How to understand the group by statement of grouping query in MySQL Database

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1: Statement order of grouping function 1 SELECT ... 2 FROM ... 3 WHERE ... 4 GROUP BY ... 5 HAVING ... 6 ORDER BY ... II: Difference between WHERE and HAVING filter conditions Data source position keyword WHERE original table Before ORDER BY statement WHERE HAVING Grouped Result Set After ORDER BY statement HAVING III: Example #1. Query the maximum age of students in each class SELECT MAX(age),class FROM STU_CLASS GROUP BY CLASS;#2. Query the maximum age of boys and girls in each class SELECT MAX(age)，class,sex FROM STU_CLASS GROUP BY class,sex;#3. Query the average age of boys and girls in each class, and the average age is greater than 15 years SELECT AVG（age）,class,sexFROM STU_CLASSGROUP BY class,sex HAVING avg(age) > 15; 4: Attention

1. Typically, the GROUP BY keyword is used in conjunction with an aggregate function to group records using the GROUP BY keyword and then evaluate each group using the aggregate function. The GROUP BY keyword and aggregate functions are often used in statistics.

COUNT() function: used to count the number of records.

SUM() function: Used to calculate the sum of the values of a field.

AVG() function: Used to calculate the average of the values of a field.

MAX() function: Used to query the maximum value of a field.

MIN() function: The minimum value used for the query field.

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