help sql query

[doraemona]

Hi

We have a table of customers, there are two columns with telephone_1 and telephone_2, normally telephone_2 are used for mobile phone numbers but sometimes telephone_1 is also a mobile, and sometimes the customer has same mobile number for telephone_1 and 2

We have to find out how many customers has mobile phone. How do we do this acurately?

I have tried

select tel_1, tel_2 from customer
where tel_1 like '6%'
or tel_2 like '6%'
group by tel_1, tel_2

which gave me 48 rows back

then

select distinct tel_1 from customer
where tel_1 like '6%'

which gave me back 5 rows

select distinct tel_2 from customer
where tel_1 like '6%'

returns 40 rows

which result is correct???

[kmesser]

1st query returns one row for each unique pair of tel1 and tel2 when both start with 6.

2nd query returns one row (of tel1) for each unique tel1 which begins with 6.

3rd query returns one row (of tel2) for each tel2 number where tel1 starts with6.

couldn't you just say:

select count(*) from customers where tel1 like '6%' or tel2 like '6%';