What is an aggregate function?

Aggregation function is a special function that performs calculations on a set of values ​​and returns a single value. These functions are mainly used to summarize and calculate data to better understand the data and make decisions. Common aggregate functions include SUM(), AVG(), MIN(), MAX(), COUNT(), etc. These functions may differ slightly in different database systems, but their basic concepts and usage are similar.

An aggregate function is a special function that performs a calculation on a set of values ​​and returns a single value. These functions are mainly used to summarize and calculate data to better understand the data and make decisions. Aggregation functions are widely used in database query languages ​​(such as SQL) to help users perform aggregation operations on data.

The characteristic of aggregate functions is that they ignore NULL values ​​in columns. This means that when performing aggregation calculations, NULL values ​​will be ignored and will not affect the calculation results. In addition, aggregate functions also have some other features, such as they return a single value, can perform calculations on multiple rows of data, can be used in conjunction with a GROUP BY clause, etc.

Common aggregate functions include SUM(), AVG(), MIN(), MAX() and COUNT(), etc. These functions may differ slightly in different database systems, but their basic concepts and usage are similar.

1. The SUM() function is used to calculate the sum of a column. It returns the sum of all values ​​in the specified column.

For example: SELECT SUM(salary) FROM employees;

This will return the total salary of all employees in the "employees" table.

2. The AVG() function is used to calculate the average of a certain column. It returns the average of all values ​​in the specified column.

For example: SELECT AVG(salary) FROM employees;

This will return the average salary of all employees in the "employees" table.

3. The MIN() function is used to return the minimum value of a certain column. It returns the minimum value in the specified column.

For example: SELECT MIN(salary) FROM employees;

This will return the minimum salary of all employees in the "employees" table.

4. The MAX() function is used to return the maximum value of a column. It returns the maximum value in the specified column.

For example: SELECT MAX(salary) FROM employees;

This will return the highest salary of all employees in the "employees" table.

5. The COUNT() function is used to count the number of rows in a column. It returns the number of values ​​in the specified column.

For example: SELECT COUNT(employee_id) FROM employees;

This will return the number of all employees in the "employees" table.

In addition to these common aggregate functions, there are other aggregate functions, such as STD(), VAR_POP(), VAR_SAMP(), etc. These functions are used to calculate statistical indicators such as standard deviation, population variance, and sample variance respectively.

Aggregation functions are very useful in data analysis, report generation and data mining. By using aggregate functions, users can extract meaningful information from large amounts of data, summarize and calculate it to better understand the data and make decisions.

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