What does "expr" do in MySQL's COUNT(expr) function?

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Consider below:

SELECT COUNT(*) FROM table;
SELECT COUNT(1) FROM table;
SELECT COUNT(-2) FROM table;
SELECT COUNT(135392) FROM table;
SELECT COUNT(field) FROM table;
SELECT COUNT(field1 + field2) FROM table;

I am not clear on what expr actually does, or what it can be used for because all of the above SQL statements return the same result. Example below:

+-----------+
| count(..) |
+-----------+
|     54542 |
+-----------+

MySQL's manual (https://dev.mysql.com/doc/refman/8.0/en/counting-rows.html) does not go into very much detail on the expr part, other than using the * symbol

COUNT(<expr>) counts the number of rows where <expr> evaluates to a non-NULL values.

Generally, it is not needed with an expression and would only be used on a single, NULL-able column -- or column that could be NULL due to an outer join.

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Expr is short for expression, itself short for "some valid block of sql that results in a single value for this row when evaluated"

It could be a constant, a column, the result of a function call, variable assignment, case statement etc

—equivalent
COUNT(*)
COUNT(1)
COUNT(‘a’)

—count only males. If the group is 1000 in number and 600 are female, this returns 400
COUNT(case when gender = ‘m’ then ‘a’ else null end)

In supplement to the other answers, the <expr> may optionally start with the word DISTINCT in which case only unique occurrences of the referenced entity/expression/function result are counted

—in a set of 1000 animals, returns 1000
COUNT(gender)

—in a set of 1000 animals, 600 female, returns 2 (only values M and F exist in the group)
COUNT(distinct gender)

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