8c How to modify a database | 现代数学启蒙

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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!