diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4d9f0a1..c53101c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -10,13 +10,14 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 if(MSVC)
   add_compile_options(/W4 /WX)
 else()
-  add_compile_options(-W -Wall -Werror)
+  add_compile_options(-Wall -Wpedantic)
 endif()
 
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 
+include(CTest)
 enable_testing()
 
 add_subdirectory(src)
diff --git a/src/core/article.cpp b/src/core/article.cpp
index 4ba0b33..3270da7 100644
--- a/src/core/article.cpp
+++ b/src/core/article.cpp
@@ -1,24 +1,17 @@
 #include "article.h"
 
-Article::Article() : Entity()
-{}
+Article::Article() : Entity() {}
 
-Article::Article(const std::shared_ptr<Seller> sellerPtr) : Entity()
-{
-    this->sellerPtr = sellerPtr;
-}
+Article::Article(std::shared_ptr<Seller> sellerPtr) : Entity() { sellerPtr_ = sellerPtr; }
 
-void Article::setArticleNo(int articleNo)
-{
-    this->articleNo = articleNo;
-}
+void Article::setArticleNo(int articleNo) { articleNo_ = articleNo; }
 
-void Article::setPrice(int price)
-{
-    this->price = price;
-}
+void Article::setPrice(int price) { price_ = price; }
 
-void Article::setDescription(const std::string& description)
-{
-    this->description = description;
-}
\ No newline at end of file
+void Article::setDescription(const std::string& description) { description_ = description; }
+
+std::string Article::getDescription() { return description_; }
+
+void Article::setSale(std::shared_ptr<Sale> salePtr) { salePtr_ = salePtr; }
+
+bool Article::isSold() { return salePtr_ ? true : false; }
\ No newline at end of file
diff --git a/src/core/article.h b/src/core/article.h
index cc25ffa..1d0487b 100644
--- a/src/core/article.h
+++ b/src/core/article.h
@@ -2,26 +2,33 @@
 #define ARTICLE_H
 
 #include "entity.h"
+#include "sale.h"
 #include "seller.h"
 
-#include <string>
 #include <memory>
+#include <string>
 
 class Seller;
 
 class Article : public Entity
 {
-public:
+  public:
     Article();
     Article(std::shared_ptr<Seller> sellerPtr);
     void setArticleNo(int articleNo);
     void setPrice(int price);
     void setDescription(const std::string& description);
-private:
-    std::shared_ptr<Seller> sellerPtr{};
-    int articleNo{};
-    int price{};
-    std::string description{};
+    std::string getDescription();
+    bool isSold();
+    void setSale(std::shared_ptr<Sale> salePtr);
+    void setSeller(std::shared_ptr<Seller> sellerPtr);
+
+  private:
+    std::shared_ptr<Seller> sellerPtr_{};
+    std::shared_ptr<Sale> salePtr_{};
+    int articleNo_{};
+    int price_{};
+    std::string description_{};
 };
 
 #endif
\ No newline at end of file
diff --git a/src/core/database.cpp b/src/core/database.cpp
index 42603ad..fc1a1c8 100644
--- a/src/core/database.cpp
+++ b/src/core/database.cpp
@@ -2,28 +2,40 @@
 
 #include <iostream>
 #include <stdexcept>
+#include <vector>
 
 Database::Database(const std::string& dbname) : db_(nullptr)
 {
-    const int errCode = sqlite3_open(dbname.c_str(), &db_);
+    dbname_ = dbname;
+    const int errCode = sqlite3_open(dbname_.c_str(), &db_);
     if (errCode) {
         throw std::runtime_error("Could not open database file.");
     }
-    exec("PRAGMA foreign_key = 1");
+    sqlite3_db_config(db_, SQLITE_DBCONFIG_ENABLE_FKEY);
 }
 
 Database::~Database() { sqlite3_close(db_); }
 
 void Database::exec(const std::string& sql)
 {
-    const int errCode = sqlite3_exec(db_, sql.c_str(), nullptr, nullptr, nullptr);
+    char* errMsg;
+    const int errCode = sqlite3_exec(db_, sql.c_str(), nullptr, nullptr, &errMsg);
     if (errCode) {
-        throw std::runtime_error("Error in SQL execution.");
+        std::string errMsgString(errMsg); // Make a C++ string of the errMsg, so that we can call
+                                          // sqlite3_free() before throwing the exception
+        sqlite3_free(errMsg);
+        throw std::runtime_error("Error in SQL execution: " + errMsgString);
     }
 }
 
-void Database::init()
+void Database::createNew()
 {
+    std::vector<std::string> sqlStrings{};
+
+    std::string sqlCreateKima2{"CREATE TABLE IF NOT EXISTS kima2 ("
+                               "version INTEGER NOT NULL);"
+                               "INSERT INTO kima2 (version) VALUES (1);"};
+    sqlStrings.push_back(sqlCreateKima2);
     std::string sqlCreateSellers{"CREATE TABLE IF NOT EXISTS sellers ("
                                  "id TEXT PRIMARY KEY NOT NULL, "
                                  "seller_no INTEGER, "
@@ -32,7 +44,7 @@ void Database::init()
                                  "offered_articles INTEGER, "
                                  "UNIQUE (seller_no)"
                                  ");"};
-
+    sqlStrings.push_back(sqlCreateSellers);
     std::string sqlCreateArticles{
         "CREATE TABLE IF NOT EXISTS articles ("
         "id TEXT PRIMARY KEY NOT NULL, "
@@ -45,13 +57,90 @@ void Database::init()
         "FOREIGN KEY (seller_id) REFERENCES sellers(id) ON DELETE CASCADE, "
         "CHECK (article_no BETWEEN 0 AND 99999)"
         ");"};
+    sqlStrings.push_back(sqlCreateArticles);
+    std::string sqlCreateSales{"CREATE TABLE IF NOT EXISTS sales ("
+                               " id TEXT PRIMARY KEY NOT NULL,"
+                               " source_no INTEGER NOT NULL,"
+                               " sold_at TEXT"
+                               ");"};
+    sqlStrings.push_back(sqlCreateSales);
+    std::string sqlCreateSalesItems{
+        "CREATE TABLE IF NOT EXISTS sales_items("
+        " sale_id TEXT NOT NULL,"
+        " article_id TEXT NOT NULL,"
+        " FOREIGN KEY (sale_id) REFERENCES sales(id) ON DELETE CASCADE,"
+        " FOREIGN KEY (article_id) REFERENCES articles(id) ON DELETE CASCADE"
+        ");"};
+    sqlStrings.push_back(sqlCreateSalesItems);
 
     beginTransaction();
-    exec(sqlCreateSellers);
-    exec(sqlCreateArticles);
+    for (const auto& sql : sqlStrings) {
+        exec(sql);
+    }
     endTransaction();
 }
 
+void Database::init()
+{
+    int version = getVersion();
+
+    switch (version) {
+    case 0:
+        createNew();
+        break;
+        // perhaps handle upgrades for db schema here...
+    default:
+        // Do nothing because we are up-to-date.
+        break;
+    }
+}
+
+int Database::getVersion()
+{
+    int retCode{};
+    sqlite3_stmt* stmt;
+
+    // Check if there's already a kima2 table available.
+    // If not, return version == 0.
+    retCode = sqlite3_prepare_v2(
+        db_, "SELECT count(*) FROM sqlite_master WHERE type='table' AND name='kima2';", -1, &stmt,
+        nullptr);
+    if (retCode != SQLITE_OK)
+        throw std::string(sqlite3_errmsg(db_));
+    retCode = sqlite3_step(stmt);
+    if (retCode != SQLITE_ROW && retCode != SQLITE_DONE) {
+        std::string errMsg(sqlite3_errmsg(db_));
+        sqlite3_finalize(stmt);
+        throw errMsg;
+    } else if (retCode != SQLITE_DONE) {
+        int count = sqlite3_column_int(stmt, 0);
+        sqlite3_finalize(stmt);
+        if (count == 0)
+            return 0; // no kima2 table, so version is 0
+    }
+
+    // Now that we know that the kima2 table is present, read and return the schema version.
+    retCode = sqlite3_prepare_v2(db_, "SELECT version FROM kima2", -1, &stmt, nullptr);
+    if (retCode != SQLITE_OK)
+        throw std::string(sqlite3_errmsg(db_));
+
+    retCode = sqlite3_step(stmt);
+
+    if (retCode != SQLITE_ROW && retCode != SQLITE_DONE) {
+        std::string errMsg(sqlite3_errmsg(db_));
+        sqlite3_finalize(stmt);
+        throw errMsg;
+    } else if (retCode == SQLITE_DONE) {
+        sqlite3_finalize(stmt);
+        return 0; // no version entry, so version is 0
+    }
+
+    int version = sqlite3_column_int(stmt, 0);
+    sqlite3_finalize(stmt);
+
+    return version;
+}
+
 void Database::beginTransaction() { exec("BEGIN TRANSACTION"); }
 
 void Database::endTransaction() { exec("END TRANSACTION"); }
\ No newline at end of file
diff --git a/src/core/database.h b/src/core/database.h
index c0b4de4..ff6f3d8 100644
--- a/src/core/database.h
+++ b/src/core/database.h
@@ -16,8 +16,11 @@ public:
     void init();
 private:
     sqlite3 *db_;
+    std::string dbname_;
     void beginTransaction();
     void endTransaction();
+    void createNew();
+    int getVersion();
 };
 
 #endif // DATABASE_H
\ No newline at end of file
diff --git a/src/core/sale.h b/src/core/sale.h
new file mode 100644
index 0000000..06f350d
--- /dev/null
+++ b/src/core/sale.h
@@ -0,0 +1,8 @@
+#ifndef SALE_H
+#define SALE_H
+
+class Sale : public Entity {
+
+};
+
+#endif
\ No newline at end of file
diff --git a/src/core/seller.cpp b/src/core/seller.cpp
index e4de084..634e585 100644
--- a/src/core/seller.cpp
+++ b/src/core/seller.cpp
@@ -1,26 +1,40 @@
 #include "seller.h"
 
-void Seller::setSellerNo(int seller_no)
-{
-    sellerNo_ = seller_no;
-}
+Seller::Seller() : Entity() {}
 
-void Seller::setFirstName(const std::string& firstName)
+Seller::Seller(const std::string& firstName, const std::string& lastName, int sellerNo,
+               int numberOfArticles)
+    : Entity()
 {
     firstName_ = firstName;
-}
-
-void Seller::setLastName(const std::string& lastName)
-{
     lastName_ = lastName;
+    sellerNo_ = sellerNo;
+    numberOfOfferedArticles_ = numberOfArticles;
 }
 
-void Seller::setNumberOfOfferedArticles(int number)
-{
-    numberOfOfferedArticles_ = number;
+inline void Seller::setSellerNo(int seller_no) { sellerNo_ = seller_no; }
+
+inline void Seller::setFirstName(const std::string& firstName) { firstName_ = firstName; }
+
+inline void Seller::setLastName(const std::string& lastName) { lastName_ = lastName; }
+
+inline void Seller::setNumberOfOfferedArticles(int number) { numberOfOfferedArticles_ = number; }
+
+inline size_t Seller::getNumberOfOfferedArticles() { return articles_.size(); }
+
+void Seller::addArticle(Article article) {
+    articles_.push_back(article);
 }
 
-size_t Seller::getNumberOfOfferedArticles()
+std::vector<Article*> Seller::getArticles(bool onlySold)
 {
-    return articles_.size();
+    std::vector<Article*> articles;
+    for (auto& article : articles_) {
+        if (onlySold && article.isSold()) {
+            articles.push_back(&article);
+        } else if (!onlySold) {
+            articles.push_back(&article);
+        }
+    }
+    return articles;
 }
\ No newline at end of file
diff --git a/src/core/seller.h b/src/core/seller.h
index 2848579..4ac6a98 100644
--- a/src/core/seller.h
+++ b/src/core/seller.h
@@ -1,8 +1,8 @@
 #ifndef SELLER_H
 #define SELLER_H
 
-#include "entity.h"
 #include "article.h"
+#include "entity.h"
 
 #include <string>
 #include <vector>
@@ -12,11 +12,17 @@ class Article;
 class Seller : public Entity
 {
   public:
+    Seller();
+    Seller(const std::string& firstName, const std::string& lastName, int sellerNo = 0,
+           int numberOfArticles = 0);
+
     void setSellerNo(int sellerNo);
     void setFirstName(const std::string& firstName);
     void setLastName(const std::string& lastName);
     void setNumberOfOfferedArticles(int number);
+    void addArticle(Article article);
 
+    std::vector<Article*> getArticles(bool onlySold = false);
     size_t getNumberOfOfferedArticles();
 
   private:
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 2c68a6d..382580f 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -4,6 +4,10 @@ add_executable(test_seller test_seller.cpp)
 target_link_libraries(test_seller core Boost::unit_test_framework)
 add_test(NAME Seller COMMAND ${CMAKE_BINARY_DIR}/bin/test_seller)
 
+add_executable(test_article test_article.cpp)
+target_link_libraries(test_article core Boost::unit_test_framework)
+add_test(NAME Article COMMAND ${CMAKE_BINARY_DIR}/bin/test_article)
+
 add_executable(test_database test_database.cpp)
 target_link_libraries(test_database core Boost::unit_test_framework stdc++fs)
 add_test(NAME Database COMMAND ${CMAKE_BINARY_DIR}/bin/test_database)
diff --git a/test/test_article.cpp b/test/test_article.cpp
new file mode 100644
index 0000000..e0792ad
--- /dev/null
+++ b/test/test_article.cpp
@@ -0,0 +1,21 @@
+#define BOOST_TEST_MODULE article
+
+#include "../src/core/article.h"
+
+#include <boost/test/included/unit_test.hpp>
+
+BOOST_AUTO_TEST_CASE(create_article)
+{
+    Article article{};
+    BOOST_TEST(article.getUuid().is_nil() == true);
+}
+
+BOOST_AUTO_TEST_CASE(check_is_sold)
+{
+    Article article{};
+    BOOST_TEST(article.isSold() == false);
+
+    auto salePtr = std::make_shared<Sale>();
+    article.setSale(salePtr);
+    BOOST_TEST(article.isSold() == true);
+}
\ No newline at end of file
diff --git a/test/test_database.cpp b/test/test_database.cpp
index 28d47dc..de5c004 100644
--- a/test/test_database.cpp
+++ b/test/test_database.cpp
@@ -8,18 +8,8 @@
 
 BOOST_AUTO_TEST_CASE(create_database)
 {
-    std::filesystem::path testPath = std::filesystem::temp_directory_path() / "kima2test.db";
-    if (std::filesystem::exists(testPath)) {
-        std::filesystem::remove(testPath);
-    }
 
-    std::string filename = testPath.generic_string();
+    Database db(":memory:");
+    BOOST_CHECK_NO_THROW(db.init());
 
-    Database db(filename);
-    db.init();
-
-    BOOST_TEST(std::filesystem::exists(testPath) == true);
-    if (std::filesystem::exists(testPath)) {
-        std::filesystem::remove(testPath);
-    }
 }
\ No newline at end of file
diff --git a/test/test_seller.cpp b/test/test_seller.cpp
index d193340..5de2ed2 100644
--- a/test/test_seller.cpp
+++ b/test/test_seller.cpp
@@ -2,17 +2,37 @@
 
 #include "../src/core/seller.h"
 
+#include <array>
+
 #include <boost/test/included/unit_test.hpp>
 
-//using namespace boost::unit_test;
-
-BOOST_AUTO_TEST_CASE( create_uuid_nil ) {
+BOOST_AUTO_TEST_CASE(create_uuid_nil)
+{
     Seller seller{};
     BOOST_TEST(seller.getUuid().is_nil() == true);
 }
 
-BOOST_AUTO_TEST_CASE(create_uuid) {
+BOOST_AUTO_TEST_CASE(create_uuid)
+{
     Seller seller{};
     seller.createUuid();
     BOOST_TEST(seller.getUuid().is_nil() == false);
+}
+
+BOOST_AUTO_TEST_CASE(create_many)
+{
+    constexpr unsigned int QUANTITY{10000};
+    std::array<Seller, QUANTITY> sellers;
+    for (unsigned i = 0; i < sellers.size(); i++) {
+        sellers[i] = Seller();
+        sellers[i].createUuid();
+    }
+}
+
+BOOST_AUTO_TEST_CASE(with_article) {
+    Seller seller("Max", "Mustermann");
+    Article article{};
+    article.setDescription("Test article");
+    seller.addArticle(article);
+    BOOST_TEST(seller.getArticles().at(0)->getDescription() == "Test article");
 }
\ No newline at end of file