Group by usage in sql

The GROUP BY statement in SQL groups the data set by specified columns and performs aggregate calculations on each group. Usage is as follows: Identify grouping columns: Specify the columns to be grouped. Aggregation functions: Use SUM, COUNT, AVG and other functions to calculate grouped values. Grouped results: The query returns grouped results, showing the aggregated calculated value of each group.

GROUP BY Usage in SQL

The GROUP BY statement in SQL is used to group rows in a data set , and aggregate the data according to the grouping. It works by grouping rows that have the same value for a specific column.

Basic syntax:

<code class="sql">SELECT aggregate_function(column_name)
FROM table_name
GROUP BY column_name</code>

Usage:

1. Identifying grouping columns: Usage The GROUP BY clause specifies the columns to be grouped.
2. Aggregation functions: Use aggregate functions (such as SUM, COUNT, AVG) to calculate values ​​in grouped columns.
3. Group results: The query returns grouped results, where each row represents a group and displays the aggregate calculation results.

Example:

Suppose we have a table containing student grades:

##3John 954Susan80

Student number	Name	Achievements
1	John	85
2	Mary	90

The following query sorts student scores by name Group and calculate the average of each student's score:

<code class="sql">SELECT AVG(成绩)
FROM students
GROUP BY 姓名</code>

Result:

##NameJohnMarySusan

Average score
90
90
80

Note:

The values ​​in the grouping column must Have equal data types.

• The GROUP BY clause must come after the FROM clause and before the HAVING clause.
• HAVING clause can be used to further filter the grouped results.

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