SQL Select Database

The USE DATABASE statement is a command in certain SQL-based database management systems that allows users to select and set a specific database as the default for the current session. By selecting a database, subsequent queries are executed within the context of that database, making it easier to interact with tables and other objects contained within it.

Additionally, the SELECT statement in SQL is used to query and retrieve data from the tables within the selected database. In this article, We will learn about SQL Select Database by understanding various examples in detail and so on.

The USE DATABASE Statement

The USE DATABASE statement is not a standard SQL command, but rather a variation of the USE command used in some SQL database management systems (DBMS) to select a specific database for the current session.
This command sets the default database for subsequent queries in that session.

Query:

USE database_name;

SQL Select Database

The SELECT statement in SQL is used to query and retrieve data from a database.
It specifies which columns of data we want to retrieve and from which tables.
However, if we are asking about “selecting” a database, we might be referring to how to choose or switch between databases in SQL.

Syntax to Select a Database in SQL

The syntax to select a database in SQL is:

USE DATABASE_NAME;

Example of SQL Select Database

Let’s go through each SQL query and show the expected output based on the provided employees table.

Table: employees

id	name	age	department	salary
1	Alice	30	Sales	50000
2	Bob	40	Marketing	60000
3	Charlie	35	Sales	55000
4	David	28	HR	45000
5	Eve	45	Marketing	65000
6	Frank	50	HR	70000
7	Grace	29	IT	48000
8	Hannah	38	IT	53000

1. Basic SELECT Statement

Query:

SELECT * FROM employees;

Output:

id	name	age	department	salary
1	Alice	30	Sales	50000
2	Bob	40	Marketing	60000
3	Charlie	35	Sales	55000
4	David	28	HR	45000
5	Eve	45	Marketing	65000
6	Frank	50	HR	70000
7	Grace	29	IT	48000
8	Hannah	38	IT	53000

Explanation: Retrieves all columns and all rows from the employees table.

2. Selecting Specific Columns

Query:

SELECT name, age FROM employees;

Output:

name	age
Alice	30
Bob	40
Charlie	35
David	28
Eve	45
Frank	50
Grace	29
Hannah	38

Explanation: Retrieves only the name and age columns for all rows.

3. Filtering Results with WHERE

Query:

SELECT name, age FROM employees WHERE age >= 35;

Output:

name	age
Bob	40
Charlie	35
Eve	45
Frank	50
Hannah	38

Explanation: Retrieves names and ages of employees older than 35.

4. Sorting Results with ORDER BY

Query:

SELECT name, age FROM employees ORDER BY age DESC;

Output:

name	age
Frank	50
Eve	45
Bob	40
Hannah	38
Charlie	35
Alice	30
Grace	29
David	28

Explanation: Retrieves names and ages of all employees sorted by age in descending order.

5. Limiting Results with LIMIT Clause

Query:

SELECT name, salary FROM employees ORDER BY salary DESC LIMIT 3;

Output:

name	salary
Frank	70000
Eve	65000
Bob	60000

Explanation: Retrieves the top 3 highest-paid employees, ordered by salary in descending order.

6. Aggregating Data with GROUP BY and Aggregation Functions

Query:

SELECT department, AVG(salary) AS average_salary FROM employees GROUP BY department;

Output:

department	average_salary
Sales	52500
Marketing	62500
HR	57500
IT	50500

Explanation: Calculates the average salary for each department.

Conclusion

The ORDER BY clause is an essential component in PL/SQL for controlling the order of query results. By using this clause, developers can enhance the readability and usefulness of their data, whether by arranging records by a single column or employing more advanced techniques like case-insensitive sorting or sorting by calculated columns.