I have a small Heroku app in which i print out name and age from each rows after query execution.
I want to avoid looping rows.Next(),Scan().. and just want to show what database returned after query execution which may be some data or error.
Can we directly dump data to a string for printing?
rows, err := db.Query("SELECT name FROM users WHERE age = $1", age)
if err != nil {
log.Fatal(err)
}
for rows.Next() {
var name string
if err := rows.Scan(&name); err != nil {
log.Fatal(err)
}
fmt.Printf("%s is %d
", name, age)
}
if err := rows.Err(); err != nil {
log.Fatal(err)
}
Pretty much: No.
The Query method is going to return a pointer to a Rows struct:
func (db *DB) Query(query string, args ...interface{}) (*Rows, error)
If you print that (fmt.Printf("%#v ", rows)) you'll see something such as:
&sql.Rows{dc:(*sql.driverConn)(0xc8201225a0), releaseConn:(func(error)(0x4802c0), rowsi:(*pq.rows)(0xc820166700), closed:false, lastcols:[]driver.Value(nil), lasterr:error(nil), closeStmt:driver.Stmt(nil)}
...probably not what you want.
Those correspond to the Rows struct from the sql package (you'll notice the fields are not exported):
type Rows struct {
dc *driverConn // owned; must call releaseConn when closed to release
releaseConn func(error)
rowsi driver.Rows
closed bool
lastcols []driver.Value
lasterr error // non-nil only if closed is true
closeStmt driver.Stmt // if non-nil, statement to Close on close
}
You'll see []driver.Value (an interface from the driver package), that looks like where we can expect to find some useful, maybe even human readable data. But when directly printed it doesn't appear useful, it's even empty... So you have to somehow get at the underlying information. The sql package gives us the Next method to start with:
Next prepares the next result row for reading with the Scan method. It returns true on success, or false if there is no next result row or an error happened while preparing it. Err should be consulted to distinguish between the two cases.
Every call to Scan, even the first one, must be preceded by a call to Next.
Next is going to make a []driver.Value the same size as the number of columns I have, which is accessible (within the sql package) through driver.Rows (the rowsi field) and populate it with values from the query.
After calling rows.Next() if you did the same fmt.Printf("%#v ", rows) you should now see that []diver.Value is no longer empty but it's still not going to be anything that you can read, more likely something resembling:[]diver.Value{[]uint8{0x47, 0x65...
And since the field isn't exported you can't even try and convert it to something more meaningful. But the sql package gives us a means to do something with the data, which is Scan.
The Scan method is pretty concise, with lengthy comments that I won't paste here, but the really important bit is that it ranges over the columns in the current row you get from the Next method and calls convertAssign(dest[i], sv), which you can see here: https://golang.org/src/database/sql/convert.go
It's pretty long but actually relatively simple, it essentially switches on the type of the source and destination and converts where it can, and copies from source to destination; the function comments tell us:
convertAssign copies to dest the value in src, converting it if possible. An error is returned if the copy would result in loss of information. dest should be a pointer type.
So now you have a method (Scan) which you can call directly and which hands you back converted values. Your code sample above is fine (except maybe the call to Fatal() on a Scan error).
It's important to realize that the sql package has to work with a specific driver, which is in turn implemented for specific database software, so there is quite some work going on behind the scenes.
I think your best bet if you want to hide/generalize the whole Query() ---> Next() ---> Scan() idiom is to drop it into another function which does it behind the scenes... write a package in which you abstract away that higher level implementation, as the sql package abstracts away some of the driver-specific details, the converting and copying, populating the Rows, etc.