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Sure! Let's break down each of these SQL queries and commands with examples to illustrate their use:
SELECT ... FROM
The
SELECT statement is used to query the database and retrieve data.Example:
This will select the
first_name and last_name columns from the employees table.WHERE
The
WHERE clause is used to filter records that meet certain conditions.Example:
This will select employees who work in the Sales department.
ORDER BY
The
ORDER BY clause is used to sort the result set by one or more columns.Example:
This will sort the employees by
last_name in ascending order.GROUP BY
The
GROUP BY clause is used to group rows that have the same values in specified columns into summary rows.Example:
This will count the number of employees in each department.
INNER JOIN
The
INNER JOIN clause is used to combine rows from two or more tables based on a related column.Example:
This will retrieve employees' first and last names along with their department names.
SUM
The
SUM function calculates the total sum of a numeric column.Example:
This will calculate the total salary for each department.
COUNT
The
COUNT function returns the number of rows that match a specified condition.Example:
This will count the number of employees in the Sales department.
AVG
The
AVG function calculates the average value of a numeric column.Example:
This will calculate the average salary for each department.
INSERT INTO
The
INSERT INTO statement is used to add new rows to a table.Example:
This will insert a new employee into the
employees table.DELETE FROM
The
DELETE FROM statement is used to delete existing records from a table.Example:
This will delete the employee with
employee_id 123 from the employees table.UPDATE
The
UPDATE statement is used to modify existing records in a table.Example:
This will update the salary of the employee with
employee_id 123 to 55,000.These examples cover the basics of querying and maintaining data in a relational database using SQL. Let me know if you need further explanations or more advanced examples!
- 作者:现代数学启蒙
- 链接:https://www.math1234567.com/article/modifydatabase
- 声明:本文采用 CC BY-NC-SA 4.0 许可协议，转载请注明出处。
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