group by usage

GROUP BY is a SQL aggregation function that groups table data by columns and performs aggregated calculations on each group. Syntax: 1. Specify grouped columns: The column to group in the GROUP BY clause specifies the column to be grouped. 2. Perform aggregation calculation: Use the aggregation function to calculate grouped columns. 3. Get results based on grouping: The query returns aggregated results, with one group for each behavior.

GROUP BY Usage

definition

GROUP BY is a SQL aggregation function that groups data in a table based on columns and aggregates specific columns in each group.

grammar

 <code class="sql">SELECT 聚合函数(列名) FROM 表名GROUP BY 分组列名;</code>

parameter

• Aggregation function: functions used to perform aggregate calculations, such as SUM, COUNT, AVG, etc.
• Column name: The column used for aggregation calculations.
• Grouped column name: The column used for grouping.

usage

The usage of GROUP BY is as follows:

1. Specify grouped columns: Specify the column to be used for grouping in the GROUP BY clause.
2. Perform aggregation calculation: Use the aggregate function to calculate the columns of each group.
3. Get results based on groups: After the grouping is completed, the SQL query returns the results of the aggregate calculation, where each row represents a group.

Example

The following example demonstrates how to use GROUP BY to calculate the number of employees in different departments:

 <code class="sql">SELECT 部门, COUNT(*) AS 人数FROM 员工表GROUP BY 部门;</code>

The query results are as follows:

department	Number of people
Sale	10
market	5
R&D	15

advantage

The main advantages of GROUP BY include:

• Data Grouping: It allows the data to be organized into different groups.
• Data Summary: Through aggregation calculation, it provides a summary view of the data.
• Performance optimization: It improves the performance of querying large datasets because it reduces the number of rows returned by the query.

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