Features | Usage | Design | License
- Supports standard SQL commands
- Persistent data storage
- Error handling
The server and client can be launched via the DBServer and DBClient classes respectively. The database server listens on port 8888 in order to receive incoming messages. These incoming commands are then passed to the handleCommand method for processing.
The client will communicate with the server using a simplified version of the SQL database query language. A brief overview of the query language is provided below:
| Class | Description |
|---|---|
| USE | changes the database against which the following queries will be run |
| CREATE | constructs a new database or table (depending on the provided parameters) |
| INSERT | adds a new record (row) to an existing table |
| SELECT | searches for records that match the given condition |
| UPDATE | changes the existing data contained within a table |
| ALTER | changes the structure (columns) of an existing table |
| DELETE | removes records that match the given condition from an existing table |
| DROP | removes a specified table from a database, or removes the entire database |
| JOIN | performs an inner join on two tables (returning all permutations of all matching records) |
The following section defines the BNF grammar that the queries would need to conform to.
The following BNF grammar can be used to construct commands to query the server.
<Command> ::= <CommandType>;
<CommandType> ::= <Use> | <Create> | <Drop> | <Alter> | <Insert> |
<Select> | <Update> | <Delete> | <Join>
<Use> ::= USE <DatabaseName>
<Create> ::= <CreateDatabase> | <CreateTable>
<CreateDatabase> ::= CREATE DATABASE <DatabaseName>
<CreateTable> ::= CREATE TABLE <TableName> | CREATE TABLE <TableName> (<AttributeList>)
<Drop> ::= DROP <Structure> <StructureName>
<Structure> ::= DATABASE | TABLE
<Alter> ::= ALTER TABLE <TableName> <AlterationType> <AttributeName>
<Insert> ::= INSERT INTO <TableName> VALUES (<ValueList>)
<Select> ::= SELECT <WildAttribList> FROM <TableName> |
SELECT <WildAttribList> FROM <TableName> WHERE <Condition>
<Update> ::= UPDATE <TableName> SET <NameValueList> WHERE <Condition>
<Delete> ::= DELETE FROM <TableName> WHERE <Condition>
<Join> ::= JOIN <TableName> AND <TableName> ON <AttributeName> AND <AttributeName>
<NameValueList> ::= <NameValuePair> | <NameValuePair>,<NameValueList>
<NameValuePair> ::= <AttributeName>=<Value>
<AlterationType> ::= ADD | DROP
<ValueList> ::= <Value> | <Value>,<ValueList>
<Value> ::= '<StringLiteral>' | <BooleanLiteral> | <FloatLiteral> | <IntegerLiteral>
<BooleanLiteral> ::= true | false
<WildAttribList> ::= <AttributeList> | *
<AttributeList> ::= <AttributeName> | <AttributeName>,<AttributeList>
<Condition> ::= (<Condition>) AND (<Condition>) |
(<Condition>) OR (<Condition>) |
<AttributeName><Operator><Value>
<Operator> ::= == | > | < | >= | <= | != | LIKE
A valid SELECT query assuming a people table exists in the database would be
SELECT * FROM people WHERE Name=='Steve';
and similarly,
SELECT * FROM people WHERE Name == 'Steve' ;
is valid and acceptable, being equivalent to the first query.
The application is split into three core modules that are needed to ensure that the server is running - DBCommands, DBEngine and DBExceptions.
The DBCommands module contains classes that implement each of the supported SQL commands.
| Class | Description |
|---|---|
| DBQuery | The parent class |
| CMDx | Child classes of DBQuery where 'x' corresponds to each supported SQL command. |
| Condition | A class that tokenizes a user-input condition and outputs a parameterized string that enables the DBEngine module to evaluate the condition. |
The DBEngine module contains the core classes that deal with file I/O and command parsing.
| Class | Description |
|---|---|
| DBCommandHandler | Performs preliminary checks on the SQL command entered by the user on the command line. |
| DBTokenizer | Tokenizes the user's input string to check if the SQL command is valid. |
| DBFileIO | Deals with writing to and from the storage media for the purposes of persistent storage. |
| DBStorage | Ensures the file system is robust and error free during read / write operations. |
| DBTable | Represents a table in a relational database management system. |
| DBExpressionCalculator | Evaluates criteria within an SQL command. |
The DBExceptions module contains exception classes and these exceptions are thrown when the RDBMS reaches an error state.
| Exception Class | Description |
|---|---|
| DBException | Parent class of any exception thrown by the server. |
| DBParseException | Error in the SQL query. |
| DBExecutionException | Invalid criteria within a valid SQL query. |
| DBStorageExecution | Error in the persistent file system. |
Copyright (c) 2021 Keane Fernandes
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