Go Wiki: SQLInterface

Introduction

The database/sql package provides a generic interface around SQL (or SQL-like) databases. See the official documentation for details.

This page provides example usage patterns.

Database driver

The database/sql package must be used in conjunction with a database driver. See https://go.dev/s/sqldrivers for a list of drivers.

The documentation below assumes a driver has been imported.

Connecting to a database

Open is used to create a database handle:

db, err := sql.Open(driver, dataSourceName)

Where driver specifies a database driver and dataSourceName specifies database-specific connection information such as database name and authentication credentials.

Note that Open does not directly open a database connection: this is deferred until a query is made. To verify that a connection can be made before making a query, use the PingContext method:

if err := db.PingContext(ctx); err != nil {
  log.Fatal(err)
}

After use, the database is closed using Close.

Executing queries

ExecContext is used for queries where no rows are returned:

result, err := db.ExecContext(ctx,
    "INSERT INTO users (name, age) VALUES ($1, $2)",
    "gopher",
    27,
)

Where result contains the last insert ID and number of rows affected. The availability of these values is dependent on the database driver.

QueryContext is used for retrieval:

rows, err := db.QueryContext(ctx, "SELECT name FROM users WHERE age = $1", age)
if err != nil {
    log.Fatal(err)
}
defer rows.Close()
for rows.Next() {
    var name string
    if err := rows.Scan(&name); err != nil {
        log.Fatal(err)
    }
    fmt.Printf("%s is %d\n", name, age)
}
if err := rows.Err(); err != nil {
    log.Fatal(err)
}

QueryRowContext is used where only a single row is expected:

var age int64
err := db.QueryRowContext(ctx, "SELECT age FROM users WHERE name = $1", name).Scan(&age)

Prepared statements can be created with PrepareContext:

age := 27
stmt, err := db.PrepareContext(ctx, "SELECT name FROM users WHERE age = $1")
if err != nil {
    log.Fatal(err)
}
rows, err := stmt.Query(age)
// process rows

ExecContext, QueryContext and QueryRowContext can be called on statements. After use, a statement should be closed with Close.

Transactions

Transactions are started with BeginTx:

tx, err := db.BeginTx(ctx, nil)
if err != nil {
    log.Fatal(err)
}

The ExecContext, QueryContext, QueryRowContext and PrepareContext methods already covered can be used in a transaction.

A transaction must end with a call to Commit or Rollback.

Dealing with NULL

If a database column is nullable, one of the types supporting null values should be passed to Scan.

For example, if the name column in the names table is nullable:

var name sql.NullString
err := db.QueryRowContext(ctx, "SELECT name FROM names WHERE id = $1", id).Scan(&name)
...
if name.Valid {
    // use name.String
} else {
    // value is NULL
}

Only NullByte, NullBool, NullFloat64, NullInt64, NullInt32 NullInt16, NullString and NullTime are implemented in database/sql. Implementations of database-specific null types are left to the database driver. User types supporting NULL can be created by implementing interfaces database/sql/driver.Valuer and database/sql.Scanner.

You can also pass pointer types. Be careful for performance issues as it requires extra memory allocations.

var name *string
err := db.QueryRowContext(ctx, "SELECT name FROM names WHERE id = $1", id).Scan(&name)

Getting a table

If you want an struct array from your SQL query.

func getTable[T any](rows *sql.Rows) (out []T) {
    var table []T
    for rows.Next() {
        var data T
        s := reflect.ValueOf(&data).Elem()
        numCols := s.NumField()
        columns := make([]interface{}, numCols)

        for i := 0; i < numCols; i++ {
            field := s.Field(i)
            columns[i] = field.Addr().Interface()
        }

        if err := rows.Scan(columns...); err != nil {
            fmt.Println("Case Read Error ", err)
        }

        table = append(table, data)
    }
    return table
}

Make sure to deal with nulls from the database.

type User struct {
  UUID  sql.NullString
  Name  sql.NullString
}

rows, err := db.Query("SELECT * FROM Users")
cases := getTable[User](rows)

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