Mysql inner join error

Databases
1

Hi

I am constructing an inner join with php - below is the code that my php code resolves to:

SELECT * FROM comp-profile-01-general a INNER JOIN comp-profile-02-social b ON a.co_num = b.co_num INNER JOIN comp-profile-03-specialties c ON a.co_num = c.co_num WHERE a.co_num = 300023

I am getting the error:
#1054 - Unknown column ‘a.co_num’ in ‘where clause’

What have i done wrong?

Thanks.

2 
SELECT
    *
FROM 
    comp-profile-01-general AS a
INNER JOIN
    comp-profile-02-social AS b
        ON a.co_num = b.co_num
INNER JOIN
    comp-profile-03-specialties AS c
        ON a.co_num = c.co_num
WHERE a.co_num = 300023

When using an alias for a table the syntax is

table_name AS alias_name

Also do you need all fields from all three tables? If not just list the required fields only in the SELECT clause

3

you’ve put backticks around the tablename.columnname combo

so a.co_num is wrong, but a.co_num is right

a.co_num is also right, and it’s “best practice” not to use backticks at all, based on naming your tables and columns so that there is no need to use backticks