First attempt at splitting the Go code into seperate files to clean things up.

category.go

@@ -0,0 +1,40 @@
+package main
+
+type Category struct {
+	Id   int32  `json:"id"`
+	Name string `json:"name"`
+}
+
+type Subcategory struct {
+	Id         int32  `json:"id"`
+	Name       string `json:"name"`
+	CategoryId int32  `json:"categoryId"`
+}
+
+func getAllCategories() []Category {
+	db := getSQLConnection()
+
+	defer db.Close()
+
+	rows, err := db.Query("SELECT * FROM category;")
+
+	if err != nil {
+		panic(err)
+	}
+
+	defer rows.Close()
+
+	var categories []Category
+
+	for rows.Next() {
+		var category Category
+
+		if err = rows.Scan(&category.Id, &category.Name); err != nil {
+			println(err.Error())
+		}
+
+		categories = append(categories, category)
+	}
+
+	return categories
+}

database.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"database/sql"
+	"fmt"
+
+	_ "github.com/lib/pq"
+)
+
+func getSQLConnection() *sql.DB {
+	//Note: The postgres driver seems to get confused when no password is supplied, so omit it in the connection sting.
+	//https://rajyavardhan.medium.com/when-you-get-relation-does-not-exist-in-postgres-7ffb0c3c674b
+	psqlInfo := fmt.Sprintf("host=%s port=%d user=%s dbname=%s sslmode=disable", "localhost", 5432, "postgres", "inventory")
+	db, err := sql.Open("postgres", psqlInfo)
+
+	if err != nil {
+		panic(err)
+	}
+
+	return db
+}

main.go

@@ -8,8 +8,6 @@ import (
 	"net/http"
 	"strconv"
 	"time"
-
-	_ "github.com/lib/pq"
 )
 
 //Go Best practices: https://github.com/golovers/effective-go
@@ -27,17 +25,6 @@ type Item struct {
 	Subcategory       Subcategory `json:"subcategory"`
 }
 
-type Category struct {
-	Id   int32  `json:"id"`
-	Name string `json:"name"`
-}
-
-type Subcategory struct {
-	Id         int32  `json:"id"`
-	Name       string `json:"name"`
-	CategoryId int32  `json:"categoryId"`
-}
-
 func main() {
 	http.HandleFunc("/categories", categories)
 	http.HandleFunc("/category", category)
@@ -84,12 +71,7 @@ func getCategory(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	psqlInfo := fmt.Sprintf("host=%s port=%d user=%s dbname=%s sslmode=disable", "localhost", 5432, "postgres", "inv")
-	db, err := sql.Open("postgres", psqlInfo)
-
-	if err != nil {
-		panic(err)
-	}
+	db := getSQLConnection()
 
 	defer db.Close()
 
@@ -128,12 +110,7 @@ func deleteCategory(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	psqlInfo := fmt.Sprintf("host=%s port=%d user=%s dbname=%s sslmode=disable", "localhost", 5432, "postgres", "inv")
-	db, err := sql.Open("postgres", psqlInfo)
-
-	if err != nil {
-		panic(err)
-	}
+	db := getSQLConnection()
 
 	defer db.Close()
 
@@ -150,36 +127,7 @@ func deleteCategory(w http.ResponseWriter, r *http.Request) {
 }
 
 func getCategories(w http.ResponseWriter, r *http.Request) {
-	//Note: The postgres driver seems to get confused when no password is supplied, so omit it in the connection sting.
-	//https://rajyavardhan.medium.com/when-you-get-relation-does-not-exist-in-postgres-7ffb0c3c674b
-	psqlInfo := fmt.Sprintf("host=%s port=%d user=%s dbname=%s sslmode=disable", "localhost", 5432, "postgres", "inv")
-	db, err := sql.Open("postgres", psqlInfo)
-
-	if err != nil {
-		panic(err)
-	}
-
-	defer db.Close()
-
-	rows, err := db.Query("SELECT * FROM category;")
-
-	if err != nil {
-		panic(err)
-	}
-
-	defer rows.Close()
-
-	var categories []Category
-
-	for rows.Next() {
-		var category Category
-
-		if err = rows.Scan(&category.Id, &category.Name); err != nil {
-			println(err.Error())
-		}
-
-		categories = append(categories, category)
-	}
+	categories := getAllCategories()
 
 	jsonBytes, err := json.Marshal(categories)
 
@@ -222,14 +170,9 @@ func insertCategory(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	psqlInfo := fmt.Sprintf("host=%s port=%d user=%s dbname=%s sslmode=disable", "localhost", 5432, "postgres", "inv")
-	db, err := sql.Open("postgres", psqlInfo)
+	db := getSQLConnection()
 
-	if err != nil {
-		w.WriteHeader(http.StatusInternalServerError)
-		w.Write([]byte(err.Error()))
-		return
-	}
+	defer db.Close()
 
 	id := 0