Question

two tables:

== customers ==
cust_id

== attachments ==
att_id
cust_id

1 customer -> MANY attachments

i would retrieve all the custmers and, adding the boolean virtual field “has_attach” to select, know if a customer have got attachments.

without GROUP BY, if this is possibile 🙂

Answer 1

Depending on how many many really means, the COUNT(*) option can impose an un-necessary burden.

In which case, the following can sometimes yield benefits.

SELECT
  *,
  CASE WHEN EXISTS (SELECT *
                      FROM attachments
                     WHERE cust_id = customers.cust_id)
       THEN 1
       ELSE 0 END  AS has_attach
FROM
  customers

This is because EXISTS doesn’t actually read all the records. It just checks that any record exists.

In fact, when using indexes this doesn’t even read any records from the table. It just checks that the index points to any matching records and stops there.