A relational database query is a request for information from a database that matches a specific set of criteria. In other words, it is a question asked of the database to retrieve data that meets certain conditions. Queries are commonly used to search for specific information, analyze data, or generate reports.
Here's an example of a simple query that retrieves all the rows from a table named 'customers' where the 'state' column is equal to 'California':
sqlSELECT * FROM customers WHERE state = 'California';In this query, the 'SELECT' keyword is used to indicate that we want to retrieve data from the 'customers' table. The '*' symbol is used to indicate that we want to retrieve all the columns in the table. The 'FROM' keyword is used to specify the name of the table we want to query. The 'WHERE' clause is used to specify the condition that must be met for a row to be included in the result set.
In this example, only the rows where the 'state' column is equal to 'California' will be included in the result set. The result set will contain all the columns from the 'customers' table for those rows.
Queries can be much more complex than this example, and can involve multiple tables, conditions, and calculations. The SQL language provides a wide range of keywords, operators, and functions to enable developers to create powerful and flexible queries to meet their specific needs.
