MYSQL计算第一个表的ID，在第二个表中找到值

Question

I have two tables,

table1 and table2 both tables has these columns

id, name, rel_id

now i would like to have a query to count ids of the table1 where name from table2 is equals to john and table1 rel_id equals to table2 rel_id. so something like this (this is not correct that's why i need help to make it work).

Select count(ids) from table1
where table2.name="john" 
and table1.rel_id=table2.rel_id

Answer 1

Well, one way is to use a join:

Select count(t1.id)
from table1 t1 join
     table2 t2
     on t1.rel_id = t2.rel_id
where t2.name = 'john';

Note that this uses table aliases to distinguish all the columns in each table. Because the tables have the same columns, you need to identify the table for each column. Also, the I changed the string constant to use single quotes rather than double quotes.

Answer 2

You need to look into joins so :

select count(ids) from table1 join table2 on table1.rel_id=table2.rel_i where table2.name="john"

A short intro from W3C schools: http://www.w3schools.com/sql/sql_join.asp

The full MySQL URL for more reference http://dev.mysql.com/doc/refman/5.0/en/join.html