What is Structured Query Language (SQL)?

What is Structured Query Language (SQL)?

SQL is a programming language that is used to interact with relational databases. Relational databases are systems that store data in tables, which are composed of rows and columns. Each row represents a record of data, and each column represents an attribute or property of the data. SQL allows users to create, manipulate, and query data in relational databases using various commands and clauses.

SQL History and Standards

SQL was developed in the 1970s by IBM researchers as a way to access data stored in a system called System R. The name SQL was derived from Structured English Query Language (SEQUEL), which was the original name of the language. SQL was influenced by the concepts of set theory and relational algebra, which are mathematical ways of describing how data can be organized and manipulated.

In 1986, the American National Standards Institute (ANSI) published the first standard for SQL, known as SQL-86 or SQL1. Since then, several revisions and extensions have been made to the standard, such as SQL-89, SQL-92, SQL:1999, SQL:2003, SQL:2008, SQL:2011, and SQL:2016. The latest version of the standard is SQL:2019, which was published in November 2019¹.

However, not all database systems implement the standard exactly the same way. Different database vendors may have their own variations and extensions of SQL, such as Microsoft SQL Server, Oracle Database, MySQL, PostgreSQL, SQLite, and others. Therefore, some SQL queries that work in one database system may not work in another.

SQL Components and Syntax

SQL consists of several components that perform different functions. The main components are:

• Data Definition Language (DDL): This component is used to define the structure and schema of the database, such as creating, altering, or dropping tables, views, indexes, constraints, triggers, etc.
• Data Manipulation Language (DML): This component is used to manipulate the data in the database, such as inserting, updating, deleting, or merging records.
• Data Query Language (DQL): This component is used to query or retrieve data from the database, such as selecting records that match certain criteria or joining data from multiple tables.
• Data Control Language (DCL): This component is used to control the access and permissions of the database, such as granting or revoking privileges or roles to users or groups.
• Transaction Control Language (TCL): This component is used to manage the transactions in the database, such as committing or rolling back changes or setting savepoints.

SQL queries are written using a series of statements or clauses that follow a specific syntax. The syntax may vary slightly depending on the database system and the standard version. However, a general syntax for a basic SQL query is:

SELECT [DISTINCT] column_list
FROM table_list
[WHERE condition]
[GROUP BY column_list]
[HAVING condition]
[ORDER BY column_list [ASC|DESC]];

The above syntax shows the order of execution of the clauses in a SQL query. The clauses inside square brackets are optional. The meaning of each clause is:

• SELECT: This clause specifies which columns or expressions to return as the result of the query. The DISTINCT keyword can be used to eliminate duplicate rows from the result.
• FROM: This clause specifies which tables or views to retrieve data from. Multiple tables or views can be joined using various join types and conditions.
• WHERE: This clause specifies which rows to filter based on a logical condition. Only the rows that satisfy the condition are returned.
• GROUP BY: This clause specifies how to group the rows based on one or more columns or expressions. The grouped rows can be aggregated using various aggregate functions such as SUM, AVG, COUNT, MIN, MAX, etc.
• HAVING: This clause specifies which groups to filter based on a logical condition. Only the groups that satisfy the condition are returned.
• ORDER BY: This clause specifies how to sort the rows based on one or more columns or expressions. The sorting order can be ascending (ASC) or descending (DESC).

SQL Examples

To illustrate how SQL works, let us consider a sample database that contains two tables: Customers and Orders. The Customers table stores information about customers such as customer ID, name, address, phone number, and email address. The Orders table stores information about orders placed by customers such as order ID, customer ID, order date, order amount, and order status.

The following are some examples of SQL queries that can be performed on this database:

• To select all columns and rows from the Customers table:

SELECT * FROM Customers;

• To select only customer ID and name from the Customers table:

SELECT CustomerID, Name FROM Customers;

• To select only distinct customer names from the Customers table:

SELECT DISTINCT Name FROM Customers;

• To select customer ID and name from the Customers table where customer ID is greater than 100:

SELECT CustomerID, Name FROM Customers WHERE CustomerID > 100;

• To select customer ID, name, and order amount from the Customers and Orders tables where customer ID is the same in both tables:

SELECT Customers.CustomerID, Customers.Name, Orders.OrderAmount
FROM Customers
INNER JOIN Orders
ON Customers.CustomerID = Orders.CustomerID;

• To select customer ID, name, and total order amount from the Customers and Orders tables where customer ID is the same in both tables, grouped by customer ID and name:

SELECT Customers.CustomerID, Customers.Name, SUM(Orders.OrderAmount) AS TotalOrderAmount
FROM Customers
INNER JOIN Orders
ON Customers.CustomerID = Orders.CustomerID
GROUP BY Customers.CustomerID, Customers.Name;

• To select customer ID, name, and total order amount from the Customers and Orders tables where customer ID is the same in both tables, grouped by customer ID and name, and having total order amount greater than 500:

SELECT Customers.CustomerID, Customers.Name, SUM(Orders.OrderAmount) AS TotalOrderAmount
FROM Customers
INNER JOIN Orders
ON Customers.CustomerID = Orders.CustomerID
GROUP BY Customers.CustomerID, Customers.Name
HAVING SUM(Orders.OrderAmount) > 500;

• To select customer ID, name, and total order amount from the Customers and Orders tables where customer ID is the same in both tables, grouped by customer ID and name, having total order amount greater than 500, and ordered by total order amount in descending order:

SELECT Customers.CustomerID, Customers.Name, SUM(Orders.OrderAmount) AS TotalOrderAmount
FROM Customers
INNER JOIN Orders
ON Customers.CustomerID = Orders.CustomerID
GROUP BY Customers.CustomerID, Customers.Name
HAVING SUM(Orders.OrderAmount) > 500
ORDER BY TotalOrderAmount DESC;

SQL Benefits and Challenges

SQL is a powerful and widely used language for managing data in relational databases. Some of the benefits of SQL are:

• SQL is a standard language that is supported by most database systems.
• SQL is a declarative language that allows users to specify what they want to do with the data without worrying about how it is done.
• SQL is a flexible language that can perform various operations on the data such as creating, manipulating, querying, analyzing, etc.
• SQL is a high-level language that can handle complex tasks with simple and concise commands.
• SQL is a portable language that can run on different platforms and devices.

However, SQL also has some challenges and limitations that users should be aware of. Some of the challenges are:

• SQL may have different syntaxes and features depending on the database system and the standard version.
• SQL may not be able to handle some types of data or operations that are not supported by relational databases, such as unstructured data or graph data.
• SQL may not be able to perform some tasks efficiently or optimally due to the limitations of the database system or the query optimizer.
• SQL may require additional skills or tools to integrate with other languages or applications.

Conclusion

SQL is a programming language that is used to interact with relational databases. It allows users to create, manipulate, and query data in tables using various commands and clauses. SQL is a standard language that is supported by most database systems. However, it may also have some variations and extensions depending on the database vendor. SQL is a powerful and flexible language that can handle various data operations. However, it may also have some challenges and limitations that users should be aware of.

Tweet

Read More..