University of Edinburgh School of Informatics
INFR11199 – Advanced Database Systems (Spring 2021)
Coursework Assignment
Due: Thursday, 18 March 2021 at 4:00pm
IMPORTANT:
• Plagiarism: Every student has to work individually on this project assignment.
All of the code for this project must be your own. You may not copy source code from other students or other sources that you find on the web. You may not share your code with other students. You may not host your code on a public code repository.
Each submission will be checked using plagiarism detection software.
Plagiarism will be reported to School and College Academic Misconduct Officers. See the University’s page on Academic Misconduct for additional information.
• Start early and proceed in steps. Read the assignment description carefully before you start programming.
• The assignment is out of 100 points and counts for 40% of your final mark.
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1 Goals and Important Points
In this project assignment, you will implement a lightweight database management system called LightDB. The assignment goals are threefold:
• to teach you how to translate from SQL queries to relational algebra query plans,
• to familiarize you with the iterator model for relational operator evaluation, and
• to build na ̈ıve implementations for the most common operators (selection, projec- tion, join, sort).
You will be starting from a bare-bone project consisting of only the main class LightDB, which defines the expected command line interface. You are free to modify this class but must preserve the command line interface. The project is also configured to use JSqlParser1, so you do not have to write your own parser (unless you want to). The main class gives an example of how to parse a SQL string into a Java object.
The reference implementation of this project is about 1100 lines of code, not including comments. Whether or not you consider this a lot, it is not a project that should be left to the last minute.
1.1 Setting Up Local Development Environment
You are free to use any text editor or IDE to complete the project. We will use Maven to compile your project. We recommend setting up a local development environment by installing Java 8 or later and using an IDE such as IntelliJ or Eclipse. To import the project into IntelliJ or Eclipse, make sure that you import as a Maven project (select the pom.xml file when importing).
2 Overview
In this project, you will implement a simple interpreter for SQL statements. That is, you will build a program that takes in a database (a set of files with data) and an input file containing one SQL query. The program will process and evaluate the SQL query on the database and write the query result in the specified output file.
1 https://github.com/JSQLParser/JSqlParser
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2.1 Supported language features
Your interpreter will not support all of SQL, but it will handle a lot of relatively complex queries. Here we give information about the queries you must support.
Your interpreter will process SELECT-FROM-WHERE queries, which may optionally also have a DISTINCT, an ORDER BY, or both. You do not need to support nested subqueries, set operators (UNION etc.), GROUP BY, aggregates like COUNT, or any other features. In addition, we make a few simplifying assumptions as below. When we say a query is valid, we mean it is a permitted input to your interpreter which you should be able to handle. When we talk about a base table, we mean a real table that exists in the database.
• You may assume all valid queries follow correct SQL syntax and that they only refer to tables that exist in the database. Also, when a query refers to a table column such as Sailors.name, you may assume the column name is valid for that table.
• You may assume there will be at least one table in the FROM clause.
• Valid queries may use aliases such as Sailors S or they may just use the names of base tables. If a query does not use aliases, all column references are fully qualified by the base table name. If a query does use aliases, all tables use aliases and all column references are qualified by an alias. Here are two examples of valid queries, the first one without aliases and the second with aliases:
1. SELECT Sailors.name, Reservations.date FROM Sailors, Reservations WHERE Sailors.id = Reservations.sid;
2. SELECT S.name, R.date FROM Sailors S, Reservations R WHERE S.id = R.sid;
You may assume that any string used as an alias will not also be the name of a base table.
• Self-joins, i.e., joining a table with itself, are valid and must be supported (and require the use of aliases).
• The WHERE clause, if present, is a conjunction (i.e., an AND) of expressions of the form A op B, where op is one of =,! =,<,>,<=,>= and A and B are either integers or column references. Thus Sailors.id = Reservations.sid, Sailors.id < 3 and 42 = 42 are all valid expressions for the WHERE cause, while for example Sailors.id < Boats.id - 1 is not a valid expression even though it would be ok in “real SQL”.
• The SELECT clause will either specify a subset of columns or have the form SELECT *. For SELECT *, order the columns in your answer following the FROM clause. Thus for SELECT * FROM R, S, each answer row has all the columns of R followed by all the columns of S. The order of columns in a table is defined by the relation schema.
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2.2
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There may be an ORDER BY clause which specifies a subset of columns for ordering. You may assume that we only want to sort in ascending order. If two tuples agree on all sort attributes, you can order them as you prefer. You may assume that no ASC, DESC, OFFSET, or LIMIT keywords will be used.
You may also assume that the attributes mentioned in the ORDER BY are a subset of those retained by the SELECT. This allows you to do the sorting last, after projection. Note that this does not mean that every attribute in ORDER BY must be mentioned in the SELECT – a query like SELECT * FROM Sailors S ORDER BY S.name is valid.
There may be a DISTINCT right after the SELECT, and it should be processed ap- propriately. Yes, SELECT DISTINCT * FROM ... is valid.
Data and output formats
We have provided you some sample data and some sample queries. Take a look at the samples directory. It has db, input, and expected output as subdirectories.
• The input directory contains SQL files with some example queries. There is one SQL query per input file.
• The db directory contains a schema.txt file specifying the schema for your database as well as a data subdirectory, where the data itself is stored. The names schema.txt and data are hard-coded and must exist in a valid database directory.
The schema.txt file contains one line per table in the database. Every line contains several strings separated by spaces. The first string on each line is the table name and all the remaining ones are attribute (column) names, in the order in which they appear in the table.
The data subdirectory contains one file per database table, and the name of the file is the same as the name of the database table with the added .csv extension. Every file contains zero or more tuples; a tuple is a line in the file with field (attribute) values separated by commas. All attribute values are integers. Using integer at- tributes simplifies your job and allows you to focus on implementing “interesting” functionality rather than boilerplate code to handle different data types. Also, you do not have to handle null values, but you do need to handle empty relations.
• The expected output directory contains the expected output files for the queries we provided. For example, query1.csv contains the expected output for the query in query1.sql. The format for the output is the same as the format for the data.
Your SQL intepreter is a program that takes three arguments: the path to a database directory, the path to an input SQL file, and the path to an output file. The program then executes the SQL query from the input file on the given database and writes the result to
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INFR11199 (Spring 2021) Coursework Assignment Page 5 of 15 the given output file. The LightDB.java file provides this command-line interface. Run
the LightDB class from your IDE and provide the required arguments.
We will run your code from the command line. We will compile your code and produce
a runnable .jar file as follows:
This command will produce target/lightdb-1.0.0-jar-with-dependencies.jar. We can run this file as follows:
The LightDB class requires passing three mandatory arguments.
Your code should handle these arguments appropriately (i.e., do not hardcode any paths).
We will test your code using our own test queries and databases with potentially different schemas. The database directory will have the same structure as described above, with files in the data directory named according to the database schema with .csv as the file extension. You may assume that prior to execution a given output file does not exist but the output directory does exist.
After we run your code, we will compare your output files with ours. For queries without an ORDER BY, it is ok if your answer file has the answer tuples in a different order to ours; for queries with an ORDER BY, your ordering must match our ordering on sort attributes, while tied tuples may have a different order to ours. As you can imagine, it is very important for you to respect the expected input and output format.
Note: We will test your code on a DICE machine with Ubuntu Linux. Remember that Linux/MacOS environments use ’/’ as path separator. The database directory will be provided with no final ’/’ symbol, as above. If you use Windows, make sure that when you form file paths, you use File.separator instead of ’\’ as path separator.
2.3 Operators and the iterator model
A key abstraction in this project will be the iterator model for relational operators. You will implement several operators:
• the bag relational algebra select, project and (tuple nested loop) join. Coursework Assignment continues. . .
mvn clean compile assembly:single
$ java -jar target/lightdb-1.0.0-jar-with-dependencies.jar Usage: LightDB database_dir input_file output_file
$ java -jar target/lightdb-1.0.0-jar-with-dependencies.jar samples/db samples/input/query1.sql samples/output/query1.csv
Read statement: SELECT * FROM Sailors
Select body is SELECT * FROM Sailors
INFR11199 (Spring 2021) Coursework Assignment Page 6 of 15 • sort and duplicate elimination operators, which are not part of the basic relational
algebra but must be added to support ORDER BY and DISTINCT.
• a scan operator which is the leaf operator for any query plan. This is really a physical operator rather than something you would add to the relational algebra, but for now we will put it in the same category as the above.
The standard way to implement all relational operators is to use an iterator API. You should create an abstract class Operator, and all your operators will extend that. Certain operators may have one or two child operators. A scan operator has no children, a join has two children, and the remaining operators have one child. Your end goal is to build a query plan that is a tree of operators.
Every operator must implement the methods getNextTuple() and reset() (put these in your abstract Operator class). The idea is that once you create a relational operator, you can call getNextTuple() repeatedly to get the next tuple of the operator’s output. This is sometimes called “pulling tuples” from the operator. If the operator still has some available output, it will return the next tuple, otherwise it should return null.
The reset() method tells the operator to reset its state and start returning its output again from the beginning; that is, after calling reset() on an operator, a subsequent call to getNextTuple() will return the first tuple in that operator’s output, even though the tuple may have been returned before. This functionality is useful if you need to process an operator’s output multiple times, e.g., for scanning the inner relation multiple times during a join.
For each of the above operators, you will implement both getNextTuple() and reset(). Remember that if your operator has a child operator, the getNextTuple() of your operator can - and probably will - call getNextTuple() on the child operator and do something useful with the output it receives from the child.
A big advantage of the iterator model, and one of the reasons it is popular, is that it supports pipelined evaluation of multi-operator plans, i.e., evaluation without material- ising (writing to disk) intermediate results.
The bulk of this project involves implementing each of the above six operators, as well as writing code to translate an SQL query (i.e., a line of text) to a query plan (i.e., a suitable tree of operators). Once you have the query plan, you can actually compute the answer to the query by repeatedly calling getNextTuple() on the root operator and putting the tuples somewhere as they come out.
We suggest you add a dump() method to your abstract Operator class. This method repeatedly calls getNextTuple() until the next tuple is null (no more output) and writes each tuple to a suitable PrintStream. That way you can dump() the results of any operator – including the root of your query plan – to your favourite PrintStream, whether it leads to a file or whether it is System.out (for testing).
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3 Implementation instructions
We recommend that you implement and test one feature at a time. Our instructions below are given in suggested implementation order.
We also recommend (but do not require) you set up a test infrastructure early on. You should do two kinds of testing – unit tests for individual components and end-to-end tests where you run your interpreter on queries and look at the output files produced to see if they match a set of expected output files. As you add more features, rerun all your tests to check that you didn’t introduce bugs that affect earlier functionality.
After you implement and test each feature, make a copy of your code and save it so if you mess up later you still have a version that works (and that you can submit for partial credit if all else fails!).
3.1 Setting up JSqlParser
For this project, you do not need to write your own SQL parser. We recommend us- ing JSqlParser, which takes care of parsing your SQL and creating a Java object. JSqlParser is an open source project:
• The project page https://github.com/JSQLParser/JSqlParser contains a wiki with examples of how to get started with JSqlParser.
• Thedocumentationisavailableathttps://javadoc.io/doc/com.github.jsqlparser/ jsqlparser/latest/index.html. The documentation is a little bare-bones but it
will be sufficient for our purposes.
The pom.xml file already has a JSqlParser dependency. You are not required to use JSqlParser, but you need to correctly parse all valid queries as defined in Section 2.1. In case you do use JSqlParser, you should play around with it on your own and read the documentation to understand the structure of the objects that it outputs.
To get you started, we have provided a simple method in LightDB.java that uses JSqlParser to read a SQL query from a file and print it out. This method illustrates the use of JSqlParser and some methods to access fields of the Statement object, such as getSelectBody() if the Statement is a Select.
You may assume all the Statements we will work with are Selects, and have a PlainSelect as the selectBody. Take a look at the PlainSelect Javadocs; the first table in the FROM clause will be in the fromItem, the remaining ones in joins. Also of relevance to you is the where field for the WHERE clause, and eventually the distinct and orderByElements fields. Write some SQL queries and write code to access and print out all these objects/fields for your queries to get an idea of “what goes where”.
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The where field of a PlainSelect contains an Expression; take a look at the docs for that. For this project, you only need to worry about AndExpression, Column, LongValue, EqualsTo, NotEqualsTo, GreaterThan, GreaterThanEquals, MinorThan and MinorThanEquals. These capture the recursive structure of an expression. The last six of the expression types mentioned are comparison expressions, the AndExpression is
a conjunction of two other Expressions, the LongValue is a numeric literal, and Column is a column reference (such as the S.id in S.id < 5). Every Column object has a column name, as well as an embedded Table object. Every Table object has a name and an alias (if aliases are used).
JSqlParser also provides a number of Visitor interfaces, which you may or may not choose to use. In particular, ExpressionVisitor and ExpressionDeParser are highly recommended to use once you get to the selection operator. Check out also the wiki page of JSqlParser on how to evaluate expressions.
The above should be enough to get you started, but you should expect to do further explorations on your own as you implement more and more SQL features.
3.2 Implement scan
Your first goal is to support queries that are full table scans, e.g., SELECT * FROM Sailors (for now assume the queries do not use aliases). To achieve this, you will need to implement your first operator – the scan operator.
Implement a ScanOperator that extends your Operator abstract class. Every in- stance of ScanOperator knows which base table it is scanning. Upon initialisation, it opens a file scan on the appropriate data file; when getNextTuple() is called, it reads the next line from the file and returns the next tuple. You probably want to have a Tuple class to handle the tuples as objects.
The ScanOperator needs to know where to find the data file for its table. It is recommended to handle this by implementing a database catalog in a separate class. The catalog can keep track of information such as where a file for a given table is located, what the schema of different tables is, and so on. Because the catalog is a global entity that various components of your system may want to access, you should consider using the singleton pattern for the catalog; if unfamiliar with the singleton pattern, see many other online references.
Once you have written your ScanOperator, test it thoroughly to be sure getNextTuple() and reset() both work as expected. Then, hook up your ScanOperator to your inter- preter. Assuming that all your queries are of the from SELECT * FROM MyTable, write code that grabs MyTable from the fromItem and constructs a ScanOperator from it.
In summary the top-level structure of your code at this point should be:
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• parse the query from the input file
• construct a ScanOperator for the table in the fromItem
• call dump() on your ScanOperator to send the results somewhere helpful, like a file or your console.
3.3 Implement selection
The next order of business is single-table selection, still with fully specified table names (no aliases). That is, you are aiming to support queries like SELECT * FROM Boats WHERE Boats.id = 4.
This means you need to implement a second Operator, which is a SelectOperator. Your query plan will now have two operators – the SelectOperator as the root and the ScanOperator as its child. During evaluation, the SelectOperator’s getNextTuple() method will grab the next tuple from its child (i.e., from the scan), check if that tuple passes the selection condition, and if so output it. If the tuple does not pass the selection condition, the selection operator will continue pulling tuples from the scan until either it finds one that passes or it receives null (i.e., the scan runs out of output).
The tricky part will be implementing the logic to check if a tuple passes the selection condition. The selection condition is an Expression which you will find in the WHERE clause of your query. The SelectOperator needs to know that Expression.
You will need to write a class to test whether a Expression holds on a given tuple. For example, if you have a table R with fields A, B and C, you may encounter a tuple (1, 42, 4) and an expression R.A < R.C AND R.B = 42, and you need to determine whether the expression is true or false on this tuple.
This is best achieved using a visitor on the Expression. You should have a class that extends JSqlParser’s ExpressionDeParser. The class will take as input a tuple and recursively walk the expression to evaluate it to true or false on that tuple. The expression may contain column references – in our example R.A < R.C AND R.B = 42 refers to all three columns of R. The visitor class needs some way to resolve the references; i.e., if our input tuple is (1, 42, 4), it needs a way to determine that R.A is 1, etc. So, your visitor class also needs to take in some schema information. It is up to you how you structure your schema information, but obviously it must allow mapping from column references like R.A to indexes into the tuple.
Once you have written your visitor class, unit-test it thoroughly. Start with simple expressions that have no column references, like 1 < 2 AND 3 = 17. Then test it with column references until you are 100% sure it works. Once your expression evaluation logic is solid, you can plug it into the getNextTuple() method of your SelectOperator.
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3.4 Implement projection
Your next task is to implement projection, i.e., you will be able to handle queries of the form SELECT Sailors.id FROM Sailors WHERE Sailors.age = 20. We still assume that the queries do not use aliases.
In comparison with selection, implementing projection is relatively easy. You need a third Operator that is a ProjectOperator. When getNextTuple() is called, it grabs the next tuple from its child. It extracts only the desired attributes, makes them into a new tuple and returns that. Note that the child could be either a SelectOperator or a ScanOperator, depending on whether your SQL query has a WHERE clause.
You get the projection columns from the selectItems field of your PlainSelect. selectItems is a list of SelectItems, where each one is either AllColumns (for a SELECT
* ) or a SelectExpressionItem. You may assume the Expression in a SelectExpressionItem will always be a Column. Once you grab these Columns you need to translate that infor- mation into something useful to the ProjectOperator.
Note that the attribute order in the SELECT does not have to match the attribute order in the table. The queries SELECT R.A, R.B FROM R and SELECT R.B, R.A FROM R are both valid, and they are different queries with different output.
By this point you should have code takes in a SQL query and produces a query plan containing:
• an optional projection operator, having as a child • an optional selection operator, having as a child • a non-optional scan operator.
Thus your query plan could have one, two or three operators. Make sure you are supporting all possibilities; try queries with/without a projection/selection. If the query is SELECT *, do not create a projection operator, and if the query has no WHERE clause, do not create a selection operator.
You are now producing relatively complex query plans; however, things are about to get much more exciting and messy as we add joins. This is a good time to pull out the logic for constructing the query plan into its own class, if you have not done so already. Thus, you should have a top-level interpreter class that reads the statement from the query file. You should also have a second class that knows how to construct a query plan for a Statement, and returns the query plan back to the interpreter so the interpreter can dump() the results of the query plan somewhere.
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3.5 Implement join
Next up, the star of the show: joins. Assume that there are still no table aliases, so you don’t have to worry about self-joins for now.
You need a JoinOperator that has both a left and right child Operator. It also has an Expression which captures the join condition. This Expression could be a single comparison such as R.A = S.B, a conjunction (AND) of comparisons, or it could be null if the join is a cross product.
Implement the simple (tuple) nested loop join algorithm: the join should scan the left (outer) child once, and for each tuple in the outer child, it should scan the inner child completely (finally a use for the reset() method!). Once the operator has obtained a tuple from the outer and a tuple from the inner, it glues them together. If there is a non-null join condition, the tuple is only returned if it matches the join condition (so you will be reusing your expression visitor class from Section 3.3). If the join is a cross product, all pairs of tuples are returned.
Once you have implemented and unit-tested your JoinOperator, you need to figure out how to translate an SQL query to a plan that includes joins.
For this project, we require that you construct a left-deep join tree that follows the order in the FROM clause. That is, a query whose FROM clause is FROM R,S,T produces a plan with the structure shown below:
The tricky part will be processing the WHERE clause to extract join conditions. The WHERE clause may contain both selections on a single table as well as join conditions linking multiple tables. For example WHERE R.A < R.B AND S.C = 1 AND R.D = S.G contains a selection expression on R, a selection expression on S, and a join condition on both R and S together. Obviously it is most efficient to evaluate the selections as early as possible and to evaluate R.D = S.G during computation of the join, rather than computing a cross product and having a selection later.
While the focus of this project is not optimisation, you do not want to compute cross products unless you have to as this is grossly inefficient. Therefore, we require that you have some strategy for extracting join conditions from the WHERE clause and evaluating them as part of the join. You do not need to be very clever about this, but you may not simply compute the cross products (unless of course the query actually asks for a
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cross product). You must explain your strategy in comments in your code and in the README that you submit with your code.
A suggested way to do this is as follows. Write another class that extends ExpressionDeParser and processes the WHERE clause. For every conjunct, the visitor determines which tables
are referenced, and adds the conjunct to an appropriate Expression. If there are k tables
being joined, there could be 2k − 1 running Expressions: k − 1 join conditions and k selection conditions on the individual tables. Once the whole WHERE clause is processed,
the 2k − 1 Expressions can be integrated into the appropriate selection and join oper- ators in the query plan. Of course some of these Expressions may turn out to be null, depending on the query.
For example, if we have SELECT * FROM R, S, T WHERE R.A = 1 AND R.B = S.C AND T.G < 5 AND T.G = S.H, the above approach would give the following query plan:
You don’t have to follow exactly this strategy. You don’t need to worry about pushing projections past the joins, you may have one big projection at the root of your query plan.
3.6 Implement aliases
ThenextstepistoimplementaliasestosupportquerieslikeSELECT R.A FROM SomeTable R. These are handy in any case, but they are essential to support self-joins.
The aliases themselves come from the FROM clause, and you can extract them from the fromItem and the joins of your PlainSelect. Unfortunately, when you reference columns in the SELECT and WHERE clauses, JSqlParser is not smart enough to know whether you are using aliases or references. Thus, if you have a SELECT R.A, the R.A is returned as a Column, and the embedded Table object has R as the table name whether or not R is a base table name or an alias. Thus you will need to keep track of these things yourself. You need to determine, when you start building your query plan, whether or not the query uses aliases. If yes, you need to figure out a way to keep track of all the aliases used in the query so you can resolve the column references throughout your code.
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Implementing aliases is not conceptually difficult, but you may find it a bit fiddly. It is a great test of how clean and modular your code is; if you have been structuring it well, you will have to modify relatively little code. You may need to modify classes that extend ExpressionDeParser, as well as the class that builds your query plan from the PlainSelect.
It may be useful to start with supporting aliases only on single-table queries, then move on to joins. Be sure to test your code on lots of queries including self-joins, as these can bring out bugs which are not apparent when each base table is used only once. Also be sure that you can still correctly handle all the “old” queries you tested (which do not use aliases).
3.7 Implement ORDER BY
Next is the ORDER BY operator. You will implement ORDER BY by adding a SortOperator. This is going to read all of the output from its child, place it into an internal buffer, sort it, and then return individual tuples when requested. You can use Collections.sort(); you will want a custom Comparator to specify the different sort orders.
You may be alarmed by the above description. Yes, sort is a blocking operator, which means it really needs to see all of its input before producing any output (think about it – what if the tuple that comes first in the desired sort order is the last one that the child operator is going to spit out?). As you imagine, buffering all the tuples in memory will not work for very large relations; for this project assignment, this is fine.
If your query has a ORDER BY clause, you should put the SortOperator as the root, followed by the rest of your plan. It is good to delay sorting as late as possible, in particular to do it after the projection(s), because there will be less data to sort that way. We are making life easier for ourselves by assuming it’s always safe to defer the ORDER BY after the projections; this is not always the case in full “real” SQL. A query like SELECT S.A FROM S ORDER BY S.B is valid SQL in the real world, we just choose not to support them in this project.
3.8 Implement DISTINCT
Given that you have just implemented sorting, it is easy to add support for dupli- cate elimination and DISTINCT. If the query doesn’t already have an ORDER BY, add a SortOperator. Then, add a new DuplicateEliminationOperator. This operator as- sumes the input from its child is in sorted order; it reads the tuples from the child and only outputs non-duplicates.
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4 Grading
We strongly suggest that you follow the architecture we described. However, we will not penalize you for making different architectural decisions, with a few exceptions:
• you must have relational Operators that implement getNextTuple() and reset() methods as outlined above. This is the standard relational algebra evaluation model and you need to learn it.
• you must construct a tree of Operators and then evaluate it by repeatedly calling getNextTuple() on the root operator.
• as explained in Section 3.5, you must build a left-deep join tree that follows the ordering of the tables in the FROM clause. Also, you must have a strategy to identify join conditions and evaluate them as part of the join rather than doing a selection after a cross product.
Disregarding any of the above three requirements will result in severe point deductions. Next we give the grading breakdown.
4.1 Code style and comments (10 points)
You must provide comments for every method you implement. At minimum, the comment must include one sentence about the purpose/logic of the method, and @params/@return annotations for every argument/return value respectively. In addition, every class must have a comment describing the class and the logic of any algorithm used in the class. If you follow the above rules and write reasonably clean code that follows our overall architecture, you are likely to get the full 10 points for code style.
4.2 Automated tests (90 points)
Be sure to read Section 2.2 carefully for information on expected input and output format.
We will run your code on our own queries and data and award you 2 points for every query that returns the correct output. The queries we provide with the assignment count for 16 out of the 90 points. You can expect that we will add additional tables to the database; of course the schema of these tables will be mentioned in the schema file and the data files will be found in the data directory.
We will test with basic queries as well as with arbitrarily complex queries that include any/all of the features you are to implement. We may also reorder the queries we gave
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you and/or intersperse them with our own, so don’t hardcode any functionality on the assumption that the queries will be run in any particular order.
If you cannot implement one or more operators, that is fine, although obviously you won’t get full points. In that case, you must clearly tell us in the README what you have not been able to implement.
5 Submission instructions
Double-check that your code compiles and runs as described in Section 2.2. You must keep the LightDB class as the top-level main class of your code.
Create a README text file containing the following information.
• an explanation of your logic for extracting join conditions from the WHERE clause. If this logic is fully explained in comments in your code, your README does not need to repeat that; however, it must mention exactly where in the code/comments the description is, so the grader can find it easily.
• any other information you want the grader to know, such as known bugs.
Create a .zip archive containing a README file and your entire project folder so that we can compile and run your code on the command line as described in Section 2.2. Do not include any .class files nor large .csv files. Upload the zip archive to Learn: Assessment (the left panel) → Assignment Submission → Coursework: LightDB.
Make sure you start early! Good luck!
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