The database/sql package

Learn the basics of the database/sql package.

The database/sql package

database/sql offers a common interface for all SQL databases; a driver does the actual talking. The driver is imported with the blank identifier for its side effect and registers itself:

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

The rest of the code is database-independent: moving from MySQL to PostgreSQL usually changes only the driver and the connection string.

sql.DB is a pool

*sql.DB is not a single connection but a connection pool. It is created once at startup and shared by all goroutines; opening and closing it per request is wrong. You tune the pool with SetMaxOpenConns, SetMaxIdleConns and SetConnMaxLifetime.

Exercise Registered drivers

sql.Drivers() returns the names of registered drivers. Without importing any driver, print len(sql.Drivers()).

Show example solution

package main

import (
    "database/sql"
    "fmt"
)

func main() {
    fmt.Println(len(sql.Drivers()))
}

Test yourself

Why is a driver package imported with <code>_</code>?
- For testing
- So that it only registers itself in init
- So it compiles faster
- To keep it secret
Show answer

So that it only registers itself in init — The driver's init function calls sql.Register; since you don't use the package name in code, the blank identifier is needed.

import (
    "database/sql"
    ____ "github.com/lib/pq"
)

Show answer

import (
    "database/sql"
    _ "github.com/lib/pq"
)

database/sql parts

database/sql

Common interface
Driver

Talks to the database
*sql.DB

Connection pool
SetMaxOpenConns

Pool limit

Next lesson: sql.Open

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