Under what circumstances is group by used in mysql?

The GROUP BY clause is used to group and aggregate data by columns. Common scenarios include: data summary calculations (sum, average, maximum, minimum, etc.) data classification (grouping by category facilitates analysis and visualization) ) Data deduplication (keep only unique records for each group) Data aggregation in subqueries Query performance optimization (by reducing the number of records scanned) Note: The grouping key must be in the query column Aggregation functions can only be applied to the grouping key or Aggregation results do not specify all non-aggregated columns, which will produce duplicate rows. GROUP BY is usually used in conjunction with the HAVING clause to further filter the grouped results

GROUP in MySQL BY usage scenarios

GROUP BY clause is used to group query results by one or more columns and aggregate the grouped results. Specific usage scenarios include:

1. Data summary

• Calculate the sum, average, maximum value, minimum value, etc. of records within a group
• For example: SELECT SUM(sales) FROM orders GROUP BY product_id;

2. Data classification

• Group data by category for convenience Analysis and visualization
• For example: SELECT category, COUNT(*) FROM products GROUP BY category;

3. Data deduplication

• Retain only the unique records in each group after grouping
• For example: SELECT DISTINCT name FROM customers GROUP BY name;

4. Subquery

• Use GROUP BY in the subquery to aggregate data, and then use it as input to the external query
• For example: SELECT name FROM customers WHERE id IN (SELECT customer_id FROM orders GROUP BY customer_id HAVING COUNT(*) > 1);

5. Performance optimization

• For situations where a large amount of data needs to be aggregated, GROUP BY can Improve query performance by reducing the number of records scanned by the database

Things to consider:

• The grouping key must be a column in the query, Otherwise an error will be raised
• Aggregation functions can only be applied to grouping keys or aggregate result columns
• If you do not specify all non-aggregated columns in the GROUP BY clause, it will result in duplicate rows in the result
• The GROUP BY clause is usually used together with the HAVING clause to further filter the grouped results

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