Queries allow access to data stored in databases.

What is a Query?

In its simplest definition, a query is a request made to a database. Queries enable communication with databases, allowing necessary data to be retrieved, new data to be written, and existing data to be modified or deleted.

In software development, databases are used to store data for later retrieval. These databases can have different structures. In widely used relational databases, data is stored in different tables that are related to each other. Non-relational databases, on the other hand, have more variety within themselves. Regardless of the type, queries allow operations on the data within databases. The query language used varies depending on the type of database.

What is a Query Language?

A query language is a language used to create queries and communicate with databases. By following the syntax rules of these languages, necessary filters are applied to the data in the database to obtain the desired data. The most well-known query language is SQL (Structured Query Language). In addition to SQL, there are dozens of different query languages such as GraphQL, JSONiq, Gremlin, FQL, DMX, and CodeQL.

Query languages are used by software developers and database administrators in software development processes. An interface is developed to prevent users from directly performing database operations, offering a more secure and easy-to-use solution. However, in various industries, direct use of query languages may also be preferred. Especially in finance, marketing, data analysis, and data science, the use of different query languages is common.

SQL Query Language

SQL, which stands for Structured Query Language, is commonly known by its acronym. Although often mistakenly thought of as a programming language, SQL is actually a query language that enables operations on relational databases. Its purpose is not to develop software but to communicate with databases. SQL can be used with many different database management systems, including MySQL, PostgreSQL, Oracle, and Microsoft SQL Server.

To prepare SQL queries, you need to be familiar with some concepts. A database is a collection that contains all tables in relational databases. Tables are structures that store data in rows and columns. A primary key is a unique value defined for a row, allowing relationships to be established between tables.

Here are some of the most commonly used SQL queries to help you better understand the structure of SQL queries:

• Create: The Create command is used to create new databases and tables. The example below creates a table named kisi_listesi and creates columns named ad (name) and soyad (surname).

CREATE TABLE kisi_listesi (ad VARCHAR(50), soyad VARCHAR(50));

• Select: The Select command is used to retrieve data from the database. The example below retrieves all data in the ad column.

SELECT ad FROM kisi_listesi;

• Insert: The Insert command is used to insert data into tables. The example below inserts data into the table named kisi_listesi.

INSERT INTO kisi_listesi (ad, soyad) VALUES (‘Ali’, ‘Ayşe’);

Queries facilitate communication with databases, allowing operations to be performed on the data stored within them. Understanding and using queries effectively is essential for efficient database management and software development.