What does group by mean in mysql

GROUP BY is a SQL aggregate function used to group data rows by a specified column and perform calculations for each group. Its syntax is: SELECT column name, aggregate function (column name) FROM table name GROUP BY column name; it allows data summary, aggregation and statistical analysis, and can optimize query performance.

GROUP BY in MySQL

What is GROUP BY?

GROUP BY is a SQL aggregate function used to group data rows with the same value and perform aggregate calculations on each group.

How to use GROUP BY?

The GROUP BY clause is used in the SELECT statement to specify the column by which to group. The syntax is as follows:

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

Example:

For example, the following query will group the customers table by the country field , and calculate the total number of customers per country:

<code class="sql">SELECT country, COUNT(*) AS total_customers
FROM customers
GROUP BY country;</code>

Benefits of GROUP BY:

• Data summary: GROUP BY allows you to Summarize large data sets into summaries that are more manageable and understandable.
• Data aggregation: Various data aggregations can be performed by using aggregate functions such as SUM, COUNT, and AVERAGE.
• Statistical Analysis: GROUP BY can be used to perform statistical analysis, such as finding the most common items or calculating averages.
• Performance Optimization: Aggregation queries are generally faster than queries that process ungrouped data because they significantly reduce the size of the result set.

Note:

• The columns specified in the GROUP BY clause must appear in the SELECT list or as parameters of an aggregate function.
• The columns in the GROUP BY clause must be of the same type.
• If the column that is grouped by contains NULL values, the NULL values ​​will form a separate group.

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