SELECT Queries

The `SELECT` statement is the cornerstone of SQL, used for retrieving or 'querying' data from one or more database tables. It is the most frequently used command in SQL. The basic syntax is `SELECT column1, column2, ... FROM table_name;`. To retrieve all columns from a table without listing them individually, you can use the asterisk wildcard: `SELECT * FROM table_name;`. The `SELECT` statement has several optional clauses that allow you to refine your query to get precisely the data you need. The `FROM` clause is mandatory and specifies the table from which to retrieve the data. The `WHERE` clause filters the records and returns only those that match a specific condition. The `GROUP BY` clause groups rows that have the same values in specified columns into summary rows. It is often used with aggregate functions (`COUNT`, `MAX`, `MIN`, `SUM`, `AVG`) to perform calculations on each group. The `HAVING` clause is used to filter the results of a `GROUP BY` clause based on a condition involving an aggregate function. Finally, the `ORDER BY` clause sorts the result set in ascending (`ASC`) or descending (`DESC`) order based on one or more columns. Mastering the structure and order of these clauses is essential for writing effective SQL queries. The logical processing order is generally FROM, WHERE, GROUP BY, HAVING, SELECT, and finally ORDER BY.