what is sql structured query language

What is SQL? Structured Query Language

SQL, or Structured Query Language, is a programming language that allows you to interact with a relational database. A relational database is a system that stores and organizes data in tables, where each table consists of rows and columns. SQL enables you to perform various operations on the data, such as creating, updating, deleting, querying, and analyzing.

In this article, you will learn:

• What are the benefits of using SQL?
• What are the main components of SQL?
• How to write and execute SQL queries?
• What are some common SQL commands and keywords?
• What are some examples of SQL queries for different scenarios?

Benefits of Using SQL

SQL is one of the most widely used and popular languages for data manipulation and analysis. Some of the benefits of using SQL are:

• Standardized: SQL is a standardized language that follows a set of rules and syntax. This means that you can use SQL with different relational database management systems (RDBMS), such as MySQL, Oracle, SQL Server, PostgreSQL, and more. You can also use SQL with various programming languages, such as Python, Java, C#, and more.
• Easy to learn: SQL is a declarative language, which means that you only need to specify what you want to do with the data, not how to do it. SQL uses simple English keywords and phrases, such as SELECT, FROM, WHERE, GROUP BY, etc., which makes it easy to understand and write.
• Powerful: SQL can handle large amounts of data efficiently and effectively. You can use SQL to perform complex calculations, aggregations, transformations, and joins on the data. You can also use SQL to create views, functions, stored procedures, triggers, and indexes to optimize the performance and functionality of the database.
• Versatile: SQL can be used for various purposes and applications. You can use SQL to create and manage databases, tables, and relationships. You can also use SQL to query and analyze data for business intelligence, reporting, data science, machine learning, web development, and more.

Components of SQL

SQL consists of several components that define the structure and functionality of the language. The main components of SQL are:

• Data Definition Language (DDL): DDL is used to create and modify the structure of the database objects, such as tables, views, indexes, etc. Some common DDL commands are CREATE, ALTER, DROP, RENAME, etc.
• Data Manipulation Language (DML): DML is used to insert, update, delete, and retrieve data from the database tables. Some common DML commands are INSERT, UPDATE, DELETE, SELECT, etc.
• Data Control Language (DCL): DCL is used to control the access and permissions of the database users and roles. Some common DCL commands are GRANT, REVOKE, etc.
• Transaction Control Language (TCL): TCL is used to manage the transactions in the database. A transaction is a logical unit of work that consists of one or more statements that either succeed or fail as a whole. Some common TCL commands are COMMIT, ROLLBACK, SAVEPOINT, etc.

Writing and Executing SQL Queries

A SQL query is a statement that requests data from the database. A query can consist of one or more clauses that specify different aspects of the query. The basic syntax of a SQL query is:

SELECT column1,column2,... FROM table1
WHERE condition1
GROUP BY column3
HAVING condition2
ORDER BY column4;

The clauses in a SQL query are:

• SELECT: This clause specifies which columns or expressions to return as the result set.
• FROM: This clause specifies which table or tables to retrieve data from.
• WHERE: This clause specifies which rows or records to filter based on a condition.
• GROUP BY: This clause specifies how to group the rows or records based on one or more columns or expressions.
• HAVING: This clause specifies which groups to filter based on a condition.
• ORDER BY: This clause specifies how to sort the result set based on one or more columns or expressions.

To execute a SQL query, you need to connect to a database server using a client application or tool. Depending on the tool you use, you may need to specify the database name, username, password, host name, port number, etc. Once you are connected to the database server,

you can write your query in a text editor or an interactive shell. Then you can run your query by pressing a button or entering a command.

The result set of your query will be displayed in a tabular format or exported to a file or another application.

Common SQL Commands and Keywords

SQL has many commands and keywords that perform different functions and operations on the data. Here are some of the most common ones:

• CREATE: This command is used to create a new database object such as a table, view, index, etc. For example, the following statement creates a table called Customers with four columns: CustomerID, CustomerName, ContactName, and Country.

CREATE TABLE Customers (
  CustomerID int,
  CustomerName varchar(255),
  ContactName varchar(255),
  Country varchar(255)
);

• INSERT: This command is used to insert one or more rows or records into a table. For example, the following statement inserts a new row into the Customers table with the values 1, ‘ABC Inc.’, ‘John Smith’, and ‘USA’.

INSERT INTO Customers (CustomerID, CustomerName, ContactName, Country)
VALUES (1, 'ABC Inc.', 'John Smith', 'USA');

• UPDATE: This command is used to modify one or more rows or records in a table based on a condition. For example, the following statement updates the ContactName column of the Customers table to ‘Jane Doe’ where the CustomerID is 1.

UPDATE Customers
SET ContactName = 'Jane Doe'
WHERE CustomerID = 1;

• DELETE: This command is used to delete one or more rows or records from a table based on a condition. For example, the following statement deletes the row from the Customers table where the CustomerID is 1.

DELETE FROM Customers
WHERE CustomerID = 1;

• SELECT: This command is used to retrieve data from one or more tables based on various criteria. For example, the following statement selects all the columns and rows from the Customers table.

SELECT * FROM Customers;

• DISTINCT: This keyword is used to eliminate duplicate values from a column or expression in a SELECT statement. For example, the following statement selects the distinct values of the Country column from the Customers table.

SELECT DISTINCT Country FROM Customers;

• AS: This keyword is used to assign an alias or a temporary name to a column or expression in a SELECT statement. For example, the following statement selects the CustomerName column as Name and the ContactName column as Contact from the Customers table.

SELECT CustomerName AS Name, ContactName AS Contact FROM Customers;

• AND: This keyword is used to combine two or more conditions in a WHERE clause. The result is true only if all the conditions are true. For example, the following statement selects the rows from the Customers table where the Country is ‘USA’ and the ContactName starts with ‘J’.

SELECT * FROM Customers
WHERE Country = 'USA' AND ContactName LIKE 'J%';

• OR: This keyword is used to combine two or more conditions in a WHERE clause. The result is true if any of the conditions is true. For example, the following statement selects the rows from the Customers table where the Country is either ‘USA’ or ‘Canada’.

SELECT * FROM Customers
WHERE Country = 'USA' OR Country = 'Canada';

• NOT: This keyword is used to negate a condition in a WHERE clause. The result is true if the condition is false. For example, the following statement selects the rows from the Customers table where the Country is not ‘USA’.

SELECT * FROM Customers
WHERE NOT Country = 'USA';

• IN: This keyword is used to check if a value belongs to a list of values in a WHERE clause. The result is true if the value matches any of the values in the list. For example, the following statement selects

the rows from the Customers table where the Country is either ‘USA’, ‘Canada’, or ‘Mexico’.

SELECT * FROM Customers
WHERE Country IN ('USA', 'Canada', 'Mexico');

• BETWEEN: This keyword is used to check if a value falls within a range of values in a WHERE clause. The result is true if the value is greater than or equal to the lower bound and less than or equal to the upper bound. For example, the following statement selects

the rows from the Customers table where the CustomerID is between 10 and 20.

SELECT * FROM Customers
WHERE CustomerID BETWEEN 10 AND 20;

• LIKE: This keyword is used to check if a value matches a pattern in a WHERE clause. The result is true if the value matches the pattern. The pattern can contain two special characters: % and _. The % character matches any sequence of zero or more characters. The _ character matches any single character. For example,

the following statement selects

the rows from the Customers table where

the CustomerName contains an ‘a’.

SELECT * FROM Customers
WHERE CustomerName LIKE '%a%';

• COUNT: This function is used to return

the number of rows or records that match

a condition or expression in a SELECT statement.

For example,

the following statement returns

the number of