SQL Select
How Select used in table?
The SELECT statement used to query and retrieve data from one or more tables.
It allows you to specify the columns you want to retrieve, apply filters, and sort the results.
Syntax:
SELECT column1, column2, ...
FROM table_name
WHERE condition;
- column1, column2, ...: The columns you want to retrieve. You can use * to select all columns.
- table_name: The name of the table from which you want to retrieve data.
- WHERE condition: Optional. It allows you to filter the rows based on a specified condition.
Example 1: Select All Columns
SELECT * FROM employees;
This retrieves all columns for all records in the employees table.
Example 2: Select Specific Columns
SELECT employee_id, first_name, last_name FROM employees;
This retrieves only the specified columns (employee_id, first_name, and last_name) for all records in the employees table.
Example 3: Filter with WHERE Clause
SELECT * FROM employees
WHERE salary > 50000;
This retrieves all columns for employees with a salary greater than 50000.
Example 4: Combining Conditions
SELECT * FROM employees
WHERE salary > 50000 AND hire_date < '2023-01-01';
This retrieves all columns for employees with a salary greater than 50000 hired before January 1, 2023.
Example 5: Sorting Results
SELECT * FROM employees
ORDER BY salary DESC;
This retrieves all columns for all employees and sorts the results by salary in descending order.
Example 6: Limiting Results
SELECT * FROM employees
LIMIT 10;
This retrieves the first 10 rows from the employees table.
Example 7: Aggregation Functions
SELECT AVG(salary) AS average_salary, MAX(salary) AS max_salary
FROM employees
WHERE hire_date > '2022-01-01';
This calculates the average and maximum salary for employees hired after January 1, 2022.
Tips:
- Use aliases (e.g., AS average_salary) to rename columns or results of calculations for better readability.
- Be mindful of data types and ensure that conditions in the WHERE clause are appropriate for the data.