A query is a request for data or information from a database table or combination of tables. This query can be thought of as providing a "view" of the database, which means it presents a specific slice or interpretation of the database data according to the query's conditions and structure. Queries that view the database contents use the keyword.
The simplest form of SELECT query uses the * wildcard operator to retrieve all database from the table. For example, a basic SQL SELECT query that retrieves all columns for all customers in the Customers table is written as: SELECT * FROM ;. If you only want to retrieve data from certain columns, then you can specify the data that you wish to retrieve using a statement like: SELECT CustomerName, EmailAddress FROM Customers;.
Queries can filter data based on specific criteria, showing only the data that meets these conditions. For instance, if you want to select employees in the Marketing department, you would use a query like: SELECT name, department, salary FROM employees WHERE department = '';. The ORDER BY clause in SQL is used to sort the result set of a query by one or more , allowing you to organize data in ascending or descending order, based on the specified (s).
The LIKE operator in SQL is used in a WHERE clause to search for a specified pattern in a column. It's particularly useful when you want to perform searches on strings, such as finding rows with columns that contain a certain substring, begin with specific characters, or end with specific characters. The % and _ are two wildcards often used with LIKE, where % represents zero, one, or multiple , and _ represents a single character. For example, you could retrieve all rows from Employees where the Name starts with 'J' by using the query: SELECT * FROM Employees WHERE Name LIKE '%';.
