Java ORM library design:

what is specific to the SQL Dialects?

Goal 1: create the interface Dialect.java.

Goal 2: create the interface DatabaseConnection.java

Goal 3: create the interface QueryBuilderFactory.java

Goal 4: create the interface Repository.java

Goal 5: create the interface Database.java


###

```java
// Dialect.java
public interface Dialect {
    // methods to return the appropriate SQL dialect string
    public String getCreateTableQuery(String tableName);
    public String getInsertQuery(String tableName);
    public String getUpdateQuery(String tableName);
    public String getSelectQuery(String tableName);
    public String getDeleteQuery(String tableName);
}

// DatabaseConnection.java
public interface DatabaseConnection {
    public Connection getConnection();
    public void close();
}

// QueryBuilderFactory.java
public interface QueryBuilderFactory {
    public QueryBuilder getQueryBuilder(Dialect dialect);
}

// Repository.java
public interface Repository {
    public void save(Object entity);
    public Object findById(Class entityClass, int id);
    public void update(Object entity);
    public void delete(Object entity);
    public List findAll(Class entityClass);
}

// Database.java
public interface Database {
    public void setConnection(DatabaseConnection connection);
    public void setQueryBuilderFactory(QueryBuilderFactory factory);
    public void setRepository(Repository repository);
}
```

###
```java
// Interface for SQL dialects
public interface SQLDialect {
    public String getSQLStatement(String query);
    public String getSQLType(String type);
}

// Implementation of SQL dialects
public class MySqlDialect implements SQLDialect {

    @Override
    public String getSQLStatement(String query) {
        // return MySql specific SQL statement
    }

    @Override
    public String getSQLType(String type) {
        // return MySql specific type
    }
}

public class OracleDialect implements SQLDialect {

    @Override
    public String getSQLStatement(String query) {
        // return Oracle specific SQL statement
    }

    @Override
    public String getSQLType(String type) {
        // return Oracle specific type
    }
}

// ORM class
public class ORMLibrary {

    private SQLDialect dialect;

    public void setSQLDialect(SQLDialect dialect) {
        this.dialect = dialect;
    }
    
    public String executeQuery(String query) {
        String sqlStatement = dialect.getSQLStatement(query);
        // execute sqlStatement
        return result;
    }

    public String getSQLType(String type) {
        return dialect.getSQLType(type);
    }
}
```


### Java ORM library design:

what different approaches are there to make the orm algorithm agnostic to the SQL Dialect (implementation) that is currently used?

compare the different approaches and name their pros and cons.

#

1. SQL Abstraction Layer: This approach involves using an abstraction layer between the application code and the database. This layer provides a unified API that lets developers write code that works with any database, regardless of its underlying SQL dialect. The pros of this approach include a decoupling of the application code from the underlying database, allowing for easier maintenance and upgrades, the ability to switch databases without needing to recompile the application, and a unified API that simplifies development. The cons include a performance hit from the abstraction layer, as well as the need to handle any inconsistencies between the different SQL dialects.

2. Database-Specific Code Generator: This approach involves writing code that is specific to a particular database. The generator can be used to produce database-specific code for any SQL dialect. The pros of this approach include better performance since the code is tailored to the specific database, as well as the ability to easily switch between databases without having to modify the code. The cons include the need to generate code for each SQL dialect, as well as the need to maintain multiple versions of code.

3. Database-Agnostic Code: This approach involves writing code that is agnostic to the underlying database. The code is written in a way that does not rely on any particular SQL dialect, and can work with any database. The pros of this approach include better maintainability and portability, as the code can be used with any database, and it is easier to switch databases. The cons include a potential performance hit, as the code may need to be adjusted to take into account any differences between the databases, and there is no unified API for developers.

##Different Approaches to Making ORM Algorithms Agnostic to SQL Dialect

1. **Parameterized Queries**
    * **Pros**: Parameterized queries allow for the same query to be used across multiple SQL dialects, as the parameterized values are translated for each dialect. This also helps with security, as the query is not exposed to SQL injection attacks.
    * **Cons**: When using parameterized queries, it is more difficult to debug since the values are not visible in the query. Additionally, if the query is very complex or includes functions, it may not be possible to parameterize the query in a way that works with all dialects.

2. **Abstract Syntax Trees**
    * **Pros**: Abstract syntax trees provide a way to write a query in a generic way that can be translated into any dialect. This provides more flexibility and allows for the same query to be used across multiple dialects.
    * **Cons**: Writing an abstract syntax tree can be difficult, as it requires knowledge of the underlying syntax of each dialect. Additionally, there is a performance penalty associated with translating the query into a dialect-specific query.

3. **Data Access Objects**
    * **Pros**: Data access objects provide an API to query data without having to write SQL. This allows the same code to be used across multiple dialects, as the data access object will abstract the underlying SQL.
    * **Cons**: Data access objects can be difficult to debug and may not provide the same level of control over the query as SQL. Additionally, data access objects may not be able to perform certain types of complex queries.
    
    
    ###
    
Java ORM library design:

what different interfaces are needed by the SQL Abstraction Layer?

create appropriate interfaces.

# SQL Abstraction Layer Interfaces

## Database Connector Interface

This interface provides methods for connecting to a database, disconnecting, and executing queries:

```java
public interface DatabaseConnector {
  public Connection connect(String url, String userName, String password);
  public void disconnect(Connection connection);
  public ResultSet executeQuery(Connection connection, String query);
}
```

## Database Mapper Interface

This interface provides methods for mapping objects to database tables, and performing CRUD operations:

```java
public interface DatabaseMapper {
  public void mapObject(Object obj, String tableName);
  public void save(Object obj);
  public Object load(String objectClass, int id);
  public void update(Object obj);
  public void delete(Object obj);
}
```

## Query Builder Interface

This interface provides methods for building SQL queries:

```java
public interface QueryBuilder {
  public String select(String tableName, String[] columnNames, Condition conditions);
  public String insert(String tableName, String[] columnNames, Object[] values);
  public String update(String tableName, String[] columnNames, Object[] values, Condition conditions);
  public String delete(String tableName, Condition conditions);
}
```

## Condition Interface

This interface provides methods for constructing conditions for SQL queries:

```java
public interface Condition {
  public Condition and(Condition condition);
  public Condition or(Condition condition);
  public Condition not(Condition condition);
  public String toString();
}
```
## Transaction Interface

This interface provides methods for performing transactions on the database:

```java
public interface Transaction {
  public void begin();
  public void commit();
  public void rollback();
}
```
## Entity Manager Interface

This interface provides methods for managing and persisting entities:

```java
public interface EntityManager {
  public void persist(Object entity);
  public void remove(Object entity);
  public <T> T find(Class<T> entityClass, Object primaryKey);
  public Query createNamedQuery(String queryName);
  public Query createQuery(String queryString);
  public <T> TypedQuery<T> createNamedQuery(String queryName, Class<T> resultClass);
  public <T> TypedQuery<T> createQuery(String queryString, Class<T> resultClass);
}
```

## Query Interface

This interface provides methods for executing queries and retrieving results:

```java
public interface Query {
  public List<Object> getResultList();
  public Object getSingleResult();
  public Query setParameter(String name, Object value);
  public Query setParameter(int position, Object value);
}
```

## TypedQuery Interface

This interface provides methods for executing typed queries and retrieving typed results:

```java
public interface TypedQuery<T> extends Query {
  public List<T> getResultList();
  public T getSingleResult();
}
```
## Criteria Query Interface

This interface provides methods for constructing type-safe queries using the Criteria API:

```java
public interface CriteriaQuery<T> {
  public CriteriaQuery<T> select(Selection<? extends T> selection);
  public CriteriaQuery<T> from(Class<T> entityClass);
  public CriteriaQuery<T> where(Expression<Boolean> predicate);
  public List<T> getResultList();
  public T getSingleResult();
}
```

## Criteria Builder Interface

This interface provides methods for constructing type-safe CriteriaQuery objects:

```java
public interface CriteriaBuilder {
  public CriteriaQuery<T> createCriteriaQuery();
  public Selection<? extends T> select(Class<T> entityClass);
  public Expression<Boolean> and(Expression<?> x, Expression<?> y);
  public Expression<Boolean> or(Expression<?> x, Expression<?> y);
  public Expression<Boolean> not(Expression<?> x);
}
```
## Data Access Object Interface

This interface provides methods for performing data access operations on a specific type of entity:

```java
public interface DataAccessObject<T> {
  public List<T> findAll();
  public T findById(int id);
  public void save(T entity);
  public void update(T entity);
  public void delete(T entity);
}
```

###

## Repository Pattern

The Repository Pattern is an abstraction layer that enables data access and manipulation without having to directly access the underlying data store. It is typically used to provide an interface for components to access data and to ensure that the data is stored and retrieved in a consistent manner.

**Pros:**

- Encapsulates data access logic
- Simplifies data access code 
- Separates data access logic from business logic 
- Makes unit testing easier

**Cons:**

- Can be complex and difficult to maintain
- Can be difficult to scale

**Interfaces:**

- IRepository 
- IRepository<T> 
- IQueryable<T> 
- IUnitOfWork 
- IUnitOfWorkFactory 

## DAO Pattern

The Data Access Object (DAO) Pattern is a design pattern that provides an abstraction layer for accessing data from a data store. It is used to separate the data access logic from the business logic.

**Pros:**

- Encapsulates data access logic
- Simplifies data access code
- Makes unit testing easier
- Easy to scale

**Cons:**

- Can be complex and difficult to maintain

**Interfaces:**

- IDataAccessObject 
- IDataAccessObject<T> 
- IQuery 
- ITransaction 
- ITransactionFactory

#

## Design Patterns Applicable to the Abstraction Layer for Accessing Data from a Data Store

### Data Access Object (DAO)

**Pros:**

- Provides an easy to use abstraction layer so that the underlying data source is not tightly coupled with the application code.
- Allows for a uniform interface to access data from a variety of data sources.
- Can be used to easily modify the underlying data source without changing the application code.

**Cons:**

- Can be difficult to maintain as the application grows and the number of data sources increase.
- Performance can suffer as the number of data sources increases due to the cost of establishing database connections.

**Interfaces:**

- `DataAccessObject` : Interface for accessing data from a data store.
- `DataSource` : Interface for providing the data source to be used by the DAO.
- `DataObject` : Interface for accessing and manipulating data objects in the data store.

### Repository Pattern

**Pros:**

- Provides a uniform interface for accessing and manipulating data objects in the data store.
- Allows for an easier way to modify the underlying data source without changing the application code.
- Allows for easier unit testing of the data access logic.

**Cons:**

- Can be difficult to maintain as the application grows and the number of data sources increase.
- Performance can suffer as the number of data sources increases due to the cost of establishing database connections.

**Interfaces:**

- `Repository` : Interface for accessing and manipulating data objects in the data store.
- `DataSource` : Interface for providing the data source to be used by the Repository.
- `DataObject` : Interface for accessing and manipulating data objects in the data store.

### Active Record

**Pros:**

- Provides an easy to use abstraction layer that allows for easy modification of the underlying data source.
- Allows for easier unit testing of the data access logic.
- Performance can be improved due to the ability to cache data objects in memory.

**Cons:**

- Can be difficult to maintain as the application grows and the number of data sources increase.
- Performance can suffer as the number of data sources increases due to the cost of establishing database connections.

**Interfaces:**

- `ActiveRecord` : Interface for accessing and manipulating data objects in the data store.
- `DataSource` : Interface for providing the data source to be used by the ActiveRecord.
- `DataObject` : Interface for accessing and manipulating data objects in the data store.

#

# Design Patterns for ORM Library

## Data Access Object (DAO)

The Data Access Object (DAO) pattern is a structural design pattern that provides an abstract interface to a data source. It provides an abstraction layer between the data access logic and the data source. This pattern helps to centralize the access logic, allowing for better code organization and maintenance.

Pros:

- Centralizes data access logic into one place, making code more maintainable
- Easily switch between different data sources by changing the DAO implementation
- Allows for more control over data access logic

Cons:

- Can become overly complex when dealing with large data sets
- High coupling between the DAO and the data source

**Interfaces:**

```java
public interface DAO {
    List<Object> getAllObjects();
    Object getObject(int id);
    void deleteObject(int id);
    void updateObject(Object object);
    void insertObject(Object object);
}
```

## Active Record

The Active Record pattern is an object-oriented design pattern that allows for easy CRUD operations on a database. This pattern simplifies the codebase by having objects in the application directly represent data in the database.

Pros:

- Simplifies codebase by having objects directly represent data
- Easy to implement and use
- Good for small to medium-sized databases

Cons:

- Can be difficult to maintain when dealing with large databases
- Heavily couples the application with the database

**Interfaces:**

```java
public interface ActiveRecord {
    void save();
    void delete();
    void update();
}
```

## Repository

The Repository pattern is a structural design pattern that provides an abstraction layer between the application and the database. This pattern allows for better code organization and maintenance by abstracting away the data access logic.

Pros:

- Centralizes data access logic into one place, making code more maintainable
- Easily switch between different data sources by changing the repository implementation
- Allows for more control over data access logic

Cons:

- Can become overly complex when dealing with large data sets
- High coupling between the repository and the data source

**Interfaces:**

```java
public interface Repository {
    List<Object> getAllObjects();
    Object getObject(int id);
    void deleteObject(int id);
    void updateObject(Object object);
    void insertObject(Object object);
}
```

#

```java
public interface Repository {
    
    // CRUD operations
    public void save(Object entity);
    public Object get(Class clazz, Object id);
    public List<Object> list(Class clazz);
    public void update(Object entity);
    public void delete(Object entity);

    // SQL operations
    public void executeSQLQuery(String sqlQuery);
    public void executeSQLUpdate(String sqlUpdate);
    public List<Object> executeSQLQueryList(String sqlQuery);
    
    // Dialect operations
    public String getCurrentSQLDialect();
    public void setCurrentSQLDialect(String dialect);

}
```