A query is a request sent to a system to retrieve, filter, sort, combine or modify data. In relational databases, SQL statements can use commands such as SELECT, INSERT, UPDATE and DELETE. The term also applies to website searches and API requests, which do not necessarily work in the same way as SQL database queries.
This guide explains query functions, examples, processing and security considerations for website administrators. Important: SQL commands such as UPDATE and DELETE can alter or remove data. Do not run historical examples against production systems without backups, testing and appropriate permissions.
What is the Definition of a Query?
Users can trigger data requests from forms or URLs, but good applications don’t hand raw SQL strings from the browser to the database. The server layer validates parameters and controls permissions. WordPress, website features can use CMS query functions that add their own search logic.
A query is a request for information or a question posed to a system or other information resource. In the context of information technology, queries are typically used to retrieve data or information from a database or other data storage system. Queries can be posed in the form of questions written in a specific programming language or using a user interface provided by the system.
In database systems, queries are often submitted using SQL (Structured Query Language), which allows users to search for data from tables stored in the database. In the context of websites, a query is a request for information submitted by a user through a user interface or through a URL (Uniform Resource Locator). For example, when a user types keywords into a website’s search box or clicks on a link, a query is sent to the website’s server to retrieve the desired information or page.
The server then processes the query and returns the results to the user through the website’s user interface. The results of this query are usually web pages generated from data or information stored on the server or other data storage system.
Query Function
The function of reading information must be distinguished from the function of modifying it. With read-only access, an administrator can test SELECTs without authorizing deletion of data. Queries can also impact performance when run repeatedly; monitor server maintenance And request distribution so that the system does not just increase the number of backends without fixing the database bottleneck.
Querying a website is used to retrieve data or information requested by users from a server or other data storage system. Some of the main functions of queries on a website include:
1. Displaying Data
Queries can be used to display data stored on a server or other data storage system. For example, when a user types a keyword into a website’s search box, the server displays data that matches that keyword.
2. Searching for Data
Queries can also be used to search for data that meets specific criteria, such as finding data that has a specific value or meets a specific condition. For example, when a user searches for a product with a specific price in an online store, a query is used to find product data that meets that price criterion.
3. Sorting Data
Queries can sort data based on specific columns, such as by name or date. For example, if a user wants to display product data on a website in alphabetical order, the query sorts the product data by product name.
4. Merging Data
Queries can combine data from multiple tables or other data storage systems, allowing the website server to present information spread across multiple tables or data storage systems on a single website page.
5. Modifying Data
Queries can also be used to modify data stored on a server or other data storage system, such as adding, deleting, or updating existing data. For example, when a user adds a new product to a website, the query modifies the product data by adding the new product.
6. Deleting Data
Queries can be used to remove unwanted or obsolete data from a database. This helps reduce the accumulation of data waste in the database and keeps the database clean and efficient.
6 Example Queries
Safety notes: The six examples in the original material use illustrative table or column names that are not always consistent. The SELECT examples can be reviewed in the practice database, but UPDATE And DELETE not a read query; both can change permanent data. Do not run them on real customer data.
The following are some examples of queries that can be used in database systems using the SQL (Structured Query Language) programming language:
1. Displaying Data
Display all data from the “Customers” table:
SELECT * FROM Customers;
2. Searching for Data
Search for customer data residing in the city of “Bandung”:
SELECT * FROM Customers WHERE City = ‘Bandung’;
3. Sorting Data
Sort customer data by city:
SELECT * FROM Customer ORDER BY City;
4. Merging Data
Combine data from the “Customers” and “Orders” tables:
SELECT Customers.*, Orders.* FROM Customers INNER JOIN Orders ON Customers.OrderID = Orders.OrderID;
5. Modifying Data
Modify customer data residing in the city of “Bandung” to “Surabaya”:
UPDATE Customer SET City = ‘Surabaya’ WHERE City = ‘Bandung’;
6. Deleting Data
Deleting data for customers residing in the city of “Bandung”:
DELETE FROM Customers WHERE City = ‘Bandung’;
These are just a few examples of queries that can be used in a database system using SQL. There are many other queries that can be used depending on specific needs and conditions.
How Queries Work
The general process involves parsing, permission validation, execution planning, index or table reading, and result delivery. Selecting the right index can reduce scan operations, but this depends on the query pattern. Open source database software have different optimizers; there is no one universal index strategy.
The way queries work varies depending on the programming language used and the data storage system available. Here’s a general overview of how queries work:
1. Submitting a Query to the System
Users submit queries to a data storage system, either through a user interface or by writing queries in a specific programming language. For example, in a database system, a user might write an SQL query and submit it to the database server.
2. Query Acceptance by the System
After a query is submitted, the data storage system receives it and processes it according to the instructions provided. The system reads the commands in the query and determines the required action, such as searching for data, displaying data, or modifying data.
3. Data Search by System
The data storage system then executes the query by searching for data that matches the specified criteria. This search process involves scanning tables or indexes within the database to find relevant data.
4. Returning Query Results
Once the requested data is found, the system returns the query results to the user. These results can be data tables, reports, or views processed according to the query instructions. The user can view and use the query results as needed.
5. Evaluate Query Results
Users evaluate query results and utilize them as needed. This evaluation can involve analyzing the data, making decisions based on the query results, or using the data for business purposes. Evaluation results can also be used to modify future queries to make them more effective and efficient.
What Is a Query Language?
SQL is a declarative language for data relations. GraphQL is a query API specification; SPARQL is for RDF; the term NoSQL refers to a family of databases with varying data models and syntax, rather than a single query language. In applications that process public input, use PDO prepared statements for data parameters to avoid stringifying raw input into SQL.
A query language is a programming language used to send questions or requests for information to systems or other information resources. In the context of information technology, query languages are often used to access and manipulate data from databases or other data storage systems. A popular example of a query language is SQL (Structured Query Language).
SQL (Structured Query Language)
SQL is the standard language for accessing and manipulating data in relational database systems. SQL provides a variety of commands that can be used to display, search, sort, merge, modify, delete, or create data from tables stored in a database. SQL is used by various database systems such as MySQL, Oracle, PostgreSQL, and SQL Server.
Other Query Languages
Besides SQL, there are also other query languages used in database systems or other data storage systems, such as:
- NoSQL: Used in NoSQL databases such as MongoDB, Cassandra, and CouchDB. NoSQL is designed to store and manage unstructured or semi-structured data.
- SPARQL: Used to access data stored in RDF (Resource Description Framework) format and used in ontology-based or linked data systems.
- GraphQL: A query language used to access and manipulate data in API-based applications, allowing clients to request specific data according to their needs.
Query languages enable users to interact with data storage systems in a structured and efficient manner. Using query languages, users can ask complex questions, perform data analysis, and obtain relevant information to support decision-making.
Difference between Read Queries and Data Change Queries
| SQL Commands | Objective | Main risks |
|---|---|---|
| SELECT | Reading and filtering rows | Can be expensive on large tables or leak data if permissions are incorrect. |
| INSERT | Add a row | Data duplication and inadequate validation |
| UPDATE | Change row | Mass change if WHERE is false |
| DELETE | Deleting rows | Data loss if there is no backup and transactions |
Safe Query Examples for Search
The following illustrative PHP example with PDO separates SQL and input data. Check the database connection and user privileges before running it in a test environment.
<?php
$stmt = $pdo->prepare(
'SELECT id, nama FROM pelanggan WHERE kota = :kota LIMIT 20'
);
$stmt->execute(['kota' => 'Bandung']);
$rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
?>Parameters are suitable for values such as city, but not substitutions for table identifiers or column names; those must be selected from the allowlist. See PHP SQL Injection security guide And technical SEO if the performance of this endpoint affects the rendering of public pages.
Database Query Testing Checklist
- Use test data and minimum access rights, not the application superuser account.
- Backup and test recovery before data change operations.
- Input validation and bind parameters for dynamic values.
- Test WHERE conditions and result limits in a non-production environment.
- Check EXPLAIN, index and response time as per DBMS.
- Restrict access to sensitive data and securely record audits.
When a query affects a public page, crawling, indexing, And canonical URL need to be monitored. Infrastructure proxy server or IP routing does not replace SQL parameterization. For broader applications, consider website performance as well as URL structure And navigation as different aspects.
Frequently Asked Questions (FAQ)
What is a query?
Requests to read, search, sort, or modify data in the system.
What is the difference between SQL query and search query?
SQL queries access databases, while search queries are search requests that can be processed by different applications.
What is the function of SELECT?
Read rows and columns according to query conditions.
What is the function of UPDATE?
Modifies the row values that satisfy the conditions.
Why is DELETE risky?
A condition error can delete unintended data.
What is SQL injection?
Vulnerability when untrusted input changes the structure of SQL commands.
How to prevent SQL injection?
Use prepared statements with minimum parameters, validation, and access rights.
Is NoSQL a query language?
No. NoSQL is a family of database models with diverse query mechanisms.
What is the function of an index in a query?
Can speed up certain searches at the cost of storage and maintenance.
Can sample queries be run in production?
Don’t just run historical examples; test in a non-production environment and audit first.
