GROUP BY Clause of a SELECT Statement

MySQL Tutorials - Herong's Tutorial Examples

∟GROUP BY Clause of a SELECT Statement

This section provides tutorial examples on how to use the GROUP BY clause to aggregate orginal rows of the base table into group rows in a SELECT statement.

"GROUP BY clause" modifies the base table by grouping original rows into group rows based on identical combined values of the specified group columns. In other words, each resulting row represents a group of original rows that has a unique combination of the values in the specified group columns. Original columns are reduced to the specified group columns only.

Group rows can also be filtered out by a specified condition in an optional "HAVINNG clause".

"GROUP BY clause" syntax is:

GROUP BY group_columns [HAVING having_condition]

where "group_columns" is a list of columns in the original base table, and "having_condition" is a predicate operation that will result a true or false condition.

Rule 1: Two types of data can be used in select expressions: 1. group columns; 2. an aggregate function of any original columns.

Here are some example of aggregate functions:

COUNT(column): Number of original records in the group represented by this resulting record. Actually, the COUNT() will produce the same number regardless of the specified field.
SUM(column): The sum of all values of the specified column in the group represented by this resulting row.
MIN(column): The minimum value of the specified column in the group represented by this resulting row.
MAX(column): The maximum value of the specified column in the group represented by this resulting row.
AVG(column): The average value of the specified column in the group represented by this resulting row.
GROUP_CONCAT(column): The string of concatenated values of the specified column in the group represented by this resulting row.
JSON_ARRAYAGG(column): The JSON array string of all values of the specified column in the group represented by this resulting row.
JSON_OBJECTAGG(key_column, value_column): The JSON object string of key-value pairs of given 2 columns in the group represented by this resulting row.

For examples, the following is nice salary statistics report per department:

SELECT Department, COUNT(Name) AS NumberOfEmployees,
 MIN(Salary) AS MinimumSalary, MAX(Salary) AS MaximumSalary,
 AVG(Salary) as AverageSalary
FROM Employee WHERE Status='Active' GROUP BY Department

Rule 2: If multiple group columns are used, rows are grouped into a single rows based on the identical combined values of the group columns, not individual identical values. For example, the following statement reports age statistics per department and per sex:

SELECT Department, Sex, COUNT(Name) AS NumberOfEmployees,
 MIN(Salary) AS MinimumSalary, MAX(Salary) AS MaximumSalary,
 AVG(Salary) as AverageSalary
FROM Employee WHERE Status='Active' GROUP BY Department, Sex

If there are 10 individual departments, you will get 20 records, assuming that every department has both sexes.

Rule 3: If a having condition is specified, it will be used to filter out the resulting group rows that do not satisfy this condition. Since the having condition is applied on the grouped rows, it can only use group columns and aggregate functions. For example, the following statement reports salary statistics only for those departments that have more than 10 active employees:

SELECT Department, COUNT(Name) AS NumberOfEmployees,
 MIN(Salary) AS MinimumSalary, MAX(Salary) AS MaximumSalary,
 AVG(Salary) as AverageSalary
FROM Employee WHERE Status='Active'
GROUP BY Department HAVING COUNT(Name)>10

The following is bad example, "Sex='Male'" can only be used in the WHERE clause, not in the HAVING clause:

SELECT Department, COUNT(Name) AS NumberOfEmployees,
 MIN(Salary) AS MinimumSalary, MAX(Salary) AS MaximumSalary,
 AVG(Salary) as AverageSalary
FROM Employee WHERE Status='Active'
GROUP BY Department HAVING sex='Male'

Rule 4: If you want to get all values of a column in the resulting group, you need to use GROUP_CONCAT(), JSON_ARRAYAGG(), and JSON_OBJECTAGG() functions. For example, the following statement reports salary statistics only for those departments that have more than 10 active employees:

SELECT Department, Sex, COUNT(Name) AS NumberOfEmployees,
 MIN(Salary) AS MinimumSalary, MAX(Salary) AS MaximumSalary,
 AVG(Salary) as AverageSalary,
 GROUP_CONCAT(Name) AS NameList,
 JSON_ARRAYAGG(Name) AS NameArray,
 JSON_OBJECTAGG(Nam, Salary) AS NameAndSalary
FROM Employee WHERE Status='Active' GROUP BY Department, Sex