How to get JSON data from MySQL with correct field type?

I am a beginner to Golang. I've read many threads about getting JSON from a mysql database, but there is a question left unanswered: the values of all fields in the JSON are strings and not their correct types, be it integer or time.

The code I'm using:

var emptyOut []byte
rows, err := db.Query(sqlString)
if err != nil {
	return emptyOut, err
}
defer rows.Close()
columns, err := rows.Columns()
if err != nil {
	return emptyOut, err
}
count := len(columns)
tableData := make([]map[string]interface{}, 0)
values := make([]interface{}, count)
valuePtrs := make([]interface{}, count)
for rows.Next() {
	for i := 0; i < count; i++ {
		valuePtrs[i] = &values[i]
	}
	rows.Scan(valuePtrs...)
	entry := make(map[string]interface{})
	for i, col := range columns {
		var v interface{}
		val := values[i]
		b, ok := val.([]byte)
		if ok {
			v = string(b)
		} else {
			v = val
		}
		entry[col] = v
	}
	tableData = append(tableData, entry)
}
jsonData, err := json.Marshal(tableData)
if err != nil {
	return emptyOut, err
}
return jsonData, nil

The part that turns data into string is surely this one:

v = string(b)

So the question is: how to turn v into the right type of value of b. This will save a lot of my effort in dealin with wrong field type later on. Thank you in advance!

I am a little unclear on the constraints of your problem.

If you know the database schema before hand, I would create a struct that represents your data. You can then use the database library gorm to get the data into a struct slice. You can add go tags to the struct and convert the struct slice to json with the fields marshaled into their correct types.
For example (untested):

type MyDataSchema struct {
    Id         int    `json:"id"`
    Name       string `json:"name"`
    Birth time.Time `json:"birth_date,omitempty"`
}
...
rows:=&DataSchema{}
result := db.Find(&rows)
b, err := json.Marshal(result)

If you dont know the schema before hand, then it is a little bit more tricky. In go there is such a thing as a type switch. You might want to consider that:
If I am following your code correctly, val := values[i] val is of type interface. So you could do something like (untested):

	switch v := i.(type) {
	case int:
		entry[col]=v
	case string:
		entry[col]=v
    case time.Time:	
        entry[col]=v
//you can add more types
    default:
		entry[col]=val
	}

This might not be a complete answer but hopefully it gets you to where you need.