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In this article I list how to do common SQL database operations with Go.

Introducing database/sql

Go offers a clean SQL database API in its standard library database/sql package, but the specific database drivers must be installed separately.

It’s a smart approach because it provides a common interface that nearly every DB driver implements.

If you want to use MySQL, you can use https://github.com/go-sql-driver/mysql.

If you’re using PostgreSQL, use https://github.com/lib/pq.

You just need to include the lib using import _, and the database/sql API will be configured to enable that driver:

import "database/sql"
import _ "github.com/go-sql-driver/mysql"

Open the database connection

Although the goal is to have it abstracted, there are still differences in some things, for example in how you connect to a database:

import "database/sql"
import _ "github.com/go-sql-driver/mysql"

//...

db, err := sql.Open("mysql", "theUser:thePassword@/theDbName")
if err != nil {
  panic(err)
}
import "database/sql"
import _ "github.com/lib/pq"

//...

db, err := sql.Open("postgres", "user=theUser dbname=theDbName sslmode=verify-full")
if err != nil {
  panic(err)
}

but much of the actual API is db-independent, and can be interchanged quite easily (not talking about SQL here, just referring to the database API).

Close the database connection

Where it makes sense, you should always close the database connection.

You can as usual use defer to close it when the function that opens the db connection ends:

db, err := sql.Open("postgres", psqlInfo)
defer db.Close()

Extract data from the database

Select a single row

Querying a table is done in 2 steps. First you call db.QueryRow(), then you call Scan() on the result.

Example:

id := 1
var col string
sqlStatement := `SELECT col FROM my_table WHERE id=$1`
row := db.QueryRow(sqlStatement, id)
err := row.Scan(&col)
if err != nil {
    if err == sql.ErrNoRows {
        fmt.Println("Zero rows found")
    } else {
        panic(err)
    }
}

db.QueryRow() is used to query a single value from a table.

Signature:

func (db *DB) QueryRow(query string, args ...interface{}) *Row

It returns a pointer to a db.Row value.

(*Row) Scan scans the row, copying the column values to the parameter passed into it.

Signature:

func (r *Row) Scan(dest ...interface{}) error

If more than one row was returned, it only scans the first one and ignore the rest.

If no row was returned, it returns a ErrNoRows error.

var ErrNoRows = errors.New("sql: no rows in result set")

Select multiple rows

To query a single row we used db.QueryRow(). To query multiple rows we use db.Query(), which returns a *Rows value.

From the docs:

//Rows is the result of a query. Its cursor starts before  the first row of the result set. Use Next to advance through the rows:

    rows, err := db.Query("SELECT ...")
    ...
    defer rows.Close()
    for rows.Next() {
        var id int
        var name string
        err = rows.Scan(&id, &name)
        ...
    }
     err = rows.Err() // get any error encountered ing iteration
    ...

// Err returns the error, if any, that was encountered during iteration. // Err may be called after an explicit or implicit Close.

We need to iterate on rows.Next(), which allows us to call rows.Scan() into the loop.

If any error arises when preparing the next row, the loop ends and we can get the error by calling rows.Err():

type Timeline struct {
    Id int
    Content string
}
rows, err := db.Query(`SELECT id, content FROM timeline`)
if err != nil {
    panic(err)
}
defer rows.Close()
for rows.Next() {
    timeline := Timeline{}
    err = rows.Scan(&timeline.Id, &timeline.Content)
    if err != nil {
        panic(err)
    }
    fmt.Println(timeline)
}
err = rows.Err()
if err != nil {
    panic(err)
}

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