SQL Fundamentals for Data Engineers

Structured Query Language (SQL) is a programming language used to manage and manipulate data in the relational database management systems (RDBMS). SQL has become a de facto standard for data management due to its simplicity, versatility, and scalability. In this blog post, we will explore the fundamentals of SQL and its usage in data engineering.

Basics of SQL

SQL can be divided into four categories, which are Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), and Transaction Control Language (TCL).

Data Definition Language (DDL)

DDL helps to define the structure of the database by creating and modifying the database objects such as tables, indexes, views, and procedures. The following are some of the essential commands of DDL.

CREATE

The CREATE command is used to create a new object in the database. For example, to create a new table named Employees with two columns EmployeeID and Name, use the following command.

CREATE TABLE Employees (EmployeeID int, Name varchar(255));

ALTER

The ALTER command is used to modify an existing object in the database. For example, to add a new column named Salary to the Employees table, use the following command.

ALTER TABLE Employees ADD Salary decimal(10,2);

DROP

The DROP command is used to delete an existing object in the database. For example, to delete the Salary column from the Employees table, use the following command.

ALTER TABLE Employees DROP COLUMN Salary;

Data Manipulation Language (DML)

DML helps to manipulate the data stored in the database by inserting, updating, and deleting records from the tables. The following are some of the essential commands of DML.

INSERT

The INSERT command is used to insert new records into a table. For example, to insert a new record with values 1 and John into the Employees table, use the following command.

INSERT INTO Employees (EmployeeID, Name) VALUES (1, 'John');

UPDATE

The UPDATE command is used to update existing records in a table. For example, to update the Name of the record with EmployeeID=1 to Jack, use the following command.

UPDATE Employees SET Name = 'Jack' WHERE EmployeeID = 1;

DELETE

The DELETE command is used to delete existing records from a table. For example, to delete the record with EmployeeID=1 from the Employees table, use the following command.

DELETE FROM Employees WHERE EmployeeID = 1;

Data Control Language (DCL)

DCL helps to control the access and permissions of database users. The following are some of the essential commands of DCL.

GRANT

The GRANT command is used to grant access permissions to database users. For example, to grant SELECT and INSERT permissions to a user named John on the Employees table, use the following command.

GRANT SELECT, INSERT ON Employees TO John;

REVOKE

The REVOKE command is used to revoke access permissions from database users. For example, to revoke SELECT and INSERT permissions from a user named John on the Employees table, use the following command.

REVOKE SELECT, INSERT ON Employees FROM John;

Transaction Control Language (TCL)

TCL helps to manage transactions in the database by committing or rolling back the changes made by DML commands. The following are some of the essential commands of TCL.

COMMIT

The COMMIT command is used to commit the changes made by DML commands to the database. For example, to commit the changes made by the previous INSERT command, use the following command.

COMMIT;

ROLLBACK

The ROLLBACK command is used to undo the changes made by DML commands in a transaction. For example, to rollback the changes made by the previous INSERT command, use the following command.

ROLLBACK;

Usage of SQL in Data Engineering

SQL is widely used in data engineering due to its capability to manage and manipulate large data sets in RDBMS. The following are some of the use cases of SQL in data engineering.

Data Warehousing

SQL is used to manage and manipulate data in data warehouse systems such as Snowflake, Amazon Redshift, and Google BigQuery. SQL provides a simple and efficient way to extract and load data from various data sources and transform them into meaningful insights.

Business Intelligence

SQL is used in business intelligence tools such as Tableau, Power BI, and Looker to create reports, dashboards, and visualizations from the data stored in RDBMS. SQL provides a flexible and powerful way to aggregate, filter, and pivot data to generate business insights.

Data Integration

SQL is used to integrate data from different sources such as databases, files, and APIs. SQL provides a standardized way to join, merge, and transform data from various sources into a unified format.

Data Visualization

SQL is used in data visualization libraries such as Matplotlib, Seaborn, and Plotly to create charts, graphs, and plots from the data stored in RDBMS. SQL provides a flexible and expressive way to aggregate and filter data before visualizing it.

Conclusion

SQL is an essential tool for data engineering due to its simplicity, versatility, and scalability. SQL provides a standardized way to manage and manipulate data in RDBMS, which makes it easy to integrate with other tools and systems. In this blog post, we explored the fundamentals of SQL and its usage in data engineering.

Category: Database