SQL count(*) only returns one row - mysql

Question

I'm a beginner in mysql. The following codes look stupid but it is why my codes cannot work. I thought it would give me n rows. However, it only gave me one row. Suppose there are n rows in the table seat, and there are two fields including id, student.
I understand count(*) will return one number. I thought that, for each row, sql will check whether id equals total number of rows. But it didn't.
select id = count(*) as id, student
from seat
The following codes did what I wanted. Could anyone explain what makes them give different results?
select id = count2 as id, student
from seat, (select count(*) as count2 from seat) seat2

Answer 1

One way:
set #num := (select count(*) from seat);
select
id
, student
, case when id = #num then 'special' else 'normal' end as x
from seat
Another way:
select
id
, student
, case when id = x.y then 'special' else 'normal' end as x
from seat
cross join (select count(*) as y from seat) as x
;
NB: In both examples you need the subquery to return just one value in one row.

Answer 2

Given
+----+----------+
| id | lastname |
+----+----------+
| 1 | aaa |
| 2 | bbb |
| 3 | ccc |
+----+----------+
3 rows in set (0.00 sec)
Your first query is an instruction to mysql to count across the entire set
MariaDB [sandbox]> select id = count(*) as id, lastname
-> from users;
+------+----------+
| id | lastname |
+------+----------+
| 0 | aaa |
+------+----------+
1 row in set (0.00 sec)
Clearly this does not return the last id or the correct count. The correct count would be returned by this
select id, lastname , count(*)
from users;
+------+----------+----------+
| id | lastname | count(*) |
+------+----------+----------+
| 1 | aaa | 3 |
+------+----------+----------+
1 row in set (0.00 sec)
And the id and lastname are indeterminate.
Your second query returns a cartesian product
select id = count2, lastname, count2
-> from users, (select count(*) as count2 from users) seat2
-> ;
+-------------+----------+--------+
| id = count2 | lastname | count2 |
+-------------+----------+--------+
| 0 | aaa | 3 |
| 0 | bbb | 3 |
| 1 | ccc | 3 |
+-------------+----------+--------+
3 rows in set (0.00 sec)
Which again does not identify the id which matches the count..
Assuming the id is a number and increments in some fashion a way which does find the last id is
MariaDB [sandbox]> select id,lastname
-> from users
-> where id = (select count(*) from users);
+----+----------+
| id | lastname |
+----+----------+
| 3 | ccc |
+----+----------+
1 row in set (0.00 sec)
But this is dangerous - if id is auto_increment then the number of rows may not match the id because of the way auto_increment is treated (it can be overridden, insert update on duplicate key etc.)
A safer way is to
MariaDB [sandbox]> select id,lastname
-> from users
-> where id = (select max(id) from users);
+----+----------+
| id | lastname |
+----+----------+
| 3 | ccc |
+----+----------+
1 row in set (0.00 sec)

Answer 3

I think you should try something like this :
select id = (select count(*) from seat) as id, student
from seat
Basically your previous request with this statement in the FROM clause (select count(*) as count2 from seat) gives you only row so it can't compare to all of your rows because you put it in your FROM clause two tables seat and seat2 without the same number of rows.
I think you don't need to use two tables.

Answer 4

I thought it [count(*)] would give me n rows. However, it only gave me one row.
Nope. You are using COUNT(*) with no GROUP BY. That makes your query an aggregation query. An aggregation query with no GROUP BY always returns one row -- even when there are no rows in the table.
Your query would be invalid in almost any other database (and in the default configuration of future versions of MySQL) because you have an aggregation query and the column id is not in the GROUP BY nor an argument to an aggregation function.
In MySQL, probably the simplest way to do what you intend uses a subquery:
select id = (select count(*) from seat) as id, student
from seat
I have no idea why you would want to call the boolean result id, but that is what your original query expresses.
Conclusion: You need to practice aggregation queries and look up what GROUP BY does.