In MySQL queries, GROUP BY and COUNT() are often used to count the number of grouped records. Its core usage includes: 1. The basic syntax is SELECT field, COUNT() AS alias FROM table GROUP BY field; 2. You can group by multiple fields, such as GROUP BY field 1 and field 2; 3. Use WHERE to filter the original data, such as filtering a specific year; 4. Use HAVING to filter the grouping results, such as keeping records with COUNT() greater than a specific value; 5. All non-aggregated fields in SELECT must be included in GROUP BY, otherwise an error may be reported.
In MySQL queries, GROUP BY and COUNT() are a set of functions that are often used together. They can help us count the number of records in each group and are commonly used combinations in scenarios such as data analysis and report generation.
For example, if you want to know the number of customers of each salesperson in a sales table, or check the number of visits per day in the user behavior log, you can use GROUP BY and COUNT() .
Basic syntax structure
GROUP BY is used to group the result set by one or more columns, while COUNT() function is used to calculate the number of rows in each group. The basic writing method is as follows:
SELECT column_name, COUNT(*) AS count_alias FROM table_name GROUP BY column_name;
For example, suppose you have an order table orders with the field salesperson_id indicating the salesperson ID. If you want to count the order quantity for each sales, you can write it like this:
SELECT salesperson_id, COUNT(*) AS order_count FROM orders GROUP BY salesperson_id;
This statement returns the order quantity corresponding to each salesperson_id .
Group by multiple fields
Sometimes you need to group by multiple fields, such as wanting to know the order volume of each salesperson in different months. At this time, you can add multiple fields after GROUP BY :
SELECT salesperson_id, order_month, COUNT(*) AS order_count FROM orders GROUP BY salesperson_id, order_month;
This will be grouped by salesperson_id first, and then further subdivided by order_month within each group.
Note: Non-aggregated fields that appear in
SELECTmust appear inGROUP BYclause, otherwise MySQL will report an error (depending on the SQL mode setting).
Filter data in combination with WHERE
Many times we don’t want to count all the data, but only care about a certain part. For example, if you want to see the order situation in 2024, you can add a WHERE condition:
SELECT salesperson_id, COUNT(*) AS order_count FROM orders WHERE order_year = 2024 GROUP BY salesperson_id;
This way, you can filter out irrelevant data first and then perform group statistics, which is more efficient.
Filter grouping results using HAVING
What if you just want to see salespeople with order quantity greater than 10? At this time, WHERE cannot be used because it is filtering the original data. HAVING needs to be used to filter the results after grouping:
SELECT salesperson_id, COUNT(*) AS order_count FROM orders GROUP BY salesperson_id HAVING order_count > 10;
Note: HAVING can be followed by an alias order_count , but WHERE cannot do it because it is a field that only exists after grouping.
Basically that's it. Mastering the combination of GROUP BY and COUNT() can help you quickly complete many data summary tasks. When using it, remember to flexibly adjust the grouping fields and filtering conditions based on business needs.
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