if idx < 0 {
        log.Fatal().Msg("no SET clause in RebindMore (" + s + ")")
    }

    prefix := s[:idx+3]
    s2 := s[idx+3:]

    idx = strings.Index(strings.ToLower(s2), "where")
    if idx < 0 {
        log.Fatal().Msg("no WHERE clause in RebindMore (" + s + ")")
    }

    suffix := s2[idx:]
    s3 := s2[:idx]

    s4 := strings.TrimSpace(s3)
    arr := strings.Split(s4, ",")

    var names = ""
    var values = ""

    for i := 0; i < len(arr); i++ {
        nameEqValue := arr[i]
        s5 := strings.ReplaceAll(nameEqValue, " ", "")
        nvArr := strings.Split(s5, "=")
        if names != "" {
            names += ","
        }
        names += nvArr[0]
        if values != "" {
            values += ","
        }
        values += nvArr[1]
    }

    s6 := prefix + " (" + names + ") = ROW (" + values + ") " + suffix
    return db.Rebind(s6)
}

Call it this way:

    // language=SQL
    s := RebindMore(db, "UPDATE table_name SET a = ?, b = ? WHERE c = ?")
    db.MustExec(s, value1, value2)

At some point I will need to add migration, and expect to just add separate code per DB to handle the differences (like for CREATE).

One final thing worth pointing out is that MySQL and Postgres handle capitalization very differently. I ended up just converting every table and column name to lower_case to avoid unnecessary complexity.

Upvotes: 1

ZAky
ZAky

Reputation: 1307

In this particular case use a place holder {{ph}} in the end of the SQL and use strings.Replace() to replace it with ? Or $1 according to the db driver.

Upvotes: 0

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