Can someone explain to me what this sql command is doing

Question

what does the m. and r . mean? Is it referencing to a table name by its first letter?also other commands like dayname

i have tables called room,employee,meeting, and participants.

select dayname(m.date), count(*) from meeting m, room r where m.room_id=r.room_id group by weekday(m.date) order by weekday(m.date);

Thanks for taking your time on this to answer me

EMPLOYEE: +-----------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-----------+----------+------+-----+---------+-------+ | staff_id | int(11) | NO |PRI | NULL | | | seniority | int(11) | YES | | NULL | | | lastname | char(30) | YES | | NULL | | | firstname | char(30) | YES | | NULL | | +-----------+----------+------+-----+---------+-------+

MEETING: +------------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +------------+----------+------+-----+---------+-------+ | meeting_id | int(11) | NO |PRI | NULL | | | room_id | int(11) | YES |MUL | NULL | | | name | tinytext | YES | | NULL | | | date | date | YES | | NULL | | +------------+----------+------+-----+---------+-------+

PARTICIPANT: +------------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +------------+---------+------+-----+---------+-------+ | meeting_id | int(11) | YES |MUL | NULL | | | staff_id | int(11) | YES |MUL | NULL | | +------------+---------+------+-----+---------+-------+

ROOM: +---------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+----------+------+-----+---------+-------+ | room_id | int(11) | NO |PRI | NULL | | | name | char(50) | YES | | NULL | | +---------+----------+------+-----+---------+-------+

Answer 1

m and r are aliases for the table names meeting and room. In this case, it's syntactically convenient to reference them by these names rather than writing the entire table name out.

SELECT * FROM my_table t WHERE t.my_column_name = 123;

In the above example, t is a convenient way to specify my_table. You can then access columns (attributes) by specifying t.my_column_name.

See this for a general explanation:
http://en.wikipedia.org/wiki/Alias_(SQL)

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Regarding dayname, I'm tempted to say that this is a user-defined SQL function, located elsewhere in the code. As far as I can tell, this is not reserved. I suggest you look through the rest of the SQL you have and see if you can find where it has been defined.

Answer 2

Yes, they're aliases for the table names. You defined them yourself in the 'from' section:

from meeting m, room r

Here you are defining m as an alias for the meeting table and r for room. This can be particularly useful if you e.g. joined against the room table twice: you can then give the two instances different aliases.