# 5.4 PostgreSQL

PostgreSQL is an object-relational database management system available for many platforms including Linux, FreeBSD, Solaris, Microsoft Windows and Mac OS X. It is released under an MIT-style license, and is thus free and open source software. It's larger than MySQL because it's designed for enterprise usage as an alternative to Oracle. Postgresql is a good choice for enterprise type projects.

## PostgreSQL drivers

There are many database drivers available for PostgreSQL. Here are three examples of them:

- [https://github.com/lib/pq](https://github.com/lib/pq) supports `database/sql`, written in pure Go.
- [https://github.com/jbarham/gopgsqldriver](https://github.com/jbarham/gopgsqldriver) supports `database/sql`, written in pure Go.
- [https://github.com/lxn/go-pgsql](https://github.com/lxn/go-pgsql) supports `database/sql`, written in pure Go.

I will use the first one in the examples that follow.

## Samples

We create the following SQL:
```sql
	CREATE TABLE userinfo
	(
	    uid serial NOT NULL,
	    username character varying(100) NOT NULL,
	    department character varying(500) NOT NULL,
	    Created date,
	    CONSTRAINT userinfo_pkey PRIMARY KEY (uid)
	)
	WITH (OIDS=FALSE);
```
An example:
```Go
	package main

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

	const (
		DB_USER     = "postgres"
		DB_PASSWORD = "postgres"
		DB_NAME     = "test"
	)

	func main() {
		dbinfo := fmt.Sprintf("user=%s password=%s dbname=%s sslmode=disable",
			DB_USER, DB_PASSWORD, DB_NAME)
		db, err := sql.Open("postgres", dbinfo)
		checkErr(err)
		defer db.Close()

		fmt.Println("# Inserting values")
	
		var lastInsertId int
		err = db.QueryRow("INSERT INTO userinfo(username,department,created) VALUES($1,$2,$3) returning uid;", "astaxie", "研发部门", "2012-12-09").Scan(&lastInsertId)
		checkErr(err)
		fmt.Println("last inserted id =", lastInsertId)

		fmt.Println("# Updating")
		stmt, err := db.Prepare("update userinfo set username=$1 where uid=$2")
		checkErr(err)

		res, err := stmt.Exec("astaxieupdate", lastInsertId)
		checkErr(err)

		affect, err := res.RowsAffected()
		checkErr(err)

		fmt.Println(affect, "rows changed")

		fmt.Println("# Querying")
		rows, err := db.Query("SELECT * FROM userinfo")
		checkErr(err)

		for rows.Next() {
			var uid int
			var username string
			var department string
			var created time.Time
			err = rows.Scan(&uid, &username, &department, &created)
			checkErr(err)
			fmt.Println("uid | username | department | created ")
			fmt.Printf("%3v | %8v | %6v | %6v\n", uid, username, department, created)
		}

		fmt.Println("# Deleting")
		stmt, err = db.Prepare("delete from userinfo where uid=$1")
		checkErr(err)

		res, err = stmt.Exec(lastInsertId)
		checkErr(err)

		affect, err = res.RowsAffected()
		checkErr(err)

		fmt.Println(affect, "rows changed")
	}

	func checkErr(err error) {
		if err != nil {
			panic(err)
		}
	}
```
Note that PostgreSQL uses the `$1, $2` format instead of the `?` that MySQL uses, and it has a different DSN format in `sql.Open`. 
Another thing is that the PostgreSQL driver does not support `sql.Result.LastInsertId()`.
So instead of this,
```Go
	stmt, err := db.Prepare("INSERT INTO userinfo(username,departname,created) VALUES($1,$2,$3);")
	res, err := stmt.Exec("astaxie", "研发部门", "2012-12-09")
	fmt.Println(res.LastInsertId())
```
use `db.QueryRow()` and `.Scan()` to get the value for the last inserted id.
```Go
	err = db.QueryRow("INSERT INTO TABLE_NAME values($1) returning uid;",	VALUE1").Scan(&lastInsertId)
	fmt.Println(lastInsertId)
```
## Links

- [Directory](preface.md)
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