MySQL count distinct statistic results go heavy

MySQL SQL statement, count this keyword can be in the number of statistics, such as

There is a TableA table with the following data in the table:

Id	Name	Age
1	Tony	18
2	Jacky	19
3	JoJo	18

SELECT COUNT (age) from TableA

The above statement can find out how many data are in table tables. The query result is 3

While the Count keyword is used together with distinct, you can count the number of fields in the data that are not duplicated. Such as:

SELECT COUNT (DISTINCT age) from TableA

The query result of the above statement is the de-weight of the age field. The result is 2.

MySQL count distinct statistic results go heavy