A query is a formal way of asking a question or requesting information. In computer science, a query is essentially the same, with the only difference being that the answer or retrieved information comes from a database (a database query).
There are two types of database queries (or a combination of these two):
Select query - this is a query that retrieves data from a database.
Action query - this requests additional actions on data. Think of inserting, updating, deleting or other types of modifications.
This does not mean users can enter random requests, as the database does not speak human language. To communicate with a database, pieces of code from a specific programming language, a query language, are used.
Queries give meaning to code. It ensures that a database understands the request and can perform its tasks correctly.
Queries can perform a number of different tasks. It is mainly used to find specific data. Queries also help automate data management tasks, summarise data, and perform calculations.
A database stores data in multiple tables. They consist of columns containing the data's characteristics, along with rows or records of information. A query then helps to retrieve data from different tables, sort it, and display it as desired by the command.
Below are some query commands:
SELECT - this allows you to retrieve data from the database. It is one of the most popular commands because every request begins with a select query.
AND - this allows you to combine data from one or more tables.
CREATE TABLE - this allows you to create different tables and specify the name of each column.
ORDER BY - this can be used to sort data results numerically or alphabetically.
SUM - this is used to summarise data from a specific column.
UPDATE - this allows you to modify existing rows in a table.
INSERT - this is used to add new data or rows to an existing table.
WHERE - this allows you to filter data and obtain its value based on a specified condition.
To specify search queries, you can also combine commands. For example, you can use SELECT and AND together to make your desired request.
A language is crucial to work with a database and queries. Generally, only one language is used, SQL.
SQL databases are relational and use predefined schemas where you must specify the data structure. On the other hand, NoSQL databases are non-relational and have dynamic schemas for unstructured data.
Other query languages exist, such as XQL, XQuery, and XPath. These are mainly used for XML data objects.
A query is a request for data or information from a database or other data repository.
The purpose of a query is to retrieve specific data from a database or other data repository based on certain criteria or conditions.
There are several types of queries, including select queries, action queries, parameter queries, and aggregate queries.
