EQL

Eanorambuena's Query Language, just for learning

What is EQL?

EQL is a query language that I'm developing for learning purposes. It's not intended to be used in production. It's a simple language that allows you to query data from a database.

How to use it?

You can use it in two ways:

Using it as a package
Using the EQL interpreter

Using the EQL interpreter

Clone the repository

git clone https://github.com/eanorambuena/EQL.git

Install the dependencies Currently this project does not have any dependencies, but it is possible that in the future it will have some.
Run the interpreter

python3 EQL

Write your query

>>> CHECKOUT MainTable;
>>> SELECT *;

Quick start

Creating a database

In this quick start, we will import a database from a CSV file. You can just run the following command:

python3 example.py

This will create a database called MainDatabase and will import the data from the CSV file http://winterolympicsmedals.com/medals.csv You will see the following output:

'Year' | 'City' | 'Sport' | 'Discipline' | 'NOC' | 'Event' | 'Event_gender' | 'Medal'
Database MainDataBase saved to example_data\database.edb

Querying the database

Now that we have a database, we can query it. To do this, we will use the EQL interpreter. To start the interpreter, just run the following command:

python3 EQL example_data/example.edb

This will start the interpreter and will load the database example_data/example.edb. Now we can start querying the database. To do this, we will use the SELECT statement. The SELECT statement is used to query data from a table. The syntax of the SELECT statement is the following:

SELECT number_of_columns

Where number_of_columns is the number of columns that you want to query. If you want to query all the columns, you can use the * character. Try to run the following query:

SELECT *

You will see the following output:

No table selected

This is because we haven't selected a table. To select a table, we will use the CHECKOUT statement. The CHECKOUT statement is used to select a table. The syntax of the CHECKOUT statement is the following:

CHECKOUT table_name

Where table_name is the name of the table that you want to select. We can see the tables that are in the database by using the LIST TABLES statement.

LIST TABLES

You will see the following output:

MainTable
medals

Let's select the MainTable table.

CHECKOUT MainTable

Now, we can query the data from the table.

SELECT *

Oh! It seems that we have an error, we can not see anything. This is because we are actually selecting the data but not printing it. To print the data, we will use the ; clause. The ; clause is used to print the data. Try to run the following query:

SELECT *;

Now we receive the following output:

----------------
0 results shown

That´s okay, MainTable is an empty table. Let's try to query the medals table.

CHECKOUT medals
SELECT *;

Now we receive the following output:

0: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'M'               | 'Silver'
1: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'W'               | 'Gold'
2: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'pairs'           | 'X'               | 'Gold'  
3: '1924'            | 'Chamonix'        | 'Bobsleigh'       | 'Bobsleigh'       | 'BEL'             | 'four-man'        | 'M'               | 'Bronze'
4: '1924'            | 'Chamonix'        | 'Ice Hockey'      | 'Ice Hockey'      | 'CAN'             | 'ice hockey'      | 'M'               | 'Gold'  
5: '1924'            | 'Chamonix'        | 'Biathlon'        | 'Biathlon'        | 'FIN'             | 'military patrol' | 'M'               | 'Silver'
6: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'FIN'             | 'pairs'           | 'X'               | 'Silver'
7: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '10000m'          | 'M'               | 'Gold'
8: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '10000m'          | 'M'               | 'Silver'
9: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '1500m'           | 'M'               | 'Gold'
-------------------
10 results shown
2301 results hidden

Now we can see the data from the medals table. We can see that we have 2311 results, but we can only see 10. This is because we have a limit of 10 results. We will see how to change this limit later.

Printing the last selection

As we have seen, we can print the data from a selection using the ; clause. But what if we want to print the data from the last selection? In every case, we can use ; to print the data from the last selection. We can also use it as a complete statement, doubling the ; character.

;;

In this case, we will print the data from the selection of the last section:

0: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'M'               | 'Silver'
1: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'W'               | 'Gold'
2: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'pairs'           | 'X'               | 'Gold'  
3: '1924'            | 'Chamonix'        | 'Bobsleigh'       | 'Bobsleigh'       | 'BEL'             | 'four-man'        | 'M'               | 'Bronze'
4: '1924'            | 'Chamonix'        | 'Ice Hockey'      | 'Ice Hockey'      | 'CAN'             | 'ice hockey'      | 'M'               | 'Gold'  
5: '1924'            | 'Chamonix'        | 'Biathlon'        | 'Biathlon'        | 'FIN'             | 'military patrol' | 'M'               | 'Silver'
6: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'FIN'             | 'pairs'           | 'X'               | 'Silver'
7: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '10000m'          | 'M'               | 'Gold'
8: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '10000m'          | 'M'               | 'Silver'
9: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '1500m'           | 'M'               | 'Gold'
-------------------
10 results shown
2301 results hidden

The ;; clause will be useful when we want to print the data from the last selection when we can not simply add the ; clause to the end of the selection.

Global clauses and Env clauses

There are two types of clauses: Global clauses and Env clauses. Global clauses are used to modify the behavior of the database. Env clauses are used to modify the behavior of the current table. We will see the global clauses first.

When we use a global clause, adding a ; clause at the end of the statement will not print the data from the last selection. This is because the global clause is not at the environment level, but at the database level. The ; clause is at the environment level, so it will not print the data from the last selection. Even though, we can use the ;; clause to print the data from the last selection, as we have seen before.

For example, CHECKOUT is a global clause. Then, if we use CHECKOUT and then the complete clause ;;, we will be printing the data from the last selection.

CHECKOUT MainTable ;;

We will receive the following output:

0: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'M'               | 'Silver'
1: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'W'               | 'Gold'
2: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'pairs'           | 'X'               | 'Gold'  
3: '1924'            | 'Chamonix'        | 'Bobsleigh'       | 'Bobsleigh'       | 'BEL'             | 'four-man'        | 'M'               | 'Bronze'
4: '1924'            | 'Chamonix'        | 'Ice Hockey'      | 'Ice Hockey'      | 'CAN'             | 'ice hockey'      | 'M'               | 'Gold'  
5: '1924'            | 'Chamonix'        | 'Biathlon'        | 'Biathlon'        | 'FIN'             | 'military patrol' | 'M'               | 'Silver'
6: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'FIN'             | 'pairs'           | 'X'               | 'Silver'
7: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '10000m'          | 'M'               | 'Gold'
8: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '10000m'          | 'M'               | 'Silver'
9: '1924'            | 'Chamonix'        | 'Skating'         | 'Speed skating'   | 'FIN'             | '1500m'           | 'M'               | 'Gold'
-------------------
10 results shown
2301 results hidden

Selecting specific rows

We can select specific rows from a table using the WHERE clause. The WHERE clause is used to filter the data from the table. We can use the WHERE clause to select specific rows from the table. The syntax of the WHERE clause is:

WHERE value_1 operator value_2

Note that we can use the WHERE clause only after the SELECT clause, and we can use it only once in a selection. For example, if we want to select the rows from the medals table where the Year column is equal to 1928, we can use the following selection:

WHERE Year = 1928;

We will receive the following output:

0: '1928'            | 'St. Moritz'      | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'M'               | 'Silver'
1: '1928'            | 'St. Moritz'      | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'W'               | 'Silver'
2: '1928'            | 'St. Moritz'      | 'Skating'         | 'Figure skating'  | 'AUT'             | 'pairs'           | 'X'               | 'Bronze'
3: '1928'            | 'St. Moritz'      | 'Skating'         | 'Figure skating'  | 'AUT'             | 'pairs'           | 'X'               | 'Silver'
4: '1928'            | 'St. Moritz'      | 'Skating'         | 'Figure skating'  | 'BEL'             | 'individual'      | 'M'               | 'Bronze'
5: '1928'            | 'St. Moritz'      | 'Ice Hockey'      | 'Ice Hockey'      | 'CAN'             | 'ice hockey'      | 'M'               | 'Gold'
6: '1928'            | 'St. Moritz'      | 'Skating'         | 'Speed skating'   | 'FIN'             | '1500m'           | 'M'               | 'Gold'
7: '1928'            | 'St. Moritz'      | 'Skating'         | 'Speed skating'   | 'FIN'             | '5000m'           | 'M'               | 'Silver'
8: '1928'            | 'St. Moritz'      | 'Skating'         | 'Speed skating'   | 'FIN'             | '500m'            | 'M'               | 'Bronze'
9: '1928'            | 'St. Moritz'      | 'Skating'         | 'Speed skating'   | 'FIN'             | '500m'            | 'M'               | 'Gold'
-----------------
10 results shown
31 results hidden

We can also concatenate multiple conditions using the AND operator. For example, if we want to select the rows where the Year is greater than 1950, the Sport is equal to Skating, and the Medal is equal to Gold, we can use the following selection:

WHERE Year > 1950 AND Sport = 'Skating' AND Medal = 'Gold';

We will receive the following output:

----------------
0 results shown

That is because there are no rows that satisfy the conditions. The WHERE clause acts as a filter of the last selection, then, the engine is concatenating the conditions as follows:

Year = 1928 AND Year > 1950 AND Sport = 'Skating' AND Medal = 'Gold';

We can see that the first condition and the second condition are not compatible, so the engine will return an empty table. If we want to get the rows where the Year is greater than 1950, the Sport is equal to Skating, and the Medal is equal to Gold, we can use the following selection:

SELECT *
WHERE Year > 1950 AND Sport = 'Skating' AND Medal = 'Gold';

Now, we will receive the following output:

0: '1952'            | 'Oslo'            | 'Skating'         | 'Figure skating'  | 'GBR'             | 'individual'      | 'W'               | 'Gold'
1: '1952'            | 'Oslo'            | 'Skating'         | 'Figure skating'  | 'GER'             | 'pairs'           | 'X'               | 'Gold'
2: '1952'            | 'Oslo'            | 'Skating'         | 'Speed skating'   | 'NOR'             | '10000m'          | 'M'               | 'Gold'
3: '1952'            | 'Oslo'            | 'Skating'         | 'Speed skating'   | 'NOR'             | '1500m'           | 'M'               | 'Gold'  
4: '1952'            | 'Oslo'            | 'Skating'         | 'Speed skating'   | 'NOR'             | '5000m'           | 'M'               | 'Gold'  
5: '1952'            | 'Oslo'            | 'Skating'         | 'Figure skating'  | 'USA'             | 'individual'      | 'M'               | 'Gold'  
6: '1952'            | 'Oslo'            | 'Skating'         | 'Speed skating'   | 'USA'             | '500m'            | 'M'               | 'Gold'  
7: '1956'            | 'Cortina dAmpezzo' | 'Skating'         | 'Figure skating'  | 'AUT'             | 'pairs'           | 'X'               | 'Gold' 
8: '1956'            | 'Cortina dAmpezzo' | 'Skating'         | 'Speed skating'   | 'SWE'             | '10000m'          | 'M'               | 'Gold'
9: '1956'            | 'Cortina dAmpezzo' | 'Skating'         | 'Speed skating'   | 'URS'             | '1500m'           | 'M'               | 'Gold'
------------------
10 results shown
208 results hidden

Managing tables

We can manage tables using the CREATE, DROP and TEMPLATE statements. The CREATE statement is used to create a new table. The DROP statement is used to delete a table. The TEMPLATE statement is used to modify the headers of the columns of a table.

Creating a table

We can create a new table using the CREATE statement. The syntax of the CREATE statement is:

CREATE table_name

For example, if we want to create a new table called income_table, we can use the following statement:

CREATE income_table

Managing columns

We can name or rename the columns of a table using the TEMPLATE statement. The syntax of the TEMPLATE statement is:

TEMPLATE "column_name_1" "column_name_2" "column_name_3" ...

For example, if we want to rename the columns of the income_table table, we can use the following statement:

TEMPLATE "Name" "Age" "Income"

Note that we do not need to specify the data type of the columns. The EQL engine will automatically detect the data type of the columns while filtering the data.

Deleting a table

We can delete a table using the DROP statement. The syntax of the DROP statement is:

DROP table_name

For example, if we want to delete the income_table table, we can use the following statement:

DROP income_table

Inserting data

We can insert data into a table using the INSERT and VALUES statements. The INSERT statement is used to insert data into a table. The VALUES statement is used to specify the values of the columns of the table. The syntax of the INSERT VALUES statement is:

INSERT
VALUES "value_1" "value_2" "value_3" ...

If we are inserting strings, we need to use quotation marks. If we are inserting numbers, we must not use quotation marks. For example, let's insert some data into the MainTable table:

CHECKOUT MainTable
INSERT VALUES "John" 25 10000
INSERT VALUES "Mary" 30 20000
INSERT VALUES "Peter" 35 30000

We can see that the MainTable table has been updated. If we select all the rows of the MainTable table using:

SELECT *;

we will receive the following output:

0: 'John'            | 25                | 10000
1: 'Mary'            | 30                | 20000
2: 'Peter'           | 35                | 30000
----------------
3 results shown

Updating the data

We can update data into a table using the UPDATE and VALUES statements. The UPDATE statement is used to update data into a row. The VALUES statement is used to specify the values of the columns of the row. The syntax of the IUPDATE VALUES statement is:

UPDATE
VALUES "value_1" "value_2" "value_3" ...

If we are updating strings, we need to use quotation marks. If we are updating numbers, we must not use quotation marks. For example, let's update some data into the MainTable table:

SELECT *
WHERE "Name" = "Mary"
UPDATE
VALUES "Mary" 30 25000

We can see that the MainTable table has been updated. If we select all the rows of the MainTable table using:

SELECT *;

we will receive the following output:

0: 'John'            | 25                | 10000
1: 'Mary'            | 30                | 25000
2: 'Peter'           | 35                | 30000
----------------
3 results shown

Deleting the data

We can delete data from a table using the DELETE statement. The syntax of the DELETE statement is:

DELETE

For example, let's delete some data from the MainTable table:

SELECT *
WHERE "Name" = "Mary"
DELETE

We can see that the MainTable table has been updated. If we select all the rows of the MainTable table using:

SELECT *;

we will receive the following output:

0: 'John'            | 25                | 10000
1: 'Peter'           | 35                | 30000
----------------
2 results shown

Listing useful commands

We can list many useful things using the LIST statement. The LIST statement is used to list the names, defined clauses and the name of variables. For example, if we want to list the names of the tables, we can use the following statement, as we have seen in a previous section:

LIST TABLES

Output:

MainTable
medals

If we want to list all the defined clauses, we can use the following statement:

LIST CLAUSES

At the time of writing this tutorial, the output of the LIST CLAUSES statement is:

;;
AND
CHECKOUT
CLAUSES
CREATE
DROP
ENV
EXIT
GET
GLOBAL
INSERT
LET
LIST
SELECT
SET
TABLES
TEMPLATE
VALUES
WHERE

If we want to list the names of the global variables, we can use the following statement:

LIST GLOBAL

Output:

NAME

If we want to list the names of the env variables, we can use the following statement:

LIST ENV

Output:

MAX_RESULTS_SHOWN
MIN_VALUE_LENGTH_SHOWN

We can list the env variables only if we are in a table. If we are not in a table, we will receive the following error:

No table selected

We will talk about EQL variables in the next section.

Using variables

In EQL, we can use variables to store data. We can use the LET statement to define a global variable. We can use the SET statement to define an env variable.

Global variables are defined for the whole database. Env variables are defined for the current table. We can use the GET statement to get the value of a variable. When we use the GET statement, the engine first tries to get the value of the variable from the env variables. If the variable is not defined in the env variables, the engine will try to get the value of the variable from the global variables. If the variable is not defined in the global variables, the engine will return nothing. The result of the GET statement is often stored as the last selection of the current table.

Defining or modifying global variables

CHECKOUT medals
LET MAX_RESULTS_SHOWN 5

We will see the following output:

GLOBAL MAX_RESULTS_SHOWN <- 5

We can see that the MAX_RESULTS_SHOWN variable has been defined or modified. If we want to get the value of the MAX_RESULTS_SHOWN variable, we can use the following statement:

GET MAX_RESULTS_SHOWN

Output:

ENV MAX_RESULTS_SHOWN -> 10

Note that the result is different because we are using the LET statement, the value of the MAX_RESULTS_SHOWN is stored in the global variables, but we are getting the value of the MAX_RESULTS_SHOWN variable from the env variables.

Defining or modifying env variables

We can modify the value of the MAX_RESULTS_SHOWN env variable using the SET statement as follows:

SET MAX_RESULTS_SHOWN 5

We will see the following output:

ENV MAX_RESULTS_SHOWN <- 5

We can see that the MAX_RESULTS_SHOWN variable has been defined or modified. If we want to get the value of the MAX_RESULTS_SHOWN variable, we can use the following statement:

GET MAX_RESULTS_SHOWN

Output:

ENV MAX_RESULTS_SHOWN -> 5

Now we can see we have successfully modified the value of the MAX_RESULTS_SHOWN env variable!

Assigning the last selection to a variable

We can assign the last selection to a variable using the LET or SET statement, followed by the name of the variable and the clause *. For example, if we want to assign the last selection to the last_selection global variable, we can use the following statement:

SELECT *
LET last_selection *

Output:

LAST_SELECTION <- (
    0: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'M'               | 'Silver'
    1: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'W'               | 'Gold'
    2: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'pairs'           | 'X'               | 'Gold'
    3: '1924'            | 'Chamonix'        | 'Bobsleigh'       | 'Bobsleigh'       | 'BEL'             | 'four-man'        | 'M'               | 'Bronze'
    4: '1924'            | 'Chamonix'        | 'Ice Hockey'      | 'Ice Hockey'      | 'CAN'             | 'ice hockey'      | 'M'               | 'Gold'
    -------------------
    5 results shown
    2306 results hidden
)

Note that variable names are not case-sensitive. We can see the value stored in last_selection as we saw in the previous subsections.

GET last_selection

Output:

GLOBAL LAST_SELECTION -> (
    0: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'M'               | 'Silver'
    1: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'individual'      | 'W'               | 'Gold'
    2: '1924'            | 'Chamonix'        | 'Skating'         | 'Figure skating'  | 'AUT'             | 'pairs'           | 'X'               | 'Gold'
    3: '1924'            | 'Chamonix'        | 'Bobsleigh'       | 'Bobsleigh'       | 'BEL'             | 'four-man'        | 'M'               | 'Bronze'
    4: '1924'            | 'Chamonix'        | 'Ice Hockey'      | 'Ice Hockey'      | 'CAN'             | 'ice hockey'      | 'M'               | 'Gold'
    -------------------
    5 results shown
    2306 results hidden
)

Exitting and saving the database

We can use the EXIT statement to exit the EQL shell. If we are running a database in a with python context, the database will be saved automatically.

If we are running the database with the EQL engine, it saves the database automatically when we use the EXIT statement, because it uses the python context manager inside.

EXIT

In this example, we will see the following output:

Database MainDataBase saved to example_data\database.edb

eanorambuena / EQL

EQL

What is EQL?

How to use it?

Using the EQL interpreter

Quick start

Creating a database

Querying the database

Printing the last selection

Global clauses and Env clauses

Selecting specific rows

Managing tables

Creating a table

Managing columns

Deleting a table

Inserting data

Updating the data

Deleting the data

Listing useful commands

Using variables

Defining or modifying global variables

Defining or modifying env variables

Assigning the last selection to a variable

Exitting and saving the database

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